Refereed Journals


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2022

  • Abstract: Recently an anti-COVID-19 therapeutic application of the drug 2-deoxy-D- glucose (2-DG) an analogue of glucose has been developed in collaboration between Institute of Nuclear Medicine and Allied Sciences (INMAS), India, Defence Research and Development Organisation (DRDO), India, and Dr Reddy’s Laboratories (DRL), India. As per the reports 2-DG is effective against SARS-COV- 2. Publication of phase 2 and phase 3 clinical trial data is pending. However, it has been shown that 2-DG reduces the supplemental oxygen dependence on covid-19 infected patients and make their recovery faster. The present outbreak of Covid-19 infection due to SARS-CoV-2, a virus from the coronavirus family, has become a major menace to human being. As the understanding of the mechanism of the therapeutic action of 2-DG on SARS-CoV-2 infected hosts is missing, in this work we have studied the possible inhibitory interaction of the drug with two different pathways (a) with non-structured viral proteins involved in translation and replication of SARS-CoV-2 and (b) its inhibition mechanism of the glycolysis pathway. We have used our fully automated novel drug designing platform with state-of-the-art free energy of binding calculator PRinMTML-ESS to evaluate the role of 2-DG as an antiviral and glycolysis pathway inhibitor in SARS-CoV-2 affected humans. Docking, all atom molecular dynamic simulation and enhanced free energy sampling methods used in PRinMTML-ESS have predicted that 2-DG effectively reduced the replication of SARS-CoV-2 in human cell by reducing the glycolytic flux, by competitive inhibition of glucose in binding with the enzyme hexokinase. 2-DG is generally administered in covid patient along with other antivirals and steroid, hence it can be used as a mild clinical therapy which can reduce the viral replication, inflammation when given in the earlier stage of the disease.

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2021

  • Abstract: One of the most ubiquitous and technologically important phenomena in nature is the nucleation of homogeneous flowing systems. The microscopic effects of shear on a nucleating system are still imperfectly understood, although in recent years a consistent picture has emerged. The opposing effects of shear can be split into two major contributions for simple atomic and molecular liquids: increase of the energetic cost of nucleation, and enhancement of the kinetics. In this perspective, we describe the latest computational and theoretical techniques which have been developed over the past two decades. We collate and unify the overarching influences of shear, temperature, and supersaturation on the process of homogeneous nucleation. Experimental techniques and capabilities are discussed, against the backdrop of results from simulations and theory. Although we primarily focus on simple systems, we also touch upon the sheared nucleation of more complex systems, including glasses and polymer melts. We speculate on the promising directions and possible advances that could come to fruition in the future.

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  • Abstract: The combined effect of surfactant and nanoparticles on the surface and interfacial tension is of great importance in various industrial processes. This study provides an insight into the combined effect of a non-ionic surfactant (Triton X-100 and Tween20) and SiO2 nanoparticles at the air–water interface using experimental and theoretical approaches. The surfactant concentration was kept constant at CMC, and nanoparticle concentration was varied from 0 wt% to 1.2 wt%. The results show that nanoparticles reduce the efficiency of Triton X-100 and Tween 20 surfactants at the air–water interface, mainly due to the adsorption of surfactants on NPs, which is also supported by the theoretical model. Furthermore, density functional theory simulations are conducted to understand the adsorption of the non-ionic surfactant on silica NPs with the variation in the degree of ionization of silica NPs. We have observed that the adsorption of the non-ionic surfactants on silica NPs reduces as the degree of ionization of NPs increases, which is in agreement with the experimental observations.

    Submitted

  • Abstract: The capture, activation, and dissociation of carbon dioxide (CO2) is of fundamental interest to overcome the ramifications of the greenhouse effect. In this regard, high-throughput screening of two-dimensional MXenes has been examined using well-resolved first-principles simulations through DFT-D3 dispersion correction. We systematically investigated different types of structural defects to understand their influence on the performance of M2X-type MXenes. Defect calculations demonstrate that the formation of M2C(VMC) and M2N(VMN) vacancies require higher energy, while M2C(VC) and M2N(VN) vacancies are favorable to form during the synthesis of M2X-type MXenes. The M2X-type MXenes from group III to VII series show remarkable behavior for active capturing of CO2, especially group IV (Ti2X and Zr2X) MXenes exhibit unprecedentedly high adsorption energies and charge transfer (>2e) from M2X to CO2. The potential CO2 capture, activation, and dissociation abilities of MXenes are emanated from Dewar interactions involving hybridization between pi orbitals of CO2 and metal d-orbitals. Our high-throughput screening demonstrates chemisorption of CO2 on pure and defective MXenes, followed by dissociation into CO and O species.

    Submitted

  • Abstract: Since the onset of global pandemic, the most focused research currently in progress is the development of potential drug candidates and clinical trials of existing FDA approved drugs for other relevant diseases, in order to repurpose them for the COVID-19. At the same time, several high throughput screenings of drugs have been reported to inhibit the viral components during the early course of infection but with little proven efficacies. Here, we investigate the drug repurposing strategies to counteract the coronavirus infection which involves several potential targetable host proteins involved in viral replication and disease progression. We report the high throughput analysis of literature-derived repurposing drug candidates that can be used to target the genetic regulators known to interact with viral proteins based on experimental and interactome studies. In this work we have performed integrated molecular docking followed by molecular dynamics (MD) simulations and free energy calculations through an expedite in silico process where the number of screened candidates reduces sequentially at every step based on physicochemical interactions. We elucidate that in addition to the pre-clinical and FDA approved drugs that targets specific regulatory proteins, a range of chemical compounds (Nafamostat, Chloramphenicol, Ponatinib) binds to the other gene transcription and translation regulatory proteins with higher affinity and may harbour potential for therapeutic uses. There is a rapid growing interest in the development of combination therapy for COVID-19 to target multiple enzymes/pathways. Our in silico approach would be useful in generating leads for experimental screening for rapid drug repurposing against SARS-CoV-2 interacting host proteins.

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  • Abstract: The study of the structural behaviour of pure and multi-component lipids at various temperatures and the interaction of these multi-component lipids with pharmaceutically important drugs carry huge importance. Here, we investigated the phase behaviour of the pure PSPC (1-palmitoyl-2-stearoyl-sn-glycero-3-phosphocholine), and multicomponent PSPC and DSPE-PEG2000(1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N[amino(polyethylene glycol)-2000]) membranes at seven different temperatures ranging from 280 K to 360 K, and calculated their structural properties. We observe a transition from the gel phase to the liquid crystalline phase between 320 K and 330 K in agreement with experimental reports for pure PSPC. PSPC remained in the tilted gel phase L\b{eta}' at 320 K and 310 K, entered the 'mixed ordered' domain with a partially interdigitated region at 300 K, and finally formed the sub gel phase at 280 K. We studied the self-assembly for the multicomponent PSPC and DSPE-PEG2000 membranes and found the coexistence of ordered and disordered phases at 320 K. In comparison to the pure PSPC, for multicomponent system, this transition was gradual, and a complete liquid crystalline to gel phase transformation occurred between 320 K and 310 K. We further studied the interaction of Paclitaxel with pure PSPC and PEGylated multicomponent lipid bilayers using umbrella sampling technique and observed PEG promotes the interaction of Paclitaxel with the later one in comparison to the former. Above the bilayer transition temperature, Paclitaxel interacts more with the bilayer and enters inside the bilayer easily for both systems. Understanding of structural and interaction behaviour of the PEGylated multicomponent lipid bilayers with Paclitaxel will help explore Paclitaxel based drug applications in the future.

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  • Abstract: Properties of crystalline and amorphous materials are characterized by the underlying long-range and local crystalline order. Deformations and defects are structural hallmarks of plasticity, ice formation, and crystal growth mechanisms. Partitioning topological networks into constituent crystal building blocks, which is the basis of topological identification criteria, is an intuitive approach for classification in both bulk and confinement. However, techniques reliant on the convex hull for assigning orientations of component units fail for non-convex blocks. Here, we propose a new framework, called Topological Unit Matching (TUM), which exploits information from topological criteria for an efficient shape-matching procedure. TUM is a general family of algorithms, capable of quantifying deformations and unambiguously determining grains of bulk and confined ice polymorphs. We show that TUM significantly improves the identification of quasi-one-dimensional ice by including deformed prism blocks. We demonstrate the efficacy of TUM by analyzing supercooled water nanoparticles, amorphous ice, and phase transitions in an ice nanotube. We also illustrate the superiority of TUM in resolving topological defect structures with minimal parameterization.

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  • Abstract: Here, we report new chemical entities that exhibit highly specific binding to the 3 chymotrypsin like cysteine protease present in the novel severe acute respiratory syndrome coronavirus 2 . Because the viral 3 CLpro protein controls coronavirus replication, 3 CLpro is identified as a target for drug molecules. We implemented an enhanced sampling method in combination with molecular dynamics and docking to reduce the computational screening search space to four molecules that could be synthesized and tested against. Our computational method is much more robust than any other method available for drug screening because of sampling of the free energy surface of the binding site of the protein and use of explicit solvent. We have considered all possible interactions between all the atoms present in the protein, ligands, and water. Using high performance computing with graphical processing units, we were able to perform a large number of simulations within a month and converge the results to the four most strongly bound ligands from a set of 17 ligands with lower docking scores. Additionally, we have considered N3 and 13b ketoamide inhibitors as controls for which experimental crystal structures are available. Out of the top four ligands, PI 06 was found to have a higher screening score compared to the controls. Based on our results and analysis, we confidently claim that we have identified four potential ligands, out of which one ligand is the best choice based on free energy and the most promising candidate for further synthesis and testing against.

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  • Abstract: The structure and dynamics of water droplets on a bilayer graphene surface are investigated using molecular dynamics simulations. The effects of solid or water and air or water interfaces on the local structure of water droplets are analyzed in terms of the hydrogen bond distribution and tetrahedral order parameter. It is found that the local structure in the core region of a water droplet is similar to that in liquid water. On the other hand, the local structure of water molecules at the solid or water and air or water interfaces, referred to as the interface and surface regions, respectively, consists mainly of three coordinated molecules that are greatly distorted from a tetrahedral structure. This study reveals that the dynamics in different regions of the water droplets affects the intermolecular vibrational density of states: It is found that in the surface and interface regions, the intensity of vibrational density of states at 50 inverse centimeters is enhanced, whereas those at 200 and 500 inverse centimeters are weakened and redshifted. These changes are attributed to the increase in the number of molecules having fewer hydrogen bonds in the interface and surface regions. Both single molecule and collective orientation relaxations are also examined. Single molecule orientation relaxation is found to be marginally slower than that in liquid water. On the other hand, the collective orientation relaxation of water droplets is found to be significantly faster than that of liquid water because of the destructive correlation of dipole moments in the droplets. The negative correlation between distinct dipole moments also yields a blueshifted libration peak in the absorption spectrum. It is also found that the water graphene interaction affects the structure and dynamics of the water droplets, such as the local water structure, collective orientation relaxation, and the correlation between dipole moments. This study reveals that the water or solid and water or air interfaces strongly affect the structure and intermolecular dynamics of water droplets and suggests that the intermolecular dynamics, such as energy relaxation dynamics, in other systems with interfaces are different from those in liquid water.

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  • Abstract: Using molecular simulations and a modified classical nucleation theory, we study the nucleation, under flow, of a variety of liquids: different water models, Lennard-Jones, and hard sphere colloids. Our approach enables us to analyze a wide range of shear rates inaccessible to brute-force simulations. Our results reveal that the variation of the nucleation rate with shear is universal. A simplified version of the theory successfully captures the nonmonotonic temperature dependence of the nucleation behavior, which is shown to originate from the violation of the Stokes-Einstein relation.

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2020

  • Abstract: We report the formation of discrete molecular rings/spirals of small molecules (1,3-dithia derivatives of ferrocene) on a highly oriented pyrolytic graphite (HOPG) surface. On the basis of microscopy and theoretical calculations, molecular level arrangement within the molecular rings is understood. The molecular rings show a limiting inner diameter, and we interpret it to be related to the critical intermolecular interaction limit. This limiting value of the inner diameter is surprisingly correlated with that observed for molecular rings/disks of a few reported molecules. The correlation reveals that molecular rings formed typically by weak van der Waals interactions should always show a limiting inner diameter and should be independent of molecular structure, size, and chemical nature.

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  • Abstract: The deterioration of the aquatic environment by the heavy metal ions contamination causes serious threat to environment and human beings. However, the treatment of complex industrial wastewater by simultaneous removal of multiple heavy metal ions via a one-step method is still extremely challenging. To this end, we synthesize ferrous sulfide (FeS) and carboxyl-functionalized ferroferric oxide (CFFO) nanoparticles, which were introduced into polyvinylidene fluoride (PVDF) matrix (individually/mixed together in an optimum ratio) through phase inversion technique. Three types of mixed matrix membranes (MMMs) were developed, viz. FeS/PVDF, CFFO/PVDF and FeS/CFFO/PVDF. The prepared membranes were characterized by SEM, TEM, AFM, FTIR, XRD, BET, and XPS techniques. The properties of the membranes were also examined for pure water flux, hydrophilicity, water uptake capacity, mechanical and thermal property, salt separation and simultaneous separation of toxic heavy metal ions such as lead, (Pb), cadmium (Cd), and chromium (Cr) from industrial ground water. The resultant membranes exhibited relatively high water flux (340–1266 L/m2h) than the unmodified PVDF membrane, due to changes in the porosity and hydrophilicity of the membranes. FeS/CFFO/PVDF membrane showed that it could effectively treat Pb, Cd, Cr and As contaminated industrial ground water, simultaneously with a high removal efficiency of about 88% for Cr(VI), 99% for Cd2+, 99% for Pb2+ and 95% for As in a single filtration process. In addition, the developed membranes conspicuously reduce their concentrations below the maximum contaminant level of WHO and BIS (India). The probable mechanism of separation of heavy metal ions through MMMs could be understood through FTIR and XPS techniques. The results of this study inferred that FeS/CFFO/PVDF membrane is a potential candidate for the simultaneous separation of Pb, Cd, Cr, and As.

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  • Abstract: Two-dimensional nanocrystals with semiconducting electronic properties are emerging as promising materials for electronic devices. Here, we present the density functional theory calculations of structural stability, Raman spectra and electronic properties of monolayer, bilayer and trilayer antimony. The cohesive energy and phonon band dispersion results revealed that free-standing Sb systems are stable materials. Calculated Raman spectra showed distinct active modes, thus facilitating the characterisation of multilayered structures in different stacking arrangements. It was found that high-frequency in-plane and out-of-plane (A1g) modes can shift as much as the layer number increases from monolayer to trilayer. Band structure calculations showed that monolayer and bilayer (AA stacked) Sb are semiconductors with band gap values, respectively, whereas bilayer (AB) and trilayer Sb displayed metallic character. Spin-orbit coupling interaction was also incorporated in band structure calculations and was found to reduce the band gap of monolayer Sb to while it does not effect on the band gap values of other systems. Moreover, it was seen that nanocrystalline Sb exhibit isotropic mechanical properties. The carrier mobility calculations showed that electron hole mobility increases by 5/32 times from monolayer to trilayer, respectively.

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  • Abstract: We develop intuitive metrics for quantifying complex nucleating systems under confinement. These are shown to arise naturally from the analysis of the topological ring network, and are amenable for use as order parameters for such systems. Drawing inspiration from qualitative visual inspection, we introduce a general topological criterion for elucidating the ordered structures of confined water, using a graph theoretic approach. Our criterion is based on primitive rings, and reinterprets the hydrogen-bond-network in terms of these primitives. This approach has no a priori assumptions, except the hydrogen bond definition, and may be used as an exploratory tool for the automated discovery of new ordered phases. We demonstrate the versatility of our criterion by applying it to analyse well-known monolayer ices. Our methodology is then extended to identify the building blocks of one-dimensional n-sided prismatic nanoribbon ices.

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  • Abstract: Structural analyses are an integral part of computational research on nucleation and supercooled water, whose accuracy and efficiency can impact the validity and feasibility of such studies. The underlying molecular mechanisms of these often elusive and computationally expensive processes can be inferred from the evolution of ice-like structures, determined using appropriate structural analysis techniques. We present d-SEAMS, a free and open-source postprocessing engine for the analysis of molecular dynamics trajectories, which is specifically able to qualitatively classify ice structures in both strong-confinement and bulk systems. For the first time, recent algorithms for confined ice structure determination have been implemented, along with topological network criteria for bulk ice structure determination. We also propose and validate a new order parameter for identifying the building blocks of quasi-one-dimensional ice. Recognizing the need for customization in structural analysis, d-SEAMS has a unique code architecture built with nix and employing a YAML Lua scripting pipeline. The software has been designed to be user-friendly and extensible. The engine outputs are compatible with popular graphics software suites, allowing for immediate visual insights into the systems studied. We demonstrate the features of d-SEAMS by using it to analyze nucleation in the bulk regime and for quasi-one- and quasi-two-dimensional systems. Structural time evolution and quantitative metrics are determined for heterogeneous ice nucleation on a silver-exposed AgI surface, homogeneous ice nucleation, flat monolayer square ice formation, and freezing of an ice nanotube.

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  • Abstract: Inspired by the discovery of high-pressure ice inclusions in diamond, we investigate the rich structural diversity of high-pressure phases of quasi-two-dimensional water constrained by diamond matrices, using molecular dynamics simulations. Monolayer ices formed are structurally similar to monolayer ices constrained by graphene. We report new bilayer and trilayer ice phases, specifically, the AB II, ABC I, ABC II, and AAB/ABB ordered phases. The relative stability of the interlayer hydrogen bonds between adjacent layers reveals differences in structural properties. We observe grain-boundary migration between pockets of AAB and ABB ices, during the pressurization process. The compression-limit phase diagram in the slit width–lateral pressure plane has been constructed and analyzed. Our results indicate that the phase behavior of confined ice is qualitatively independent of the diamond–water force-field parameters and is significantly influenced by the lattice structure and geometry of the confining surfaces.

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  • Abstract: We report the application of Fe3O4-functionalized boron nitride nanosheets for the remediation of As(III) ions from contaminated water. The specific surface area of the nanocomposite has been found as 179.5 m2 g–1. Due to its superparamagnetic nature at room temperature, the nanocomposite can be easily isolated from the solution under an external magnetic field. For As(III) ions, the maximum adsorption capacity of the nanocomposite is obtained as 30.3 mg g–1, which is approximately 4 times more than that of the bare BNNSs (8.5 mg g–1). The results from density functional theory calculations are also in close agreement with experimental findings and show that As(OH)3 binds more (4 times) efficiently to the BNNS-Fe3O4 nanocomposite than the bare BNNSs, implying a 4 times higher adsorption capacity of the nanocomposite. Especially, it is found that the synthesized nanocomposite could lessen the concentration of As(III) ions from 134 to 2.67 ppb in a solution at 25 °C. On increasing the temperature to 35 °C, the level of As(III) ions could be reduced from 556 to 10.29 ppb, which is close to the limit prescribed by the World Health Organization. The adsorbent was easily separable and showed regeneration properties. These outcomes depict the prospect of using BNNS-Fe3O4 nanocomposites as commercial adsorbents for the removal of As(III) ions from contaminated water.

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  • Abstract: n this work, we performed ab initio calculations to investigate the structural stability, carrier mobility, and CO2 separation and capture ability of mono-layered group III nitrides (XN) and phosphides (XP) (X = Al, Ga, In). The results showed that all the six two-dimensional sheets exhibit indirect band gaps, ranging from 1.35 eV for InP to 4.02 eV for AlN by using HSE functional. Mobility calculations performed using deformation potential theory shows that the mobility is dominated by holes as compared to electrons and reaches a value of 1.7 ×103 cm2V-1s-1 for AlN and 6.9 ×102 cm2 V-1s-1 AlP. Density functional perturbation theory was used to predict the frequency of Raman active modes, the results showed a red shift in the calculated Raman peak frequency with increase in the mass of metal ion. The calculated adsorption energy of CO2 is in the range of -0.19 eV to -0.22 eV over XN, whereas the adsorption energy varies from -0.51 eV to -1.12 eV over XP, which is larger than that of graphene and hexagonal boron nitride. The adsorption energy of CO2 on various nanostructures follows the order as EInN >EGaN >EAlN and >EInP >EGaP >EAlP . On the other hand, it is seen that N2 show significantly weaker interaction with the surfaces of XN and XP as compared to CO2, indicating high selectivity of sheets towards CO2 capture.

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  • Abstract: Early detection and easier follow up of oral squamous cell carcinoma OSCC would significantly improve the morbidity and mortality associated with it. With newer technologies, it has become possible to validate cancer biomarkers in saliva with high sensitivity and specificity. There is however a need to further validate these biomarkers in cohorts of different ethnic groups. Our objective was to validate previously evaluated salivary biomarkers in Indian population. The study enrolled 117 patients. These were grouped into subcatergories of 31 early TNM stage I to II and 27 late stage OSCC TNM stage III to IV, 30 PMOD and 29 post treatment patients. There were 42 control subjects. We evaluated 3 protein markers, IL 1, IL 8 and LGALS3BP using ELISA, from unstimulated saliva samples.

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  • Abstract: Since the breakthrough of graphene, 2D materials have engrossed tremendous research attention due to their extraordinary properties and potential applications in electronic and optoelectronic devices. The high carrier mobility in the semiconducting material is critical to guarantee a high switching speed and low power dissipation in the corresponding device. Here, we review significant recent advances and important new developments in the carrier mobility of 2D materials based on theoretical investigations. We focus on some of the most widely studied 2D materials, their development, and future applications. Based on the current progress in this field, we conclude the review by providing challenges and an outlook in this field.

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  • Abstract: Molecular dynamics simulations are performed to study the separation of CO2 from flue gas using carbon and boron nitride nanotube membranes. Flue gas is considered as a binary mixture of CO2 and N2 with CO2 molar concentrations of 25 and 50%. Nanotubes of boron nitride and carbon with three different chiralities of (10,0), (14,0), and (18,0) are considered for the investigation of the effect of pore size on gas separation. The permeance of CO2 is found to be higher in the boron nitride nanotube (BNNT) membrane compared to that in the carbon nanotube (CNT) membrane. The estimated CO2 permeance is of the order of 107 GPU in both types of membranes at an initial applied pressure of 50 bar. The gas permeance decreases with a decrease in membrane pore size. The optimum pore size is determined on the basis of gas permeance and the corresponding selectivity data. The free-energy changes for N2 molecules to pass through from the gas phase to the BNNT and CNT membranes are 19.36 and 9.06 kJ/mol, respectively, indicative of a significant barrier for N2 permeance in the case of BNNT. Selectivity analysis also shows that the performance of boron nitride is better than that of carbon nanotube under same conditions. This work suggests that the direct use of boron nitride nanotube as a membrane can be useful for separating CO2 from flue gas with high permeance.

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  • Abstract: Two-dimensional covalent organic frameworks (2D-COFs) belong to a new class of molecular materials that have attracted huge attention in recent years due to their analogous nature to graphene. In this work, we present a systematic study of the electronic structure, carrier mobility and work function of imine based 2D-COFs. We identify these 2D-COFs as a new class of semiconducting materials with tunable electronic/optoelectronic properties and significant mobility. The results show that by rationally doping 2D-COFs at the molecular level, it is possible to control their structural and optoelectronic responses. Cohesive energy calculations revealed that all the studied 2D-COFs are thermodynamically stable. Also, the calculated binding energy of 2D-COFs on HOPG was found to be less than 1 eV, which indicates that the COFs do not interact strongly with HOPG, and it will not affect their electronic properties. Additionally, we have synthesized a 2,4,6-pyrimidinetriamine based 2D-COF and experimentally measured its band gap using scanning tunnelling spectroscopy. The experimentally measured band gap is found to be in good agreement with theoretical results.

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  • Abstract: Hydrodynamic flow can have complex and far-reaching consequences on the rate of homogeneous nucleation. We present a general formalism for calculating the nucleation rates of simply sheared systems. We have derived an extension to the conventional Classical Nucleation Theory, explicitly embodying the shear rate. Seeded molecular dynamics simulations form the backbone of our approach. The framework can be used for moderate supercooling, at which temperatures brute-force methods are practically infeasible. The competing energetic and kinetic effects of shear arise naturally from the equations. We show how the theory can be used to identify shear regimes of ice nucleation behavior for the mW water model, unifying disparate trends reported in the literature.

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  • Abstract: Two dimensional surface-confined metal–organic networks are metal doped self-assembled monolayers of molecules on solid surfaces. We report the formation of uniform large area solution-processed semiconducting SMONs of Pd and Zn with mellitic acid on a highly oriented pyrolytic graphite surface under ambient conditions. The microscopic structure is determined using scanning tunneling microscopy , atomic force microscopy , and X-ray photoelectron spectroscopy . Using tunneling spectroscopy, we observed a reduction in the band gap of 900 , respectively, compared to the pure MA assembly. Concomitant density functional theory calculations reveal that the coordination geometry and microscopic arrangement give rise to the observed reduction in the band gap. The dispersion of the frontier bands and their delocalization due to strong electronic coupling suggest that the MA–Pd SMON could potentially be a 2D electronic material.

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  • Abstract: A hybrid approach combining machine learning algorithms with molecular simulation is utilized to screen hypothetical metal–organic framework (h-MOF) database for the best material to separate ethane (C2H6) and ethylene (C2H4). In particular, we rationalized the relation between structural and chemical properties of h-MOF with the C2H6/C2H4 selectivity. 8% h-MOFs were chosen randomly from the h-MOF dataset as a training set. The simulations were conducted at 298 K and 1 bar using a multicomponent grand-canonical Monte Carlo method to obtain the C2H6/C2H4 selectivity. Based on the training set, the random forest (RF) model was developed to predict the selectivity of the rest of the h-MOFs. Among all the chemical and structural properties, void fraction plays a significant role in predicting the equilibrium C2H6/C2H4 selectivity. The trained machine learning model can reasonably predict the C2H6/C2H4 selectivity of the remaining h-MOF materials with an RF score of 0.89. Four h-MOFs have shown the best performance, which was compared with the previously discovered materials. The top four h-MOFs were further simulated at different pressures to obtain the adsorption isotherms. Further, the energy contribution of secondary building units and the local density profiles were analyzed to understand the enhanced interaction between h-MOF atoms and C2H6.

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2019

  • Abstract: Covalent organic frameworks (COFs), a class of carbon-based polymeric materials have the potential to be used as hydrogen adsorbent. Three dimensional (3D) COFs, due to their low density and high surface area, although have higher hydrogen adsorption, they have less stability than two dimensional (2D) COFs. Here we studied porphyrin group containing 2D COF, namely H2,P-COF for hydrogen storage using density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations and the results were compared with the most common 2D COFs, COF-1 and COF-5. Cylindrical shaped 2D COFs where isolated unit blocks are stacked in multiple layers due to van der Waals interactions between individual layers, increase the effective surface area for hydrogen storage. A further modification has been done by bridging the inter-layer gap by pyridine molecules. Insertion of pyridine increases the separation distance of layers of 2D COFs as well as the free volume. Feasibility of the structure formation and stability of all the structures were checked using DFT study. To ensure the structural stability of bridged COFs after hydrogen loading, alternating layers of COF were bridged. Single, bi, tri and tetra -pyridine molecules were chemically bonded with the existing carbon ring present in between two C2O2B rings to form pyridine bridged H2,P-COFs. Our GCMC results show a significant increase in storage capacity which is mainly due to an increase in the free volume of the material. The highest capacity of 5.1 wt% and 20 g H2/L at 298 K and 100 bar, above the gravimetric DOE goal, has been found at room temperature for tetra-pyridine doped porphyrin COF structure.

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  • Abstract: Molecular dynamics simulations of Lennard Jones particles have been performed to study the self assembled structure of nanoparticles formed upon evaporation of nanofluid droplets on a heated surface. Different shapes of NPs such as a sphere, cube, triangle, and rod are considered in this work for the nanofluid. The influence of solvent surface and NP surface interaction strengths, size, and shape of NPs is analyzed on the structure of the NP deposit formed upon evaporation. The solvophilic substrate leads to the formation of different structures such as the hemispherical clump, monolayer, and ring depending on the size, shape, and interaction between other pairs of atoms. On the other hand, the solvophobic substrate always leads to a clump of NPs. Structural and thermodynamic properties are calculated to characterize the self-assembled structures. The low pair energy and high excess entropy are the characteristics of a ring structure. Furthermore, the mean square displacement of NPs is found to be lower for the ring structure compared to the hemispherical clump structure, and this observation is independent of the shape and size of the NP. The change in arrangement from disorder to order is observed for rod shaped NPs during evaporation.

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  • Abstract: We explored the aspirin adsorption and their hydrogen evolution reaction activity in waste water of borocarbonitride sheets. Our results indicate that BCN sheets considered here show HER activity and exhibit superior performance regarding adsorption of aspirin in waste water in comparison with graphene and hexagonal boron nitride. The drug molecule possesses a strong affinity to BCN, with the order of binding energy on following the order BCN h BN graphene.

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  • Abstract: Ice formation causes numerous problems in many industrial fields as well as in our daily life. The control of ice nucleation and rational design of anti-icing surface with low ice adhesion are desirable in various industries such as aircraft, power line, ships, building, and cryopreservation. However, despite considerable attention in the development of ice or water-repellent surfaces, it is still challenging to design icephobic or anti-icing surfaces with high resistance to icing. In this study, coarse-grained molecular dynamics simulation is utilised to investigate the ice adhesion mechanism on lubricant-infused nanotextured surfaces. Using steered molecular dynamics simulation, we find that the adhesion strength of ice on nanotextured surfaces impregnated with lubricant films to be higher compared to that on textured surfaces in presence excess lubricant films. We illustrate that the ice adhesion strength depends on the texture density and the ice adhesion strength increases with nanoposts density. Lubricant-impregnated surfaces (LISs) with higher posts density exhibit greater adhesive interaction energy due to the large contact area between the icecube and the textured surface. This systematic study enhances our understanding of ice adhesion mechanism on LISs which can apply for designing novel anti-icing surfaces with extremely weak ice adhesion strength

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  • Abstract: Understanding the interaction between nanoscale materials and nucleobases is essential for their use in nanobiotechnology and nanomedicine. Our ab initio calculations indicate that the interaction of nucleobases with boron–carbon–nitride (BCN) is mainly governed by van der Waals interactions. The adsorption energies, ranging from decrease in the order of, which can be attributed to interactions and different side groups of the nucleobases. We found that anions (N and O atoms) of nucleobases prefer to stay on top of cation of the substrate. The results also showed that BCN exhibits superior binding strength than graphene and boron–nitride-based materials. We also found that upon adsorption, the fundamental properties of BCN and nucleobases remains unaltered, which suggests that BCN is a promising template for self-assembly of nucleobases.

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  • Abstract: In this work, a comparative study on water-stable microporous adsorbents is conducted computationally in the quest of a suitable adsorbent for post-combustion CO2 capture. In this regard, three metal–organic frameworks (MOFs), two covalent organic frameworks (COFs), and a single-wall carbon nanotube (SWCNT) are investigated under the same flue gas conditions. The simulation results show that the pure component adsorption capacity for CO2 follows the order SWCNT > InOF-1 > COF-300 > UiO-66 > COF-108 > ZIF-8 at post-combustion conditions. Further, these materials are impregnated with ionic liquids to examine the effect on the CO2 separation ability of these materials. The adsorption capacity enhances by incorporating ionic liquids, especially [EMIM][SCN] compared to [EMIM][BF4] as a result of a stronger interaction and being less bulky in nature. We further tested the effect of the presence of other components of flue gas on the selectivity of CO2 over N2, and we found that the presence of SO2 and water vapor reduces the CO2 selectivity in all of the materials considered in this work.

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  • Abstract: In this study, we report the fabrication of hydrophobic cotton fibers via simple and cost-effective one step easy fabrication approach by utilizing waste recycled polyethylene (PE) gloves. Also, we prepared silres-modified cotton fibers with hydrophobic and oleophilic properties via a simple dip coating process. The resultant both coated cotton (PE and silres) fibers exhibited high repellency towards the water with water contact angle < 135° and superoleophilicity with an oil contact angle ˜ 0°. Due to such special wettability, the as-prepared cotton fibers exhibited high selective oil absorption capacity when used as absorptive materials for separating various oil/water mixtures under acidic, alkaline, and salt aqueous solutions. It was observed that oil/organic solvents absorption capacity of the PE coated cotton fibers was better (16.8–59.7 %, depending on the type of oil) than that of silres modifications. The fabricated PE fiber shows oils and organic solvents absorption ability in the range of 22–61 times their weight as well as good reusability in oil/water separation even after ten absorption-desorption repeated cycles by simple squeezing method. Thus, the utilization of PE waste materials provides an extremely low cost and better approach in the fabrication of promising surfaces for oil/water separation applications.

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  • Abstract: Underwater superoleophilicity involves interactions between a solid surface and two immiscible liquids, viz., water and oils, in which water remains in the completely wetted and oils in the non-wetted state. Materials with underwater superoleophilicity have drawn significant interest due to their superior performance in selective separation of oil and organic solvents from an aqueous phase. However, the development of such materials with special wettability for water and oils are hindered by (1) complex fabrication process (2) long processing duration with high cost, and (3) use of environmentally unfriendly and expensive fluorochemicals to lower the surface energy. Herein, we demonstrate the use of waste potato peels (WPP) to fabricate simple, economical and eco-friendly materials with superhydrophilic and underwater superoleophobic properties. Initially, powder of WPP was prepared and accumulated into a layer via a simple cleaning, smashing, one step inexpensive chemical treatment and stacking procedures. The developed WPP layer was efficient for the gravity-driven separation of various oil/water mixtures (including hexane, toluene, dodecylbenzene, and kerosene) and water-in-oil emulsions, with high efficiency in single unit operation. During the oil/water separation process, the WPP layer was also found to serve as an adsorbent material for efficient removal of various water-soluble dyes contaminants, simultaneously. Thus, the developed WPP layer is not only a good biomaterial for water remediation by the oil/water separation and dye adsorption simultaneously, but can also contribute in reducing environmental pollution and wastage.

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  • Abstract: It is widely known that the existence of arsenic (As) in water negatively affects humans and the environment. We report the synthesis, characterization, and application of boron nitride nanosheets (BNNSs) and Fe3O4-functionalized BNNS (BNNS–Fe3O4) nanocomposite for removal of As(V) ions from aqueous systems. The morphology, surface properties, and compositions of synthesized nanomaterials were examined using scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, surface area analysis, zero-point charge, and magnetic moment determination. The BNNS–Fe3O4 nanocomposites have a specific surface area of 119 m2 g–1 and a high saturation magnetization of 49.19 emu g–1. Due to this strong magnetic property at room temperature, BNNS–Fe3O4 can be easily separated in solution by applying an external magnetic field. From the activation energies, it was found that the adsorption of As(V) ions on BNNSs and BNNS–Fe3O4 was due to physical and chemical adsorption, respectively. The maximum adsorption capacity of BNNS–Fe3O4 nanocomposite for As(V) ions has been found to be 26.3 mg g–1, which is 5 times higher than that of unmodified BNNSs (5.3 mg g–1). This closely matches density functional theory simulations, where it is found that binding energies between BNNS–Fe3O4 nanocomposite and As(OH)5 are 5 times higher than those between BNNSs and As(OH)5, implying 5 times higher adsorption capacity of BNNS–Fe3O4 nanocomposite than unmodified BNNSs. More importantly, it was observed that the synthesized BNNS–Fe3O4 nanocomposite could reduce As(V) ion concentration from 856 ppb in a solution to below 10 ppb (>98.83% removal), which is the permissible limit according to World Health Organization recommendations. Finally, the synthesized adsorbent showed both separation and regeneration properties. These findings demonstrate the potential of BNNS–Fe3O4 nanocomposite for commercial application in separation of As(V) ions from potable and waste water streams.

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  • Abstract: All atom molecular dynamics simulations and experiments were performed to understand the adsorption behavior of gadolinium ion on the crown ethers grafted polystyrene surface. Two different types of crown ethers, viz, dibenzo crown ether and dicyclo hexano crown ether, were grafted separately on the PS surface to understand the adsorption behavior. We investigate the roles of ion concentration and grafting density of the crown ether on the adsorption behavior of ion on the PS surface. The adsorption of shows an increasing trend with increasing salt concentration, for all cases of crown ether grafting densities. The adsorption behavior follows the Langmuir isotherm model. We further investigate the dynamics of the ion by evaluating the diffusion coefficient and mean residence time. It was found that D decreases with increasing salt concentration for both DBCE and DCHCE. On the contrary, as expected, the value of increases with an increase in salt concentration. Overall, a 3-fold increase in was seen with increasing salt concentration. The potential of mean force analysis using umbrella sampling reveals favorable binding energy for higher grafting density of DCHCE compared to that of DBCE

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  • Abstract: Opening a bandgap in graphene by doping with lighter elements plays a vital role in the next generation nanoelectronic devices. Here, we present the structural, electronic, and mobility of graphene co-doped with boron nitrogen and boron phosphorus using density functional theory with the inclusion of van der Waals interactions. By analyzing the band structure, it is found that BCP shows a direct bandgap whereas BCN exhibits an indirect bandgap. The bandgap values calculated using PBE functional are for BCN and for BCP. From phonon dispersion results, it is apparent that both BCN and BCP shows positive frequencies within the Brillouin zone demonstrating the lattice dynamical stability of these materials. Deformation potential theory is applied to calculate the electron/hole mobility by applying the uniaxial strain along x- and y-directions. It is seen that BCP possess significantly larger mobility compared to BCN. For BCP, the mobility of electron is 1588 and that of the hole is and 1607 along x direction in units of , respectively, which are larger than MoS2. Also, Boltzmann theory within the constant relaxation time approximation is used to determine the temperature dependent conductivity, and the results are consistent with the deformation theory.

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  • Abstract: The ultrathin films of 2-ferrocenyl-1,3-dithiolane (FcS2C3) and 2-ferrocenyl-1,3-dithiane (FcS2C4) drop-casted from toluene on highly oriented pyrolytic graphite (HOPG) surface are investigated using atomic force microscopy (AFM). Two types of growth polymorphs have been observed, which are distinctly different based on their nature of growth and the molecular level packing. We have developed a new type of temperature-dependent desorption experiment named “microscopic thermal desorption analysis” (MTDA) for understanding the adsorption energetics related to the observed growth polymorphs on the surface. Using MTDA, we have calculated the adsorption energies of growth polymorphs of both molecules and further revealed that their formation requires an activation energy. The subtle relation between the adsorption energies and activation energies of growth polymorphs account for their average abundance on the surface. The experimental observations are further supported by density functional theory (DFT) calculations

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  • Abstract: In this work, we have considered the crystallisation behaviour of supercooled water in the presence of surface defects of varying size. Ice nucleation on Ag exposed surface is investigated by molecular dynamics simulation at a temperature of. For systems with , the surface layers crystallise within. In the system with defects, we observe two distinct stacking patterns in the layers near the surface and find that systems with AA stacking cause a monotonic decrease in the early nucleation dynamics with an increase in defect size. Where AB stacking is observed, the effect of the defect is diminished and the dynamics are similar to the plain surface. This is supported by the variation in the orientational dynamics, hydrogen bond network stability, and tetrahedrality with respect to the defects. We quantify results in terms of the network topology using double diamond cages and hexagonal cages. The configurations of the initially formed layers of ice strongly affect the subsequent growth even at long timescales. We assert that the retarded ice growth due to defects can be explained by the relative increase in DDCs with respect to HCs.

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  • Abstract: The adsorption and diffusion of water in realistic carbon models at 300 K has been studied via grand canonical Monte Carlo and molecular dynamics simulations. The presence of functional groups has been found to be crucial to describe the adsorption process, while the models without functional groups are unable to capture the host–guest interactions. Functional groups were attached in cylindrical shells on the outer walls of CMK models in 2 and 4 cylindrical shells to study the effect of their concentration and location on water adsorption. The adsorption isotherm starts at a lower chemical potential, with water adsorbed near functional groups forming small clusters. On increasing chemical potential the water cluster grows and merges to form bigger clusters and completely fills the pores. We also analyzed the isosteric heat, radial distribution functions, hydrogen bond, and cluster size of water molecules. It indicates that the adsorption occurs due to the formation and growth of water clusters. For models with functional groups, the pore filling happens at lower chemical potential, when compared to the models without functional groups, owing to the presence of active sites which favors the nucleation of water molecules. The effect of functional groups is also remarkable in the diffusion of water inside the pores of models, lowering the mobility of the adsorbed molecules. The agreement between the results of the models with functional groups and the experimental observations makes the presence of these groups necessary to study the adsorption and diffusion of water in carbon models.

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  • Abstract: Atomistic molecular dynamics (MD) simulations are performed in order to understand the adsorption behavior of UO22+ ions from an aqueous medium. The dibenzo crown ether (DBCE) and dicyclo hexano crown ether (DCHCE) grafted on the polystyrene surface is used as the adsorbent. We investigated the role of grafting density s (mol/nm2) of DBCE and DCHCE on the adsorption behavior with increasing UO22+ ion concentration Cs (M). The amount of adsorption (qe) (mg/g) increases with an increase in UO22+ salt concentration and follows the Langmuir adsorption isotherm model....

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  • Abstract: Imine COF (covalent organic framework) based on the Schiff base reaction between p-phenylenediamine and benzene-1,3,5-tricarboxaldehyde was prepared on the HOPG-air (air=humid N2) interface and characterized using different probe microscopies. The role of the molar ratio of TCA and PDA has been explored, and smooth domains of imine COF up to a few m are formed for a high TCA ratio compared to PDA. It is also observed that the microscopic roughness of imine COF is strongly influenced by the presence of water (in the reaction chamber) during the Schiff base reaction. The electronic property of imine COF obtained by tunneling spectroscopy and dispersion corrected density functional theory calculation are comparable and show semiconducting nature with a band gap of. Further, we show that the frontier orbitals are delocalized entirely over the framework of imine COF. The calculated cohesive energy shows that the stability of imine COF is comparable to that of graphene.

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2018

  • Abstract: Atomistic molecular dynamics simulations are performed in order to derive thermodynamic properties important to understand the extraction of gadolinium and uranium dioxide with dibenzo crown ether in nitrobenzene and octanol solvents. The effect of polystyrene graft length, on DBCE, on the binding behavior of is investigated for the first time. Our simulation results demonstrate that the binding of onto the oxygens of crown ethers is favorable for polystyrene grafted crown ether in the organic solvents OCT and NB. The metal ion binding free energy in different solvent environments is calculated using the thermodynamic integration method. GBinding becomes more favorable in both solvents, NB and OCT, with an increase in the polystyrene monomer length. The metal ion transferability from an aqueous phase to an organic phase is estimated by calculating transfer free-energy calculations . GTransfer is significantly favorable for both Gd3+ and UO22+ for the transfer from the aqueous phase to the organic phase (i.e., NB and OCT) via ion-complexation to DBCE with an increase in polystyrene length. The partition coefficient (log P) values for Gd3+ and UO22+ show a 5-fold increase in separation capacity with polystyrene grafted DBCE. We corroborate the observed behavior by further analyzing the structural and dynamical properties of the ions in different phases.

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  • Abstract: Graphene–metal nanocomposites are promising materials to address the heat dissipation problems in nanoscale electronic and computing devices. A low resistance interface between metal and graphene contact is crucial for the development of highly efficient nanodevices. In this direction, we have investigated the thermal conductance (TC) across the gold–graphene interface for various thicknesses of the graphene layer and temperatures using molecular dynamics (MD) simulations. The TC is found to decrease with the increase in graphene layer number from one to three. Further increase in the number of layers has no effect on the TC. The TC is also found to increase monotonously with the temperature in the range from 50 to 300 K. However, there is no effect of temperature on TC beyond 300 K. In order to enhance the TC value, we have investigated the thermal transport in the defect mediated gold–graphene interface for various defect sizes and defect densities. TC is found to increase significantly with the increase in the vacancy defect size and density of defects in the graphene sheet. The TC obtained for graphene containing defects of size 2.24 and 3.16 Å at 300 K is found to be 5 and 26% higher than the TC obtained for defect free graphene. The vibrational density of states (VDOS) of interface forming materials shows that the defects in the graphene sheet enhance the out-of-plane low frequency vibrational modes within graphene. This process facilitates high vibrational coupling between the gold and graphene, and enhances the heat transfer across the interface. This demonstrates that the TC across the gold–graphene interface can be tuned by adjusting the defect vacancy size and density of the defects.

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  • Abstract: The freezing of water in the presence of salts is very common and widely investigated phenomena. However, the role of the substrate during crystallization, and in particular the molecular-level resolution of nucleation mechanism, is still not well-understood. In this work, we investigated the freezing behavior of supercooled water and aqueous lithium chloride solutions on smooth graphitic surfaces. We illustrate the role of solid surfaces in the freezing process of aqueous solutions as a function of mole fraction of a salt exhibiting lowering of freezing temperature, irrespective of a water-binding affinity to the surface. Our molecular dynamics simulations show that the hydrophobic surface is a better nucleating surface when the mole fraction of the salt is over 5%. Our findings reveal that nucleation of ice occurs heterogeneously at the liquid–solid interface. Consequently, propagation of the ice front yields phase-segregated brine near the liquid–vapor interface. Furthermore, we have investigated the effect of salt–surface interaction on the freezing process. We observe lowering of the freezing point with an increase in the water–surface interaction. The simulations demonstrate that nucleation of ice occurs heterogeneously at the liquid–solid interface for low values of interaction, whereas homogeneous nucleation of ice takes place away from the substrate at higher interaction strengths.

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  • Abstract: Grand canonical Monte Carlo (GCMC) simulations have been performed to investigate the adsorption and separation behavior of ternary and quaternary gaseous mixtures of CO2, along with H2S, SO2, and N2, in bundles of aligned double-walled carbon nanotubes with a diameter of 3 nm and an intertube distance of 0.5 nm. All of the simulations are performed at 303 K and at pressures varying between 0 and 3 bar. The GCMC results are then compared to the ideal adsorbed solution theory (IAST) predictions. For the ternary mixture H2S–CO2–N2, the results show that CO2 has the highest adsorption among the three components. The IAST predictions agree reasonably well with the GCMC data for the ternary mixture, except for H2S. For the quaternary mixture H2S–SO2–CO2–N2, it is observed that initially CO2 has the highest adsorption up until around 2 bar, whereafter there is a crossover by SO2 to have the highest adsorption. IAST fails to predict the adsorption behavior of the quaternary mixture involving SO2.

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  • Abstract: In this study, the adsorption of nonionic surfactant, triethylene glycol monododecyl ether (C12E3), on a surface of silica nanoparticle (NP) has been studied with variation in the degree of ionisation (DI) of silica NP using all-atom molecular dynamic simulations in hexadecane–water system. Hydrogen bonding is found to be responsible for the adsorption of C12E3 on NP, particularly at low DI. We observe that with increasing DI of NP, the amount of adsorption of C12E3 on NP reduces, which is negligible beyond DI 0.5. The decrease in the adsorption with increasing DI is due to the decrease in the number of hydrogen bonds formed by the silica NP with surfactant molecules. Potential of mean force (PMF) profiles indicate attractive interactions between NP and C12E3 for DI < 0.5, and for larger DI depletion effect is observed. This work explains the unusual effect of nonionic surfactant on interfacial tension in the presence of silica particles as observed in recent experiments.

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  • Abstract: Grand canonical Monte Carlo simulations are conducted to investigate the adsorption ability of a 3-D graphene sponge (GS) to separate acidic gases from flue gas stream. To assess the adsorption capacity of GS, first, adsorption of pure component flue gas is studied at a temperature of 303 K and varying pressure up to 2.5 bar. Subsequently, the adsorption capacity and selectivity of GS are investigated for a ternary mixture of flue gas under the same conditions. This study shows that the maximum adsorption capacity of GS for pure component flue gas is observed for SO2 followed by CO2 and N2. The adsorption uptake decreases with an increase in pore size of GS. At 1 bar, the amount of adsorption of SO2 and CO2 are 13 mmol/g and 2.6 mmol/g, respectively. Upon increasing the average pore size to 20 Å, the excess amount decreases by 56% and 58% for SO2 and CO2, respectively. The adsorption capacities of GS for CO2 and SO2 are better than other carbon-based adsorbents except for CNT bundles. In the case of a ternary mixture of N2, CO2, and SO2 in the mole ratios of 0.8, 0.15, and 0.05, we found that the adsorption behavior follows the same order as in the pure component flue gas adsorption. However, the adsorption amount decreases significantly from that of pure component adsorption amount in GS. The adsorption amount of SO2 and CO2 at postcombustion conditions decreases to 1.3 mmol/g and 0.5 mmol/g, respectively, which further decreases upon increasing the average pore size. Selectivity analysis of adsorption shows that the adsorption selectivity of SO2 over N2 is the maximum followed by the selectivity of CO2 over N2 and SO2 over CO2. Both selectivity and uptake capacity decreases with increase in average pore size of GS.

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  • Abstract: The free energy, formation work and entropy dependences for water condensate formed from the vapor over the defect free and containing surface defects basal face of crystal at initial stage of condensation at temperatures of are calculated using the bicanonical statistical ensemble method, with Ewald summation for long range electrostatic and polarization interactions with the substrate. The effect of surface defects in form of rectangular towers as a part of regular structure on the stability of condensed phase embryos is investigated. In contrast to small scale structures, relatively larger coarse grained nanostructure of crystal surface demonstrates an unconditional advantage in the ability to stimulate condensation compared to defect free surface. The formation of condensed phase embryos on the surface with multiple defects begins at vapor pressures 5 to 6 orders of magnitude lower than that on corresponding defect-free surface, and this effect is resistant to temperature variations. The condensate on the surface of the crystal is thermodynamically stable, both on defect free and nanostructured surfaces, with the exception of short initial stage of the monomolecular film on the defect free surface.

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  • Abstract: Grand canonical Monte Carlo simulations are performed to study the adsorption of water in realistic CMK-3 and CMK-5 models at 300 K. The adsorption isotherms are characterized by negligible uptake at lower chemical potentials and complete pore filling once the threshold chemical potential is increased. Results for the isosteric heat of adsorption, radial distribution function (O–O and O–H), hydrogen bond statistics and the cluster size distribution of water molecules are presented. The snapshots of GCMC simulations in CMK-3 and CMK-5 models show that the adsorption happens via the formation of water clusters. For the CMK-3 model, it was found that the pore filling occurred via the formation of a single water cluster and a few very small clusters. The water cluster size increased with an increase in pore size of the CMK-3 model. For the CMK-5 model, it was found that the adsorption first occurred in the inner porosity (via cluster formation). There was no adsorption of water in the outer porosity during the filling of the inner porosity. After the inner porosity was completely filled, the water begins to fill the outer porosity. Snapshots from GCMC simulations of the CMK-5 model clearly show that the water adsorption in the outer porosity occurs via the formation and growth of clusters and there was no formation of layers of water in the porosity as seen for nonpolar fluids like nitrogen.

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  • Abstract: In this work, we have carried out a systematic study of nucleation of a supercooled nanofluid droplet on a graphite substrate using molecular dynamics simulations. In particular, the effect of nanoparticle loading in the supercooled liquid and the interaction strength between water and NP on the behavior of ice nucleation is investigated. At lower, the nucleation rate is indifferent, while at higher, the nucleation rate is found to reduce with the addition of nanoparticles. We found the maximum rate of ice nucleation is at and, which is approximately 45 times more than that seen in the bulk water. We present in detail the effect of nanoparticle and nanoparticle water interactions on the structure and composition of ice. The results demonstrate that the number of ice like water molecules in the nanofluid droplet decreases with increasing, which correlates well with the lowering of the rate of ice nucleation at higher vol of particle and stronger water–nanoparticle interaction. Therefore, the hydrophilicity of the nanoparticles inhibits nucleation. We further investigate the effect of the shape of nanoparticles on ice nucleation. The results suggest that the rate of ice nucleation is independent of particle shape of size 1.2 nm. Finally, we try to draw a quantitative comparison with the water activity based ice nucleation theory.

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  • Abstract: Atomistic molecular dynamics (MD) simulations are performed in order to derive thermodynamic properties important to understand the extraction of gadolinium (Gd3+) and uranium dioxide (UO2) with dibenzo crown ether (DBCE) in nitrobenzene (NB) and octanol (OCT) solvents. The effect of polystyrene graft length, on DBCE, on the binding behavior of Gd3+ and UO22+ is investigated for the first time. Our simulation results demonstrate that the binding of Gd3+ and UO22+ onto the oxygens of crown ethers is favorable for polystyrene grafted crown ether in the organic solvents OCT and NB. The metal ion binding free energy in different solvent environments is calculated using the thermodynamic integration (TI) method. GBinding becomes more favorable in both solvents, NB and OCT, with an increase in the polystyrene monomer length. The metal ion transferability from an aqueous phase to an organic phase is estimated by calculating transfer free-energy calculations. GTransfer is significantly favorable for both Gd3+ and UO22+ for the transfer from the aqueous phase to the organic phase via ion-complexation to DBCE with an increase in polystyrene length. The partition coefficient values for Gd3+ and UO22+ show a 5-fold increase in separation capacity with polystyrene grafted DBCE. We corroborate the observed behavior by further analyzing the structural and dynamical properties of the ions in different phases.

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2017

  • Abstract: Grand canonical Monte Carlo (GCMC) simulation is used to study the adsorption of pure SO2 using a functionalized bilayer graphene nanoribbon (GNR) at 303 K. The functional groups considered in this work are OH, COOH, NH2, NO2, and CH3. The mole percent of functionalization considered in this work is in the range of 3.125%–6.25%. GCMC simulation is further used to study the selective adsorption of SO2 from binary and ternary mixtures of SO2, CO2, and N2, of variable composition using the functionalized bilayer graphene nanoribbon at 303 K. This study shows that the adsorption and selectivity of SO2 increase after the functionalization of the nanoribbon compared to the hydrogen terminated nanoribbon. The order of adsorption capacity and selectivity of the functionalized nanoribbon is found to follow the order COOH > NO2 > NH2 > CH3 > OH > H. The selectivity of SO2 is found to be maximum at a pressure less than 0.2 bar. Furthermore, SO2 selectivity and adsorption capacity decrease with increase in the molar ratio of SO2/N2 mixture from 1:1 to 1:9. In the case of ternary mixture of SO2, CO2, N2, having compositions of 0.05, 0.15, 0.8, the selectivity of SO2 over N2 is higher than that of CO2 over N2. The maximum selectivity of SO2 over CO2 is observed for the COOH functionalized GNR followed by NO2 and other functionalized GNRs.

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  • Abstract: Janus particles provide an asymmetry in structure, which can impart diverse functionalities leading to immense importance in various applications, ranging from targeted delivery to interfacial phenomena, including catalysis, electronics, and optics. In this work, we present results of a molecular dynamics study of the growth mechanism of coating on gold nanoparticles from droplets of n-alkyl thiols with different chain lengths and terminal groups. The effect of chain lengths and functional groups on the formation of a monolayer of alkyl thiols on AuNPs is investigated. A two-step mechanism, initiated by the binding of the droplet to the nanoparticle surface with a time constant on the order of 1 ns, followed by the diffusion-driven growth with a larger time constant, is shown to capture the growth dynamics of the monolayer. It is observed that the time required for complete wetting increases with an increase in the chain length. Moreover, the monolayer formation is slowed down in the presence of carboxyl groups because of strong hydrogen bonding. The kinetics of the n-alkyl thiols coating on the nanoparticles is found to be independent of the droplet size but carboxyl-terminated thiols spread more with increasing droplet size. Furthermore, different time constants for different chains and functional groups yield Janus coating when two droplets of alkyl thiols with different terminal groups are allowed to form monolayers on the nanoparticle. The Janus balance for different combinations of alkyl thiols and nanoparticle sizes varies in the range of 0.42–0.71.

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  • Abstract: The industries discharge a variety of pollutants, such as heavy metals, organic toxins, and oils, in water resources. Exposure of these contaminants in water causes adverse health effects on various forms of life. Novel materials are needed for the effective removal of pollutants from industrial wastewater. Graphene and hexagonal boron nitride (hBN) sheets are promising materials for removal of organic pollutants. In this work, the suitability of the sheets for the separation of the ethanol–water mixture is investigated by studying the adsorption and structural behavior of ethanol–water mixtures in slit pores with variable width (7–13 Å) using molecular dynamics simulations. The selectivity of ethanol is found to depend on the pore-width and nature of the pore walls. The selectivity of ethanol is highest for 9 Å pores and lowest for 7 Å pores, irrespective of the nature of the pore walls. However, selectivity of ethanol is relatively higher for hBN pores compared to the graphene pores, for all the considered pore widths. At a lower pore width, molecular sieving plays an important role for selective adsorption of ethanol molecules. On the other hand, at a higher pore width, selective adsorption of ethanol molecules is affected by the nature of the pore walls. The diffusion coefficients of water and ethanol molecules substantially decrease with a decrease in pore width for both graphene and hBN surfaces. The resident time of water and ethanol molecules decreases with increase in the slit-width. Furthermore, water and ethanol molecules confined in hBN pores show higher residence time and lower. diffusion coefficient values compared to graphene pores. The adsorption behavior of water and ethanol molecules in the slit pores are analyzed using the potential mean forces, for water and ethanol molecules on the graphene and hBN surfaces, which are determined by umbrella sampling technique.

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  • Abstract: New experiments with multigrain mixtures in a laterally shaken, horizontal channel show complete axial segregation of species. The channel consists of multiple concatenated trapeziums, and superficially resembles microratchets wherein asymmetric geometries and potentials transport, and sort, randomly agitated microscopic particles. However, the physics of our macroscale granular ratchet is fundamentally different, as macroscopic segregation is gravity driven. Our observations are not explained by classical granular segregation theories either. Motivated by the experiments, extensive parallelized discrete element simulations reveal that the macroratchet differentiates grains through hierarchical bidirectional segregation over two different time scales: Grains rapidly sort vertically into horizontal bands spanning the channel's length that, subsequently, slowly separate axially, driven by strikingly gentle, average interfacial pressure gradients acting over long distances. At its maximum, the pressure gradient responsible for axial separation was due to a change in height of about two big grain diameters over a meter-long channel. The strong directional segregation achieved by the granular macroratchet has practical importance, while identifying the underlying new physics will further our understanding of granular segregation in industrial and geophysical processes.

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  • Abstract: The presence of high concentration of arsenic in conventional water sources can cause serious health and environmental hazards. An urgent need is to find an efficient adsorbent for the removal of arsenic ions (As) from contaminated water. In the present study, molecular dynamics simulation is used to understand the adsorption behaviour of As on hexagonal boron nitride (h-BN) and graphene nanosheets. The adsorption of As follows the Langmuir isotherm and the maximum adsorption capacities are found to be 270.1 and 211.7 mg/g for h-BN and graphene nanosheets, respectively. Further, potential of mean force (PMF) of As revealed that the h-BN nanosheet possesses lower contact minima (1.35 kcal/mol) for arsenic ion compared to graphene nanosheet (1.2 kcal/mol). These results indicate strong interaction between arsenic ion and h-BN nanosheet. On the other hand, desorption of As on h-BN nanosheet showed higher energy barriers (2.3 kcal/mol) compared to graphene nanosheet (1.5 kcal/mol). Correspondingly, the residence time of As is approximately threefold higher on h-BN nanosheet compared to graphene nanosheet. We also report that the presence of partial charges on B and N atoms in the h-BN sheet influence the adsorption behaviour As ions and the maximum adsorption capacity of h-BN nanosheet with partial charges is found to be 311.7 mg/g. Thus, our study strongly suggests the potential applicability of h-BN nanosheet as an efficient adsorbent for the removal of arsenic ions.

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  • Abstract: In this study, we have developed super-repellent surface on cotton fabric via a facile and eco-friendly strategy using zirconia particles with water-soluble siloxane emulsion. The coated fabric using zirconia-siloxane (ZS) coating showed super-repellency of liquids with surface tension , like water, mixtures of isopropyl alcohol with deionized water, and ethylene glycol with contact angle of respectively. Furthermore, the coated fabric displays low sliding angle, with these liquids. The super-repellency of the coated fabric is attributed due to its lower surface energy. The produced coating exhibited excellent durability and retained its super-repelling properties under harsh environment conditions like acidic, alkaline, salty, ultraviolet irradiation, mechanical abrasion and repeated tear test with an adhesive tape. In addition, in a mixture of water and oil, the developed coated fabric exhibited dual nature viz., superhydrophobicity and superoleophilicity, maintaining the super-repellency with water even they are wetted with oily liquids. The materials with ability to repel water in the presence of oily pollutants are very useful in application related to sea water. Thus as-prepared coated fabric, with dual functionality, is a promising material for many applications including anti-wetting, self-cleaning, support for aquatic floating devices and as a filtration material for rapid and continuous oil-water separation.

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  • Abstract: Superhydrophobic/superoleophilic fabrics have been the subject of profound research for self-cleaning and oil–water separation applications. However, the practical application of these materials is still limited due to the complex preparation process using fluorinated chemicals and poor stability of the developed coatings on the surface of fabrics under harsh environmental conditions. In this study, we fabricated a dual-functional coating on cotton fabric with superhydrophobicity and visible light photocatalytic activity via an inexpensive dip coating method. The in situ synthesized poly-triethoxyvinylsilane (PTEVS) with the combination of polydimethylsiloxane (PDMS) was used to obtain a superhydrophobic coating and subsequently a AgBr coating to integrate the photocatalytic activity. The surface morphology and chemical structure of the coated fabric were characterized by field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy.

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  • Abstract: An interaction potential model has been developed, for the first time, for Cu2Se using the ab initio derived data. The structure and elastic constants of Cu2Se using the derived force field are within a few percent of DFT derived structure and elastic constants and reported experimental structure. The derived force field also shows remarkable ability to reproduce temperature dependent behavior of the specific heat and thermal expansion coefficient. The thermal structure evolution of the Cu2Se is studied by performing the molecular dynamic simulations using the derived force field. The evaluated thermodynamic properties such as free energy and excess entropy, show that the increased Cu Se interaction with the temperature makes the system more thermodynamically stable 2017 Wiley Periodicals, Inc.

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  • Abstract: Adsorption and separation of gases in porous carbon models are studied using molecular simulations. We use three porous carbon models (named as cs400, cs1000, and cs1000a) developed in a previous work obtained from Hybrid Reverse Monte Carlo simulations. The density of carbon atoms as well as the presence of heteroatoms (hydrogen and oxygen) differ between the three carbon models. Gas adsorption in the carbon models were studied using Grand Canonical Monte Carlo simulations. We found that cs1000 sample (with highest carbon density) shows the largest separation ability for N2/CH4, CH4/CO2, and N2/CO2 systems. cs1000a sample (with larger pore width up to 1.2 nm) shows higher selectivity for SO2/N2 and SO2/CO2 systems. We also studied the influence of surface chemistry (presence of carbonyl and carboxyl groups) in the porous carbon models on adsorption and separation of gases. We found that the presence of carbonyl and carboxyl groups has a significant effect on the adsorption and separation of polar gas molecules. Interestingly, the presence of functional groups does not seem to have much impact on SO2/CO2 separation at moderate to high pressures for carbonyl functional groups. For carboxyl functional groups, this is not the case, and the selectivity curves remain flat or decrease slightly at moderate to high pressures. We found increasing selectivity for all binary gas systems except for the N2/CH4 system, which is expected, as both the gases are nonpolar. For all binary gas systems studied, the maximum selectivity was found for SO2/N2 system.

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  • Abstract: In this work, we have studied the effect of hydrophilic silica nanoparticles (NPs), in the presence of nonionic surfactants (Triethylene glycol monododecyl ether and Tween 20), on the oil–water (n-octane–water, n-dodecane–water and n-hexadecane–water) interfacial tensions (IFTs) at 300 K, using coarse-grained molecular dynamics simulations based on the MARTINI force field. Simulation results indicate that silica NPs solely do not affect the IFT. However, the silica NPs may or may not increase the IFT of oil–water containing nonionic surfactant, depending on the tendency of the surfactant to adsorb on the surface of NPs. The adsorption occurs due to the formation of hydrogen bonds, and adsorption increases with a decrease in pH, as seen in experimental studies. In this work, we found that the oil–water IFT increases with an increasing amount of adsorption of the surfactant on NPs. At a fixed amount of adsorption of the surfactant on NPs, the IFT behavior is indifferent to the change in concentration of NPs. However, the IFT decreases with an increase in surfactant concentration. We present a detailed analysis of the density profile and intrinsic width of the interface. The IFT behavior is found to correlate extremely well with the intrinsic width of the interface. The current study provides an explanation for the increase in IFT observed in a recent experiment [N. R. Biswal et al., J. Phys. Chem. B 120, 7265–7274 (2016)] for various types of NPs and nonionic surfactant systems.

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  • Abstract: We study the effect of nanoparticles on the vapour-liquid surface tension of TIP4P/2005 water model using molecular dynamic simulations. The interactions of nanoparticles with water and volume percentage of nanoparticles are varied. It is found that the surface tension increases with increasing hydrophilicity and decreases with increasing hydrophobicity, as the nanoparticle volume percentage is increased. However, the surface tension values do not increase at higher volume fraction, both for hydrophilic and hydrophobic interactions. We also present the effect of temperature on the surface tension of water with nanoparticles. It is found that surface tension of water with hydrophobic nanoparticles reduces substantially faster with increasing temperature compared to that containing hydrophilic nanoparticles. The behaviour is well supported by the interfacial width data.

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  • Abstract: This study presents the preparation and characterization of natural waste material (orange peel) derived low cost, bio-based, recyclable, highly hydrophobic/superoleophilic magnetic sorbents to treat environmental pollution caused by oils and organic contaminants from water. Orange peels were dried, powdered and converted into magnetic and highly hydrophobic/superoleophilic sorbents by the incorporation of Fe3O4 nanoparticles using co-precipitation approach and subsequently modifying with low surface energy polydimethylsiloxane (non-fluorinated hydrophobic reagent) layers. SEM, EDS, XRD, FTIR, XPS, TGA, VSM and contact angle measurements analysis were used for characterization of the developed materials. The as-prepared sorbents exhibit strong magnetic behavior with saturation magnetization (Ms) value of 14.9 emu/g, excellent hydrophobicity with water contact angle of 149.2° and very high capacity for the selective absorption of various oils and organic solvents from oil contaminated water. It has high absorption capacity up to 5.11 and 6.90 times its original weight for diesel oil and engine oil respectively, along with high recyclability of more than eight cycles and the advantage of magnetic separation from water surface after absorption of oils. Thus, the orange peel waste derived absorbent with magnetic and high hydrophobic properties is a promising alternative for the capture of oil and organic pollutants from water.

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  • Abstract: Thermal transport properties associated with the thermal structure evolution of are studied using density functional theory and molecular dynamics simulations. Thermal conductivity of is calculated over a temperature range of using reverse non equilibrium molecular dynamics simulations. The thermal conductivity found through MD simulations decreases monotonically with increasing temperature, which is in line with the reported experimental data and our calculated DFT data. The average phonon mean free path evaluated using the kinetic theory, found to be within the range of decreases with increasing temperature. Furthermore, we have investigated the temperature dependent heat transport phenomena using phonon density of states, calculated using MD simulations. The phonon modes are found to shift towards the low frequency numbers with increasing temperature, indicating lower heat carrying capacity of the material and in agreement with the computed thermal conductivity.

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  • Abstract: The dendrimer polyamidoamine (PAMAM) has been widely applied in environmental applications as adsorbents for wastewater treatment. In this work, molecular dynamics simulations are conducted to understand the effect of dendrimer grafted graphene and graphene oxide on the structural and dynamical properties of the Pb2+ ion. The adsorption capacity of the metal ion is improved significantly, over 60%, using carboxyl terminal groups of a dendrimer molecule grafted on a graphene oxide surface. We examine the self-diffusion coefficient and residence time of Pb2+ ion near graphene and graphene oxide surfaces grafted with PAMAM dendrimers using terminal groups. Further, the potential of mean force is analyzed to understand the role of different surface groups in enhancing the adsorption of the metal ion.

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2016

  • Abstract: The dynamics of dewetting of gold films on graphene surfaces is investigated using molecular dynamics simulation. The effect of temperature, film diameter and film thickness on the dewetting mechanism, leading to the formation of nanoparticles, is reported. The dewetting behavior for films is in contrast to the behavior seen for thicker films. The retraction velocity, in the order of 300 m for nm film, decreases with an increase in film thickness, whereas it increases with temperature. However at no point do nanoparticles detach from the surface within the temperature range considered in this work. We further investigated the self-assembly behavior of nanoparticles on graphene at different temperatures. The process of self-assembly of gold nanoparticles is favorable at lower temperatures than at higher temperatures, based on the free energy landscape analysis. Furthermore, the shape of an assembled structure is found to change from spherical to hexagonal, with a marked propensity towards an icosahedral structure based on the bond-orientational order parameters.

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  • Abstract: The adsorption and separation behavior of SO2–CO2, SO2–N2 and CO2–N2 binary mixtures in bundles of aligned double-walled carbon nanotubes is investigated using the grand-canonical Monte Carlo (GCMC) method and ideal adsorbed solution theory. Simulations were performed at 303 K with nanotubes of 3 nm inner diameter and various intertube distances. The results showed that the packing with an intertube distance d = 0 has the highest selectivity for SO2–N2 and CO2–N2 binary mixtures. For the SO2–CO2 case, the optimum intertube distance for having the maximum selectivity depends on the applied pressure, so that at p < 0.8 bar d = 0 shows the highest selectivity and at 0.8 bar < p < 2.5 bar, the highest selectivity belongs to d = 0.5 nm. Ideal adsorbed solution theory cannot predict the adsorption of the binary systems containing SO2, especially when d = 0. As the intertube distance is increased, the ideal adsorbed solution theory based predictions become closer to those of GCMC simulations. Only in the case of CO2–N2, ideal adsorbed solution theory is everywhere in good agreement with simulations. In a ternary mixture of all three gases, the behavior of SO2 and CO2 remains similar to that in a SO2–CO2 binary mixture because of the weak interaction between N2 molecules and CNTs.

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  • Abstract: Carbon nanotubes (CNTs) have been identified as extremely promising candidates for gas capture and storage. Therefore, an understanding of the adsorption mechanisms is crucial to the improvement of CNT applications. In this work, grand-canonical Monte Carlo simulations and analytical models are used to study, at the temperature of T = 303 K, the adsorption and condensation of SO2 in hexagonal arrays of double-walled CNTs of different inner nanotube radii Rin and intertube distances d. For both the inner and the outer adsorption, type I and type IV adsorption isotherms (IUPAC classification) are observed; they can be described adequately by analytical models. At a given pressure, the maximum adsorption among different CNT geometries depends strongly on the applied pressure. For the inner adsorption, three stages of adsorption are identified with increasing pressures. At low pressures, only one monolayer is formed, where the adsorption energy dominates the adsorption. At intermediate and high pressures, multilayers are formed until finally condensation is achieved; now it is the surface area or the available volume per CNT mass unit that dominates the adsorption. The nonlinear dependence of the outer adsorption on Rin and d can be explained by similar arguments as adopted for the inner adsorption. The effective number density of SO2 molecules and isosteric heat of adsorption are also analyzed to deepen our understanding of the adsorption behavior.

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  • Abstract: In this work, the effect of temperature on the contact angle of a water droplet on grafted thermo-responsive poly-(N-isopropylacrylamide) (PNIPAAm) polymer brushes is studied using all-atom molecular dynamics simulations in the temperature range of 270–330 K. A shift from 55° to 65° in contact angle values is observed as the temperature increases from 300 K to 310 K, which is in line with the experimental reports. The behavior of a water droplet on PNIPAAm brushes is analyzed using hydrogen bond analysis, water diffusion, radial distribution functions, the potential of mean force, excess entropy and the second virial coefficient (B2). The thermo-responsive behavior of PNIPAAm brushes, quantified using the excess entropy and B2 of PNIPAAm–water and water–water interactions, is mainly governed by polymer–water interactions. In particular, the excess entropy and B2 of PNIPAAm resulting from the PNIPAAm–water interactions are found to increase with increasing temperature. The dehydration of PNIPAAm brushes and the increase in the contact angle of water were confirmed to be entropy driven processes.

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  • Abstract: The suspension and adsorption of silica nanoparticles on a cellulose surface, in an aqueous medium is investigated using Brownian dynamic simulations. The inter particle and particle–surface interactions are modeled within the framework of the DLVO theory. Our analysis predicts the accumulation of negatively charged nanoparticles near a negatively charged surface depending on the Debye screening length of the medium. A crossover from the suspension to the adsorption of negatively charged silica nanoparticles onto a negatively charged cellulose surface has been reported as the screening length () of the medium increases. The crossover is observed at , due to the interplay between the nanoparticle–nanoparticle and the nanoparticle–surface interactions. The adsorption behavior of nanoparticles is explained using the potential of mean force analysis. The amount of nanoparticles adsorbed depends on their bulk volume fraction and the screening length of the medium. Further, the effects of electrical potentials of nanoparticle and surface on the adsorption are reported. The data suggests that the adsorption of nanoparticles increases either with increasing magnitude, or/and, with decreasing magnitude. The adsorbed particles form a disordered monolayer, and undergo subdiffusive motion. We have also observed a transition from the gas-like structure to the liquid-like structure of nanoparticles in the adsorbed monolayer as their bulk volume fraction increases.

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  • Abstract: The selective adsorption behaviours of carbon dioxide, methane and nitrogen on bundles of functionalized CMK-5 are investigated at 303 K using grand-canonical Monte Carlo simulations. Functional groups cause a significant enhancement in CO2 uptake. On the other hand, the adsorption amount of methane decreases with respect to bare CMK-5 by 13% (at 38.13 bar) upon functionalization. Furthermore, functionalized CMK-5 with different pore sizes (4 nm, 6 nm, 8 nm) and inter-tube distances (d = 0 to 1.5 nm) are used to investigate the adsorption behaviour of flue gases.

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  • Abstract: The selective adsorption behaviours of carbon dioxide, methane and nitrogen on bundles of functionalized CMK-5 are investigated at 303 K using grand-canonical Monte Carlo simulations. Functional groups cause a significant enhancement in CO2 uptake . On the other hand, the adsorption amount of methane decreases with respect to bare CMK-5 by 13% (at 38.13 bar) upon functionalization. Furthermore, functionalized CMK-5 with different pore sizes (4 nm, 6 nm, 8 nm) and inter-tube distances (d = 0 to 1.5 nm) are used to investigate the adsorption behaviour of flue gases.

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  • Abstract: Industrial wastewater contains toxic metals such as copper, lead, cadmium, zinc and cobalt. Hence, it is necessary to eliminate the toxic heavy metal ions from wastewater before it is released into the environment. In this study, we have used classical molecular dynamics simulations and density functional theory calculations to investigate the desalination performance of nanoporous graphene (NPG) membranes for different pore sizes and chemical functionalization (hydroxyl, nitrogen and fluorine) of the pore. The underlying mechanism involved in the separation process is explained using potential of mean force (PMF) calculations and plane-wave DFT calculations. The estimated energy barriers using DFT calculations were found to be in good agreement with the results obtained using classical MD simulations. This study shows that the NPG functionalized with N (NPG-N) shows higher salt rejection with intermediate permeability compared NPG functionalized with F (NPG-F) and OH (NPG-OH). NPG-OH shows higher water permeability with lower salt rejection compared to NPG-N and NPG-F. However, NPG-F shows lowest permeability compared to other two NPGs considered in this study. Even at high pressures like 500 MPa the salt rejection percentage is not less than 90 and the minimum permeability is 270. Overall, our results indicate that the water permeability of NPG membranes is about 4–5 orders of magnitude higher than the existing technologies. Thus, the NPG membrane may have a valuable role to play in industrial waste water purification.

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  • Abstract: This paper presents the effect of negatively charged silica nanoparticles (NPs) on the interfacial tension of the n-hexane–water system at variable concentrations of four different surfactants, viz., an anionic surfactant, sodium dodecyl sulfate (SDS), a cationic surfactant, cetyltrimethylammonium bromide (CTAB), and two nonionic surfactants, Tween 20 and Triton X-100 (TX-100). The presence of negatively charged silica nanoparticles is found to have a different effect depending on the type of surfactant. In the case of ionic surfactants, SDS and CTAB, silica NPs reduce the interfacial tension of the system. On the contrary, for nonionic surfactants, Tween 20 and TX-100, silica NPs increase the interfacial tension. The increasing/decreasing nature of the interfacial tension in the presence of NPs is well supported by the calculated surface excess concentrations. The diffusion kinetic control (DKC) and statistical rate theory (SRT) models are used to understand the behavior of dynamic interfacial tension of the surfactant–NP–oil–water system. The DKC model is found to describe the studied surfactant–NP–oil–water systems more aptly.

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  • Abstract: Recent simulations have improved our knowledge of the molecular-level structure and hydration properties of mixed self-assembled monolayers (SAMs) with equal and unequal alkyl thiols at three different arrangements, namely, random, patchy, and Janus. In our previous work [V. Vasumathi et al., J. Phys. Chem. C 119, 3199–3209 (2015)], we showed that the bending of longer thiols over shorter ones clearly depends on the thiols’ arrangements and chemical nature of their terminal groups. In addition, such a thiol bending revealed to have a strong impact on the structural and hydration properties of SAMs coated on gold nanoparticles (AuNPs). In this paper, we extend our previous atomistic simulation study to investigate the bending of longer thiols by increasing the stripe thickness of mixed SAMs of equal and unequal lengths coated on AuNPs. We study also the effect of stripe thickness on the structural morphology and hydration of the coated SAMs. Our results show that the structural and hydration properties of SAMs are affected by the stripe thickness for mixtures of alkyl thiols with unequal chain length but not for equal length. Hence, the stability of the stripe configuration depends on the alkyl’s chain length, the length difference between the thiol mixtures, and solvent properties

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  • Abstract: In this work, we address the nucleation behavior of a supercooled monatomic cylindrical water droplet on nanoscale textured surfaces using molecular dynamics simulations. The ice nucleation rate at 203 K on graphite based textured surfaces with nanoscale roughness is evaluated using the mean fast-passage time method. The simulation results show that the nucleation rate depends on the surface fraction as well as the wetting states. The nucleation rate enhances with increasing surface fraction for water in the Cassie–Baxter state, while contrary behavior is observed for the case of Wenzel state. Based on the spatial histogram distribution of ice formation, we observed two pathways for ice nucleation. Heterogeneous nucleation is observed at a high surface fraction. However, the probability of homogeneous ice nucleation events increases with decreasing surface fraction. We further investigate the role of the nanopillar height in ice nucleation. The nucleation rate is enhanced with increasing nanopillar height. This is attributed to the enhanced contact area with increasing nanopillar height and the shift in nucleation events towards the three-phase contact line associated with the nanotextured surface. The ice-surface work of adhesion for the Wenzel state is found to be 1–2 times higher than that in the Cassie–Baxter state. Furthermore, the work of adhesion of ice in the Wenzel state is found to be linearly dependent on the contour length of the droplet, which is in line with that reported for liquid droplets.

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  • Abstract: In the present work, simple, inexpensive (without using any sophisticated equipment), durable superhydrophobic coatings on cotton fabrics with photocatalytic properties were achieved by the application of non-fluorinated hydrophobic reagents in combination with zirconia particles and subsequently AgBr modification. A hybrid mixture of hexadecyltrimethoxy silane and stearic acid was used as a hydrophobic reagent. The as-prepared coated fabrics not only displayed superhydrophobicity (water contact angle of 153, water sliding angle 7) and superoleophilic properties (oil contact angle of 0°) but also showed photocatalytic degradation of methylene blue under visible light illumination. The modified fabric can effectively separate a series of oil–water mixtures with high efficiency (>99%) even after repeated use for 10 cycles through an ordinary filtering process, without any noticeable change in efficiency. More importantly, the as-prepared coated fabric retained its superhydrophobicity and superoleophilicity under harsh environmental conditions (acidic, alkaline, salty, ultraviolet irradiation, and mechanical abrasion) and repeated tear testing with an adhesive tape. Thus, the superhydrophobic material presented in this work, with dual functionality, is a promising material for degrading organic pollutants in the water phase and in the treatment of oil spills.

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  • Abstract: In this paper, we have studied the effect of three different types of nanoparticles (NPs) (e.g. SiO2, TiO2, and ZnO) on the interfacial tension (IFT) of different oil–water systems (e.g. oil: n-hexane, n-heptane, n-decane, toluene). The IFT of different oil–water systems, at variable concentrations of a nonionic surfactant, Tween 20, in the absence and presence of three different NPs was examined. As expected, the presence of Tween 20 surfactant effectively reduces the initial as well as final IFT of the n-hexane–water system. However, inclusion of NPs, irrespective of charge, alters the efficacy of Tween 20 surfactant in further reducing the IFT. In order to investigate the retarding efficiency of NPs on Tween 20 surfactant, the surface excess concentration of surfactants in the presence of 0.1 weight% of different NPs was also inspected along with apparent diffusion coefficients (Da). It has been found that the surface excess of surfactants at the interface decreases in the presence of NPs. Also increasing the concentration of Tween 20 surfactant increases the Da, leading to a higher adsorption rate. However, similar to a surface excess of surfactant, Da values are less in the presence of NPs compared to the particle free system.

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2015

  • Abstract: We present a molecular dynamics study on the self-assembly of anisotropic nanoparticles—triangles and tetrahedrons on a flat surface. We observe ordered and disordered aggregates of nanoparticles depending on the particle–particle and surface–particle interactions. Anisotropic particles induce directionality in the assembly process. In particular, a cross over from the isotropic (spherical) assembly to the anisotropic (non-spherical) assembly of nanoparticles is identified as their size increases for weak nanoparticle–surface interactions. However, at strong nanoparticle–surface interactions, clusters of nanoparticles grow uniformly on the surface. We present phase diagrams that depict all possible structures of triangles and tetrahedrons depending on their size (L) and the nanoparticle–surface interaction strength. We show a disorder to order transition in the plane, as L and increase.

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  • Abstract: We studied the structural and dynamical properties of methane and ethane in montmorillonite MMT slit pore of sizes 10, 20 and 30 using grand canonical Monte Carlo and classical molecular dynamics MD simulations. The isotherm, at 298.15 K, is generated for pressures up to 60 bar. The molecules preferentially adsorb at the surface as indicated by the density profile. In case of methane, we observe only a single layer, at the pore wall, whose density increases with increasing pressure. However, ethane also displays a second layer, though of low density in case of pore widths 20 and 30. In plane self-diffusion coefficient, of methane and ethane is of the order of At low pressure, increases significantly with the pore size. However, decreases rapidly with increasing pressure. Furthermore, the effect of pore size on diminishes at high pressure. Ideal adsorbed solution theory is used to understand the adsorption behaviour of the binary mixture of methane 80 percent and ethane 20 percent at 298.15 K. Furthermore, we calculate the selectivity of the gases at various pressures of the mixture, and found high selectivity for ethane in MMT pores. However, selectivity of ethane decreases with increase in pressure or pore size.

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  • Abstract: In this work, we performed atomistic simulations to study the structural properties of mixed self-assembled monolayers (SAM) of hydrophilic and hydrophobic alkylthiols, with two different chain lengths (C5 and C11), on gold nanoparticles (NPs) at three different arrangements, namely: random, patchy, and Janus domains. In particular, we report the effect of mixing of thiols with unequal carbon chain lengths (C5 and C11) at three different arrangements on the structural properties and hydration of SAMs. Our simulation study reveals that the arrangement of thiols having unequal carbon chains in mixed SAMs is a key parameter in deciding the hydrophilicity of the coated gold NPs. Thus, our findings suggest that the hydration of the SAMs-protected gold NPs is not only dependent on the molecular composition of the thiols, but also on the organization of their mixing. In addition, our results show that the bending of longer thiols, when these are mixed with shorter thiols, depends on the arrangement of thiols as well as the chemical nature of their terminal groups.

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  • Abstract: An ab initio derived transferable polarizable force-field has been developed for Zinc sulphide (ZnS) nanoparticle (NP) and ZnS NP-PMMA nanocomposite. The structure and elastic constants of bulk ZnS using the new force-field are within a few percent of experimental observables. The new force-field show remarkable ability to reproduce structures and nucleation energies of nanoclusters (Zn1S1-Zn12S12) as validated with that of the density functional theory calculations. A qualitative agreement of the radial distribution functions of Zn-O, in a ZnS nanocluster-PMMA system, obtained using molecular mechanics molecular dynamics (MD) and ab initio MD (AIMD) simulations indicates that the ZnS–PMMA interaction through Zn-O bonding is explained satisfactorily by our force-field. 2015 Wiley Periodicals, Inc.

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  • Abstract: The adsorption and orientational ordering of carbon dioxide molecules on parallel bundles of charged as well as uncharged carbon nanotubes were investigated at 300 K using grand canonical Monte Carlo and molecular dynamics. Zigzag and armchair single-walled carbon nanotubes of radius 1.5 nm and different intertube distances between 0 and 2 nm were used. A fixed charge of 0.01–0.04e was placed on each carbon to investigate the effect of charge. Adding a positive charge to the carbon nanotubes causes a significant increase in adsorption (up to 35% at a pressure of 1.88 bar), while the gas adsorption decreases by up to 15% on negatively charged carbon nanotubes. The increase or decrease of adsorption upon charging is attributed to the change in potential energy for the interaction between the individual carbon dioxide molecule and the nanotube. The higher adsorption on positively charged nanotubes leads to thicker adsorbed layers and a lower orientational order of the adsorbed molecules.

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  • Abstract: Fluids in confined environment are increasingly playing important roles in new technologies. Hence, understanding the nature of nano-confined fluids is of direct relevance in various applications. This special issue is composed of 14 papers reflecting advances in the molecular simulation of confined fluids. The first part of the issue is devoted to review articles. Eslami and co-workers provide a comprehensive review of simulation methods used for investigating the structure and dynamics of nano-confined fluids. While the article is focused on polymers, the methods based on molecular dynamics can also be used for non-polymeric fluids. The authors have also described the thermal properties of the confined polymers, such as thermal conductivity and the Kaptiza length, using non-equilibrium molecular dynamics. Das, on the other hand, provides a comprehensive review of the thermodynamics and kinetics of liquid–liquid coexistence under confinement using molecular simulation. The author describes the use of Monte Carlo and molecular dynamics methods to obtain the contact angle and phase coexistence information. Furthermore, kinetics of phase separation of confined fluids is also discussed by Das.

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  • Abstract: The adsorption behaviors of heavy metal ions Cd2+, Cu2+, Pb2+, and Hg2+, in aqueous media using functionalized single-walled carbon nanotube (SWCNT) with functional groups and are studied using molecular dynamics (MD) simulations. The results show that adsorption capacity is improved significantly using surface modification of SWCNT with carboxyl, hydroxyl, and amide functional groups. In addition, the adsorption capacity is found to increase with increasing metal-ion concentration. It is observed that the surface effectively adsorbs over 150 to 230 percent more metal ions than the bare CNT surface. On the contrary, are relatively weak functional groups where excess metal-ion adsorption compared to the bare CNT is in the range 10 to 47 percent. The structural properties, self-diffusion coefficients, and adsorption isotherms of the metal ions are computed and analyzed in detail. Moreover, the potential of mean force (PMF) is computed to understand the free energy of metal ions, in the presence of functional groups, which is remarkly higher in absolute terms, leading to significant affinity for adsorption compared to the case for the bare CNT. In general, the following order of adsorption of the metal ions on functionalized CNT is observed

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  • Abstract: Grand-canonical Monte Carlo simulations and adsorption experiments are combined to find the optimized carbon nanotube (CNT) arrays for gas adsorption at low pressures and 303 K. Bundles of 3D aligned double-walled carbon nanotube (DWCNT) with inner diameter of 8 nm and different intertube distances were made experimentally. The experimental results show that decreasing intertube distance leads to a significant enhancement in carbon-dioxide (CO2) adsorption capacity at 1 bar. The molecular simulation study on CO2 adsorption onto bundles of 3D aligned DWCNT with inner diameters of 1, 3, and 8 nm and intertube distance of 0-15 nm shows that the intertube distance plays a more important role than the CNT diameter.

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  • Abstract: Electric field induced phase transitions of confined water have an important role in cryopreservation and electrocrystallization. In this study, the structural and dynamical properties of nano-confined water in nano-slit pores under the influence of an electric field varying from 0 to 10 V are investigated under ambient conditions using molecular dynamics simulations. In order to replicate the nature of different materials, a systematic approach is adopted, including pore-size and lattice constant variations in different lattice arrangements viz., triangular, square and hexagonal, with hydrophilic and hydrophobic surface–fluid interactions. The structural behavior of water is investigated using radial distribution functions, bond order parameters and hydrogen bond calculations; the dynamical properties are analyzed using lateral and rotational diffusivity calculations. The lateral diffusivity with increasing electric field E increases by order of magnitude during electromelting. The pore-size, lattice constant, lattice arrangement and hydrophobic/hydrophilic nature of the pore surface strongly influence the electromelting behavior. Higher values of lattice constants and/or hydrophobic pores enhance the electromelting behavior of nanoconfined water.

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2014

  • Abstract: The aggregation and dispersion of two anisotropic nanoparticles (NPs), cubes and tetrahedrons, in a polymer matrix are studied in this work using coarse-grained molecular dynamics simulations. We present the phase diagrams of NP–polymer composites, depicting microscopically phase-separated, dispersed, and bridged cubes and tetrahedrons in a polymer matrix, which depend on the interaction between the NPs and polymer, along with the NPs’ volume fraction. The microscopic phase separation occurs at very low, where NPs self-organize into multidimensional structures, depending on. In particular, for tetrahedrons, a cross-over from an ordered spherical aggregate to a disordered sheet-like aggregate is observed with increasing. The free energy profile for a structured assembly is estimated, which clearly shows that the successful assembly of NPs is energetically favorable at a lower temperature. However, there exists an energy barrier for the successful assembly of all the NPs in the system. At intermediate, a transition from a clustered state to a state comprising dispersed cubes and tetrahedrons in a polymer matrix is observed. At higher, a further transition takes place, where gas-like dispersed NPs form a liquid-like aggregate via polymer layers. Therefore, the findings in this work illustrate that the effective interaction between anisotropic NPs in a polymer matrix is very diverse, which can generate multidimensional structured assemblies, with the disordered clustering, dispersion, and bridging-induced aggregation of NPs.

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  • Abstract: Coarse-grained molecular dynamics is utilized to quantify the behavior of a supercooled water drop on smooth and rough surfaces. Crystallization on rough surface is characterized based on wetting states. Freezing temperature and work of adhesion of water droplet are linearly associated with roughness parameters corresponding to the Cassie-Baxter and Wenzel states. The behavior is insensitive to different surface-fluid affinity. We show in general, for Wenzel states, work of adhesion is higher than that of Cassie-Baxter state for surfaces that have identical freezing temperatures. Formation of ice on surfaces can adversely affect industrial processes and applications in daily life such as in power lines, aircraft, ships, and buildings.1,2 Thus, there have recently been efforts to design such ice-repellant surfaces, and many of them have been inspired by the successes in the design of super-hydrophobic surfaces.3–14 However, the influence of surface texture is less understood for de-icing application, and attempts have been made to find linear correlation between the equilibrium work of adhesion of ice and the wettability of the substrate by liquid wate.r where is the equilibrium (Young's) contact angle.15,16 Early attempts by Murase et al.17 resulted in a significant scatter in the data, leading to weak correlation of the above expression. However, Meuler et al.18 have recently confirmed the above correlation to hold between the work of ice adhesion (work to remove ice from the surface) and the receding contact angle of liquid water, for various surface coatings. They concluded that a reduction of the ice adhesion strength requires a surface on which water droplets exist in the Cassie-Baxter state (drop suspended on surface protrusions) before freezing occurs. While some experiments indicate that superhydrophobic surface can minimize or eliminate ice formation, under some conditions,19 other experiments, for example of Jung et al.,20 conclude that anti-icing design needs optimization of the competing influence of both freezing delay and liquid-shedding ability, i.e., low adhesion. Recent work of Nosonovsky and Hejazi,21 based on the theoretical analysis of mechanical forces acting on a liquid droplet and ice, suggests that it depends on the size of the roughness whether or not a superhydrophobic surface is at the same time ice-phobic. This is in line with the prediction22 of a hard-sphere model that there is a optimal pit size on the surface for unrestrained growth of crystals. Furthermore, the lattice commensurability and incommensurability can affect crystal nucleation dramatically.23 Moreover, the claim of Chen and co-workers that superhydrophobic surfaces cannot reduce ice adhesion24 further corroborates other findings. Hence, the apparent contradictions of experiments confuse the picture of the role of roughness for the design of anti-icing surfaces. In this Letter, we demonstrate, using molecular dynamics, that freezing temperature and work of adhesion of liquid water, can be directly correlated with parameters of nanoscale roughness, if dependence on the wetting state is allowed for. Furthermore, we demonstrate clearly that the crystallization behavior on the rough surface can be characterized based on the wetting state of the liquid drops, and we extend the validity of an earlier continuum equation25,26 to the supercooled drop-surface systems.

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  • Abstract: The crossover behaviour of water droplet's state from the Wenzel state to the Cassie state with varying pillar height and surface fraction is examined critically using molecular dynamics. We report the effect of the system size on the wetting behaviour of water droplets by examining the contact angle for both regimes. We observe that when the droplet size is comparable to the pillar dimension, the contact angle of droplets fluctuates with increasing droplet size because of the contact line pinning, which is more pronounced in the Wenzel regime. We further demonstrate the phantom-wall method to evaluate free energy of intermediate wetting states.

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  • Abstract: The wetting behavior of an ethanol–water droplet is investigated on graphitic smooth and rough surfaces using molecular dynamics simulations. On a smooth surface, ethanol molecules prefer to stay at the vapor–liquid and solid–liquid interfaces. The contact angle of a droplet on a smooth surface decreases with an increase in the ethanol concentration from 0 to 30 wt percent. The corresponding line tension increases from 3 × 10–11 to 9.4 × 10–11 N at 300 K. The critical weight percentage for complete wetting is found to be approximately 50 percent. In the case of a textured graphite surface, with the addition of ethanol molecules, the Cassie–Baxter state of a drop is transformed into the Wenzel state via the partial Wenzel state, with ethanol molecules filling the rough region, leading to an increase in its wettability. A linear relation of 1 + cos with the roughness parameter associated with the Cassie–Baxter and Wenzel states is observed, indicating that the solid–liquid interfacial tension is directly proportional to the roughness parameter. This behavior is akin to that seen for the case of pure liquid. The hydrogen bonding and density profile are analyzed to understand the wetting states of the blended drop.

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  • Abstract: All-atom molecular dynamics simulations are conducted to understand the structural and dynamical behavior of self-assembled monolayer of n-alkanols on a mica surface. In particular, we report the effect of increasing carbon chain length (C1–C10) on the self-assembly, surface diffusion, and preferential tilting of n-alkanol monolayer, for monolayer surface coverage ranging from 6 × 10–5 to 3.54 × 10–3 mol/m2. The adsorption phenomena typically follow the Langmuir adsorption isotherm. However, the maximum adsorption is observed for n-hexanol, and it drops with further increase in the chain length. The surface diffusion coefficient, Ds, within monolayer, is nonmonotonic in nature. The maximum value of Ds decreases with increasing carbon chain length, with an exception of methanol owing to its preferential attachment with the cage of mica due to the presence of K+. The behavior of Ds is clearly explained using instantaneous autocorrelation function of hydrogen bonds with the surface. Further, Ds, is found to vary inversely proportional to the lifetime of hydrogen bond of alkanols with the surface. Most probable tilt angle of molecules with increasing alkyl group (C1, C2, C4, and C6) is in the order 71° > 38° > 29° > 19°. However, for octanol we observed molecules to attain a preferential tilt angle of 80°. The self-assembly behavior of lower alkanols, i.e., C1–C6 is contrary to that seen for higher alkanols.

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  • Abstract: Three-stage pseudo-supercritical transformation path and multiple-histogram reweighting technique are employed for the determination of solid-liquid coexistence of the Lennard-Jones (12-6) fluid, in a structureless cylindrical pore of radius, R, ranging from 4 to 20 molecular diameters. The Gibbs free energy difference is evaluated using thermodynamic integration method by connecting solid and liquid phases under confinement via one or more intermediate states without any first order phase transition among them. The thermodynamic melting temperature, Tm, is found to oscillate for pore size, R < 8, which is in agreement with the behavior observed for the melting temperature in slit pores. However, Tm for almost all pore sizes is less than the bulk case, which is contrary to the behavior seen for the slit pore. The oscillation in Tm decays at around pore radius R = 8, and beyond that shift in the melting temperature with respect to the bulk case is in line with the prediction of the Gibbs-Thomson equation.

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  • Abstract: We present a molecular dynamics study on the stretching of a linear polymer chain that is adsorbed at the junction of two intersecting flat surfaces of varying alignments. We observe a transition from a two-dimensional to one-dimensional structure of the adsorbed polymer when the alignment, i.e., the angle between the two surfaces that form a groove, is below 135 degree.

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  • Abstract: The effects of the electric field on the vapor–liquid equilibria of methanol and ethanol confined in a graphitic slit pore of width 4 nm using molecular dynamics simulations are reported. The vapor–liquid critical temperature of methanol gets suppressed under confinement. The external electrical field further decreases the critical temperature with increasing electric field strength up to E = 1.5 V·nm–1. Surprisingly, a further increase in the electric field strength reverses the critical temperature behavior and is seen to increase with increasing electric field. The reversible behavior of the critical temperature with the electric field is also seen for nanoconfined ethanol at approximately 1.5 V·nm–1. The critical density, on the other hand, is found to continuously decrease with increasing electric field strength. Application of an external electric field results in the decrease in vapor and liquid densities. The coordination number in the liquid phase is found to decrease first with increasing electric field until E = 1.5 V·nm–1 and then increases with a further increase in the electric field, confirming the observed trend in the critical temperature according to the mean field theory. Orientational order of nanoconfined methanol and ethanol, on the other hand, is found to increase with increasing electric field.

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  • Abstract: Performance of a lithium-ion based rechargeable battery is investigated using coupled battery model including heat and stress models via finite element method simulations. An effort is made to elucidate the importance of using diffusivity equation, in the model, as a function of lithium-ion concentration and temperature. Diffusivity expressions for both anode and cathode material are developed using atomistic simulations. Simulation results show 10% drop in the battery potential after 100 charge-discharge cycles. This decline in performance is attributed to the concentration gradient, heat generation and stress accumulation, substantiating the need to address these effects simultaneously. Finally, intercalation stress values due to the modified diffusivity expression are found to differ considerably with that due to the constant diffusion values used in earlier works. The findings validate the assertion that intercalation stress values depend greatly on the lithium-ion concentration based diffusivity expression.

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  • Abstract: Microstructure of dibenzo-18-crown-6 (DB18C6) and DB18C6/Li+ complex in different solvents (water, methanol, chloroform, and nitrobenzene) have been analyzed using radial distribution function (RDF), coordination number (CN), and orientation profiles, in order to identify the role of solvents on complexation of DB18C6 with Li+, using molecular dynamics (MD) simulations. In contrast to aqueous solution of LiCl, no clear solvation pattern is found around Li+ in the presence of DB18C6. The effect of DB18C6 has been visualized in terms of reduction in peak height and shift in peak positions of gLi-Ow. The appearance of damped oscillations in velocity autocorrelation function (VACF) of complexed Li+ described the high frequency motion to a “rattling” of the ion in the cage of DB18C6. The solvent-complex interaction is found to be higher for water and methanol due to hydrogen bond (HB) interactions with DB18C6. However, the stability of DB18C6/Li+ complex is found to be almost similar for each solvent due to weak complex-solvent interactions. Further, Li+ complex of DB18C6 at the liquid/liquid interface of two immiscible solvents confirm the high interfacial activity of DB18C6 and DB18C6/Li+ complex. The complexed Li+ shows higher affinity for water than organic solvents; still they remain at the interface rather than migrating toward water due to higher surface tension of water as compared to organic solvents. These simulation results shed light on the role of counter-ions and spatial orientation of species in pure and hybrid solvents in the complexation of DB18C6 with Li+. Graphical Abstract DB18C6/Li+ complex in pure solvents (water, methanol, chloroform, and nitrobenzene) and water/nitrobenzene interface

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  • Abstract: The melting transition of bulk and confined Stockmayer fluids is analyzed using molecular dynamic simulations. The solid–liquid coexistence temperature is evaluated using a modified three-stage pseudo-supercritical transformation path. The bulk melting temperature calculated using the aforementioned method agrees well with the literature value. Melting temperatures of the Stockmayer fluid confined in Lennard-Jones (LJ) 9–3 slit pore of pore size, H, varying from 6 to 20 molecular diameters are reported. For molecular diameters, the shift in the melting temperature for the Stockmayer fluid is oscillatory in nature with the inverse of the pore size. However, for higher H the shift in melting temperature obeys the Gibbs–Thomson equation. The thermodynamic melting temperatures of the Stockmayer fluid under confinement, for variable pore sizes, are found to be usually higher than that of the bulk fluid. The structural and orientational order parameters are also presented, which suggest similarity in the structures of confined LJ and confined Stockmayer fluids.

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  • Abstract: Ice and water droplets on graphite have been studied by quantum path integral and classical molecular dynamics simulations. The point-charge q-TIP4P/F potential was used to model the interaction between flexible water molecules, while the water-graphite interaction was described by a Lennard-Jones potential previously used to reproduce the macroscopic contact angle of water droplets on graphite. Several energetic and structural properties of water droplets with sizes between 102 and 103 molecules were analyzed in a temperature interval of 50–350 K. The vibrational density of states of crystalline and amorphous ice drops was correlated to the one of ice Ih to assess the influence of the droplet interface and molecular disorder on the vibrational properties. The average distance of covalent OH bonds is found 0.01 larger in the quantum limit than in the classical one. The OO distances are elongated by 0.03 in the quantum simulations at 50 K. Bond distance fluctuations are large as a consequence of the zero-point vibrations. The analysis of the H-bond network shows that the liquid droplet is more structured in the classical limit than in the quantum case. The average kinetic and potential energy of the ice and water droplets on graphite has been compared with the values of ice Ih and liquid water as a function of temperature. The droplet kinetic energy shows a temperature dependence similar to the one of liquid water, without apparent discontinuity at temperatures where the droplet is solid. However, the droplet potential energy becomes significantly larger than the one of ice or water at the same temperature. In the quantum limit, the ice droplet is more expanded than in a classical description. Liquid droplets display identical density profiles and liquid-vapor interfaces in the quantum and classical limits. The value of the contact angle is not influenced by quantum effects. Contact angles of droplets decrease as the size of the water droplet increases which implies a positive sign of the line tension of the droplet

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2013

  • Abstract: Grand-canonical transition matrix Monte Carlo simulation with histogram reweighting and finite-size scaling technique are used to calculate fourth-order Binder cumulant of order parameter along the vapour–liquid coexistence line to calculate the critical temperature of bulk and confined square-well fluid in slit pore of two pore sizes. Further, this approach is utilised to estimate the critical temperatures of relatively more complex fluids such as n-alkanes confined in graphite and mica slit pores of different slit widths. The estimated critical temperatures are compared with the critical temperature obtained for the same systems using simplified form of the scaling law. This investigation reveals that critical temperatures of simple and complex fluids in bulk state and under confinement, estimated using the scaling law, are within reasonable accuracy with that obtained using more accurate and rigorous approach of fourth-order Binder's cumulant.

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  • Abstract: Abstract A novel nanocomposite polyvinyl alcohol precursor-based material dispersed with the web of carbon microfibers and carbon nanofibers is developed as lithium (Li)-ion electrolyte battery separator. The primary synthesis steps of the separator material consist of esterification of polyvinyl acetate to produce polyvinyl alcohol gel, ball-milling of the surfactant dispersed carbon micro-nanofibers, mixing of the milled micron size (500 nm) fibers to the reactant mixture at the incipience of the polyvinyl alcohol gel formation, and the mixing of hydrophobic reagents along with polyethylene glycol as a plasticizer, to produce a thin film of 25 mu. The produced film, uniformly dispersed with carbon micro-nanofibers, has dramatically improved performance as a battery separator, with the ion conductivity of the electrolytes (LiPF6) saturated film measured as 0.119 S cm 1, approximately two orders of magnitude higher than that of polyvinyl alcohol. The other primary characteristics of the produced film, such as tensile strength, contact angle, and thermal stability, are also found to be superior to the materials made of other precursors, including polypropylene and polyethylene, discussed in the literature. The method of producing the films in this study is novel, simple, environmentally benign, and economically viable.

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  • Abstract: The melting transition of a Lennard-Jones system confined in slit pores of variable pore size, H, is studied using molecular dynamics simulations. We examine various mechanisms to locate the pore melting temperature under confinement using molecular simulations. Three types of structure-less pore walls are considered, namely strongly attractive walls, weakly attractive walls, and repulsive walls. In particular, we present details of the density–temperature hysteresis, Lindemann parameter, and non-Gaussian parameter for various pore sizes ranging from 8 to 3 molecular diameters. The methods as used in this work are found applicable for repulsive, weak, and moderately attractive pores. Using the above criteria, we estimated the melting temperature for various pore surfaces and pore sizes. The melting temperature, for an attractive surface, is observed to be elevated or depressed depending on the pore size. In contrast, depression in the melting temperature is observed in the case of weakly attractive and repulsive surfaces. Crossover behavior from three-dimensional to two-dimensional for weakly attractive and repulsive surfaces is proposed using the relation.

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  • Abstract: In this work, we study the influence of polymer chain length (m), based on Lennard-Jones potential, and nanoparticle (NP)-polymer interaction strength on aggregation and dispersion of soft repulsive spherically structured NPs in polymer melt using coarse-grain molecular dynamics simulations. A phase diagram is proposed where transitions between different structures in the NP-polymer system are shown to depend on m and np. At a very weak interaction strength, a transition from dispersed state to collapsed state of NPs is found with increasing m, due to the polymer's excluded volume effect. NPs are well dispersed at intermediate interaction strengths, independent of m. A transition from dispersion to agglomeration of NPs, at a moderately high NP-polymer interaction strength, is identified by a significant decrease in the second virial coefficient, excess entropy, and potential energy, and a sharp increase in the Kirkwood-Buff integral. We also find that NPs undergo the following transitions with increasing

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  • Abstract: Grand canonical Monte Carlo simulations and adsorption experiments are conducted to understand the adsorption of CO2 onto bundles of 3D aligned double walled carbon nanotubes of diameter 5 nm at 303 K. The simulation of partial adsorption isotherms, only inner tube volume, only interstices between tubes, and unrestricted, allows a breakdown of the experimental adsorption isotherms into contributions of different regions. The results are compatible with microscopic observations of the majority of the inner tube volumes being accessible for CO2. Further, the unrestricted adsorption isotherm is quantitatively equivalent to the sum of inner and outer adsorption for the pressure range considered in this work, p 40 bar, indicating no significant interference between inner and outer regions. The intertube distance, which is varied from 0 to 15 nm, dramatically affects the isosteric heat of adsorption and adsorption capacity. Excess adsorption is found to display a nonlinear behavior with d, for unrestricted and outer cases. For low pressures p 14 bar, maximum adsorption occurs at d 0.5 nm. However, for higher pressures, 14 40 bar, the adsorption peaks at d 1 nm. The Freundlich isotherm is found to fit the experimental and simulation data. The adsorption sequence changes with the intertube distance for the unrestricted case. At d 0.5 nm, adsorption proceeds with increasing loading in the following order grooves and inner surface adsorption fill interstitial region fill inner region. However, at higher distances, d 0.5 nm, the sequence changes the following inner surface adsorption partial outer surface adsorption complete outer surface adsorption fill interstitial, groove, inner adsorption. The change in mechanism of adsorption is clearly reflected in the behavior of the heat of adsorption, where we observed a crossover behavior at around d 0.5 nm.

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  • Abstract: We investigate segregation in a horizontally vibrated binary granular mixture in a closed offset-Christmas tree channel. The segregation phenomenon occurs in two steps vertical sorting followed by axial segregation. In the first step, sorting occurs via Brazil nut effect or reverse Brazil nut effect depending on the particles size and density ratios. The two layers thus formed then separate axially towards opposite ends of the channel with the top layer always moving towards root of the Christmas tree. We discuss the segregation mechanism responsible for axial segregation.

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  • Abstract: The solid liquid coexistence of a Lennard Jones fluid confined in slit pores of variable pore size, H, is studied using molecular dynamics simulations. Three stage pseudo supercritical transformation path of Grochola J. Chem. Phys. 120, and multiple histogram reweighting are employed for the confined system, for various pore sizes ranging from 20 to 5 molecular diameters, to compute the solid liquid coexistence. The Gibbs free energy difference is evaluated using thermodynamic integration method by connecting solid-liquid phases under confinement via one or more intermediate states without any first order phase transition among them. Thermodynamic melting temperature is found to oscillate with wall separation, which is in agreement with the behavior seen for kinetic melting temperature evaluated in an earlier study. However, thermodynamic melting temperature for almost all wall separations is higher than the bulk case, which is contrary to the behavior seen for the kinetic melting temperature. The oscillation founds to decay at around, and beyond that pore size dependency of the shift in melting point is well represented by the Gibbs Thompson equation.

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2012

  • Abstract: We present the equilibrium and dynamical properties of liquid sodium (Na) and lithium (Li), based on embedded atom models, using molecular dynamics simulations. In particular, we present vapor–liquid equilibria, critical properties, diffusivity, shear viscosity and excess entropy of liquid Na and Li. Critical temperatures obtained in the current work are 2462 K and 5649 K for Na and Li, respectively. On the other hand, critical density for Na is 0.3493 g/cm3 and that for Li is 0.1553 g/cm3. Critical pressures based on the exiting EAM models for Na and Li are 113 bar and 1686 bar, respectively. The relation of excess entropy and dynamical properties is examined in the framework of existing scaling laws. We observed an exponential nature between the dimensionless scaled diffusion constant and the approximate excess entropy for liquid Na and Li, as also observed for other liquid metals.

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  • Abstract: To extend the work of binary fluid mixtures and their associated bridge-like structures, the adsorption of gas-like molecules (interacting via hard-sphere potentials) on self-assembled fluid channels was examined. We examined the morphological evolution of an initial random binary mixture under confinement of chemically patterned substrates with strong, long-range preferential attraction to the pure square-well component. Gas-like molecules were presumed to have a weak attraction to the square-well fluid. The morphology and corresponding density profiles revealed the underlying chemical and physical adsorption of gas-like molecules to off-strip voids and to the interface of the self-assembled fluid channels. The entropic effects drive the non-interaction hard-sphere molecules to assemble or reorganize in the voids left between the self-assembled square-well fluids. Such studies can help in the study of formation of nano-liquid structures and enhanced adsorption of gas-like molecules for storage purposes.

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  • Abstract: We capture the effects of the structured surface on a phase transition of hard-sphere fluids. The confining environment follows single-walled carbon nanotube (SWCNT) configuration. For careful discrimination of the surface-chirality effect, hard-core potentials are applied to carbon atoms, and further their positions are fixed. In this way, equation of states and microstructures of the confined particles are intrinsically obtained based on the SWCNT chirality as well as the diameter. We observed three branches indicating fluid-like and solid-like phases with onsets of freezing and melting. We found that freezing and melting of fluid particles are very sensitive to the surface chirality in small-diameter SWCNT, which especially holds a single layer of fluid particles. In those SWCNTs, spreading pressures are found to be lower than those of smooth-surface cylindrical pores. The surface chirality has less impact on the phase change of confined fluids for large-diameter SWCNT, of which diameter is a dominant factor.

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  • Abstract: Surface phase transitions of Lennard–Jones (LJ) based two- and four-site associating fluids have been studied for various associating strengths using grand-canonical transition matrix Monte Carlo simulations. Our results suggest that, in the case of a smooth surface, represented by a LJ 9-3-type potential, multiple-site associating fluids display a prewetting transition within a certain temperature range. However, the range of the prewetting transition decreases with increasing associating strength and increasing number of sites on the fluid molecules. With the addition of associating sites on the surface, a quasi-2D vapor–liquid transition may appear, which is observed at a higher surface site density for weaker associating fluids. The prewetting transition at lower associating strength is found to shift towards the quasi-2D vapor–liquid transition with increasing surface site density. However, for highly associating fluids, the prewetting transition is still intact, but shifts slightly towards the lower temperature range. Adsorption isotherms, chemical potentials and density profiles are used to characterize surface phase transitions.

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  • Abstract: This paper presents the results of quantum chemical and classical molecular dynamics (MD) simulations of the microhydration states of the Sr2+ ion. The quantum chemical results strongly suggest a coordination number (CN) of 8 for the first hydration shell of Sr2+, which is in quantitative agreement with data available from X-ray absorption fine structure (XAFS) measurements. The calculated theoretical Srsingle bondO bond distance of 2.59 Å is also in excellent agreement with the XAFS results (2.60 Å). Classical MD simulations are conducted on various water models to predict the hydration structure of the Sr2+ ion. The CN is found to be in the range of 8–9 using SPC, TIP3P, and TIP4P-2005 water models, with the probability more skewed toward 8. MD–EXAFS study and coordination number analyses reveal that TIP4P-2005 is the best model potential for simulating water molecules to reproduce the experimentally observed absorption spectra and coordination numbers.

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  • Abstract: We examined the influence of external electric field on the vapor–liquid coexistence curve and on structural properties of TIP4P/2005 water confined in hydrophobic and hydrophilic pores using all-atom molecular dynamics simulations. While the electric field increases the critical temperature of bulk water, the effect is contrary on the confined water. We clearly observe that the critical temperature of confined water decreases with increasing field strength. The current work strongly indicates that using electric field results in a decrease in the saturated liquid phase density and increase in the saturated vapor phase density of the confined water and can be used to modulate the evaporation rate of water under confinement. The effect of an electric field on the confined fluids is more pronounced at higher temperature. We also report that the critical density of water behaves differently in hydrophobic and hydrophilic pores. With increasing electric field, the critical density increases in hydrophobic pores; however, it is found to decrease in hydrophilic pores. We analyze the results using pair correlation functions and orientational, tetrahedral, and hydrogen bond distributions. Our investigation indicates that the presence of an electric field enhances the coordination number N(r) of the bulk phase. In contrast, the presence of an electric field reduces N(r) of the confined fluid. This is clearly reflected in the behavior of the critical temperature of bulk and confined water. Our structural analysis reveals that the application of an external field induces orientational order of dipole vector parallel to the field direction in bulk water, whereas its effects on dipole orientation is much less in confined systems. We also report that the hydrogen-bonding behavior in the vapor phase is responsible for the difference in critical density of water confined in hydrophobic and hydrophilic pores.

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  • Abstract: Grand-canonical transition-matrix Monte Carlo and histogram reweighting techniques are used herein to study the vapor-liquid coexistence properties of two-dimensional (2D) flexible oligomers with varying chain lengths (m = 1–8). The phase diagrams of the various 2D oligomers follow the correspondence state (CS) principle, akin to the behavior observed for bulk oligomers. The 2D critical density is not influenced by the oligomer chain length, which contrasts with the observation for the bulk oligomers. Line tension, calculated using Binder's formalism, in the reduced plot is found to be independent of chain length in contrast to the 3D behavior. The dynamical properties of 2D fluids are evaluated using molecular dynamics simulations, and the velocity and pressure autocorrelation functions are investigated using Green-Kubo (GK) relations to yield the diffusion and viscosity. The viscosity determined from 2D non-equilibrium molecular dynamics simulation is compared with the viscosity estimated from the GK relations. The GK relations prove to be reliable and efficient for the calculation of 2D transport properties. Normal diffusive regions are identified in dense oligomeric fluid systems. The influence of molecular size on the diffusivity and viscosity is found to be diminished at specific CS points for the 2D oligomers considered herein. In contrast, the viscosity and diffusion of the 3D bulk fluid, at a reduced temperature and density, are strongly dependent on the molecular size at the same CS points. Furthermore, the viscosity increases and the diffusion decreases multifold in the 2D system relative to those in the 3D system, at the CS points

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2011

  • Abstract: Wetting transition of water on graphite and boron-nitride surfaces is investigated by molecular dynamics simulation. In particular, we report the effect of temperature and system size on the contact angle of water droplet on the two surfaces. Wetting temperature of water on graphite is found to be 470 ± 5 K, which is in good agreement with the estimate of Zhao (Phys. Rev. B 76 (2007) 041402) using grand-canonical Monte Carlo simulations. On the other hand, wetting temperature of water on BN surface is estimated to be lower, 438 ± 5 K. Temperature dependence of line tension of water droplet on both the surfaces is also studied, and found to vary between 10 to 10 and 10 to 9 N for temperature in the range of 300 to 420 K. In this work, line tension for water on graphite and BN surfaces is observed to have a logarithmic proportional behavior with the contact angle.

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  • Abstract: We present the influence of surface heterogeneity on the vapour–liquid phase behaviour of square-well fluids in slit pores using grand-canonical transition-matrix Monte Carlo simulations along with the histogram-reweighting method. Properties such as phase coexistence envelopes, critical properties and local density profiles of the confined SW fluid are reported for chemically and physically patterned slit surfaces. It is observed that in the chemically patterned pores, fluid–fluid and surface attraction parameters along with the width of attractive and inert stripes play fundamentally different roles in the phase coexistence and critical properties. On the other hand, pillar gap and height significantly affect the vapour–liquid equilibria in the physically patterned slit pores. We also present the effect of chemically and physically patterned slit surfaces on the spreading pressure.

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  • Abstract: Grand-canonical transition-matrix Monte Carlo (GC-TMMC) simulation, is used to investigate the effect of pore shape and surface-fluid strength on the vapor–liquid phase transition and crossover behavior of critical properties from 3D to 2D of a square well (SW) fluid. We present the vapor–liquid coexistence phase diagram in hard and attractive cylindrical pores of varying slit width from 4 to 50 molecular diameters. This investigation indicates that having same pore shape but different surface nature can significantly alter the coexistence envelopes and hence the critical point. Critical temperature is found to approach the 3D bulk value monotonically irrespective of the pore shape and surface nature. However, the rate of approach of critical point towards the 3D bulk value decreases as the effective confinement increases. On the other hand, approach of pore critical density towards the bulk 3D value follows a non-monotonic path, irrespective of pore shape and surface strength. Interestingly, with the same pore shape, attractive wall surface follows an opposite trend to approach the bulk critical density as compared to that of hard wall surface. Crossover from 3D to 2D behavior in the hard cylindrical pores is observed around 28 molecular diameters, which is significantly larger than that observed by earlier workers for hard and attractive slit pores.

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  • Abstract: A gel free microchannel device made up of polydimethyl siloxane is fabricated for the surface based electrophoresis of double stranded deoxy ribonucleic acid molecules. In the presence of directional external electric field, DNA fragments near the corners of the microchannel are found to separate faster as compared to those over the base of the channel. This is in spite of the reduction in the mobility of molecules over the channel corners. We performed coarse grained molecular dynamics simulations which reveal that, though the adsorption energy of the DNA fragments increases near the corner, it is the increase in the relative mobility which enhances the separation of the fragments over the corner. Fractionation of ds DNA based on different molecular lengths by passing them through a sieving medium has been regularly explored for all diagnostics. Some of the widely used methods include slab gel electrophoresis,1 zone electrophoresis, 2,3 capillary electrophoresis 4 to 7 in microcapillaries, microfabricated channel arrays, etc.8,9 Fragment sizes above 10 kbp are resolved very poorly by the above bulk electrophoresis techniques.5 Thus, new schemes for DNA fractionation have emerged involving electrostatic interactions between nucleic acid molecules and surfaces using its inherent chemical, topological, or structural properties,10 to 13 as demonstrated for the first time by Pernodet et al.10 over a perfectly flat silicon wafer and later by Han and Craighead.14 Literature shows the influence of various parameters like electric field intensity, ionic strength, and migration distances on the mobility of ds DNA as they are transported along surfaces.15 Lee and Kuo16 fabricated a gel-free microchannel electrophoresis device for fractionating larger DNA fragments where they chemically modified the channel’s bottom surface. The process although widely explored has not been fully understood. In this letter, we have investigated the surface electrophoresis process for ds DNA in polydimethyl siloxane microchannels. A peculiar behavior of ds DNA is observed as they move along the channel corners instead of the channel base in a few trials. The channel length and time needed for fractionating a 1 kb ladder decreased substantially along the channel corners as compared to the base. In other words, orthogonally placed pair of surfaces is found to be more favorable for the fractionation process in comparison to flat surfaces. We have utilized molecular dynamics simulations to understand the physics of interaction of the ds DNA with the surface to explain the above behavior. The microchannel electrophoresis device for our experiments was fabricated in PDMS by using a carbon dioxide laser etched pattern on a poly methyl methacrylate substrate.

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  • Abstract: Bridge-like structures have been reported in recent studies for a binary mixture with square well fluids in 2D and 3D using different techniques. In this paper, we present our NVT simulation results in 3D with symmetric additive water and oil-like molecules. We use Dirac's delta function defined using theta step function for evaluating pressure components and surface tension values. Our investigation reveals that though 3D NVT MC results are in qualitative agreement with the published results showing all the structures as bridge-like, complete wetting of the walls by the preferred component and micelle structures but the critical parameters as surface attraction strength significantly alter to lower values for PW and CW cases in comparison to the 2D MD cases. We have studied cases with pore width as Our pressure and surface tension values show clear signature of confinement effect and dependence on other parameters for the bridge cases but it is concluded that different structural transitions depend only on the predefined energetically favorable and unfavorable interfaces which in turn comes from the separation of the two walls and average initial density. Our density profiles confirm the evolved structures.

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  • Abstract: We report all-atom molecular dynamics simulations of water confined in graphite and mica slit pores of variable size ranging from 10 to 60 Å. For each pore size, we demonstrate that the confinement not only reduces the critical temperature of the water but also introduces inhomogeneity in the system that, in turn, results in different vapor–liquid coexistence densities at different layers of the pore. We report, in detail, the contribution of different layers toward the vapor–liquid phase diagram of the confined water in graphite and mica slit pores. We also present the hydrogen bonding (HB) distribution in various layers and the ordering of water molecules near the surface of pore. Bond orientational order calculations of water near the surface of the pores indicate that water molecules tend to order near the mica surface whereas the ordering is absent for the case of graphite pores.

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  • Abstract: Surface phase transitions are studied for Lennard-Jones (LJ) based dimer forming associating fluids on modified surfaces with active sites for various association strengths using grand-canonical transition matrix Monte Carlo. We examine adsorption isotherm, density, energy, and monomer profiles to differentiate layering, quasi-2D vapor–liquid and prewetting transitions. Prewetting transition is found for association strengths, whereas, for weaker associating fluids, we observe quasi-2D vapor–liquid transition. The growth of thick films in the case of quasi-2D vapor liquid transitions is found to suppress with decrease in temperature and eventually splits in layering transitions. For systems exhibiting prewetting transition, wetting temperature and prewetting critical temperature increase with increasing association strength. In addition, we examine boundary tension of quasi-2D and prewetting transitions using finite size scaling formalism of Binder. Our results indicate that the quasi-2D boundary tension is lower than that of the prewetting transition. Surface sites are found to reduce the boundary tension; however, the effect of active sites diminishes with stronger fluid–fluid associating strength.

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2010

  • Abstract: Nanoconfined fluids — that is, fluids confined between surfaces separated by nanometers — play important roles in many natural and man-made processes and products. One example is hard disk drive lubrication where, as data density has increased exponentially, the distance between the read head and rotating platen has been exponentially decreasing for several decades. This distance is now at 10–12 nm, and in the next generation of disk drives will be at 8 nm;1 currently, monolayers of lubricant are used to protect disk drives in abnormal situations (e.g., power loss), but in the future it is expected that they will be lubricated at all times, including during read/write operations. Additional examples include the lubrication of microelectromechanical systems (MEMS), and nanoelectromechanical systems (NEMS),2 and a model3 for the natural lubrication of synovial joints, all of which can involve moving surfaces separated by distances of the order of nm. The latter exhibit very low-sliding friction at normal pressures up to 5 MPa or more; the model system, consisting of polyzwitterionic brushes polymerized directly onto the mica sheets in a surface force balance (SFB), exhibits very similar low-sliding friction (within a factor of 2 of the natural synovial joints) at pressures as high as 7.55 MPa. These three examples highlight the desirability of being able to lubricate effectively between surfaces moving relative to each other while separated by distances on the order of nm. If the lubricant undergoes a fluid-solid phase transition under nanoconfinement, resulting in a many order of magnitude increase in the effective viscosity and the onset of a nonzero yield stress,* then it is clearly not useful as a lubricant. In addition to lubrication at the nanoscale, phase transitions under nanoconfinement are also clearly important in industrial adsorption and catalytic processes (micro- and mesoporous adsorbents, with pore widths of under 2 nm and 2–50 nm, respectively, are widely used in the chemical, petrochemical, gas processing, and pharmaceutical industries for separations, pollution abatement, and as catalysts and catalyst supports).4 Additional application areas (e.g., in geology, oil recovery and nanofabrication, including nanotemplating through nanoconfinement) are described in the excellent review article by Gelb etal.4

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  • Abstract: We present the effect of surface attraction on the vapor liquid equilibria of square well fluids in slit pores of varying slit width from quasi 3 D to 2 D regime using molecular simulation methodologies. Four to five distinct linear regimes are found for shift in the critical temperature with inverse slit width, which is more prominent at higher surface fluid interaction strength. On the other hand, shift in the critical density and the critical pressure does not show any specific trend. Nevertheless, critical density and pressure show the sign of approaching toward the 3 D bulk value with increase in the slit pore width, H, beyond 40 molecular diameters. The crossover from 3 D to 2 D behavior for attractive pores is observed around 14 to 16 molecular diameters, which is significantly different from the crossover behavior in the hydrophobic slit pore. Critical properties for molecular diameters are indifferent to the surface characteristics. Corresponding state plot displays fluctuating positive deviation of spreading pressure for large pores and negative deviation for small pores from the bulk saturation value. Such behavior is more accentuated at stronger surface fluid interaction strength. We also present vapor liquid surface tensions of the SW fluid for different attractive planar slit pores of variable slit widths. Vapor liquid surface tension or interfacial width values are insensitive to the surface fluid interaction strength for slit width, H less than or equal 2 molecular diameters. At a given slit width and temperature, vapor liquid interfacial width is found to decrease with increasing wall fluid interaction for H greater 2. However, interfacial properties approaches to the bulk value with increasing slit width. On the other hand, surface tension at a reduced temperature displays a nonmonotonic behavior with the change in H, which is in good agreement with the nature of the corresponding scaled interfacial width.

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  • Abstract: Prewetting transitions are studied for Lennard Jones based dimer forming associating fluids, on a structureless surface represented by LJ 9 to 3 type potential, for various association strengths using grand canonical transition matrix Monte Carlo and histogram reweighting techniques. Occurrences of prewetting transition are observed for association strengths and 10.0. Structural properties, monomer fraction, and orientation order profile of thin thick film of one-site associating fluids are presented. Wetting temperature, and prewetting critical temperature, increases with increasing association strength, which is in agreement with the results of the density functional theory. Length of prewetting line, on the other hand, is found to decrease first with increasing association energy until and subsequently found to increase substantially for. This behavior is contrary to the prediction from the DFT. We observe that the boundary tension of thin-thick film via GC TMMC and finite size scaling exhibits a maximum with respect to association strength.

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  • Abstract: Fluid-solid phase transition and coexistence of square-well fluids confined in narrow cylindrical hard pores are characterized using molecular simulation methods. The equation of state containing a fluid phase, a solid phase and a fluid-solid coexistence state was separately obtained for different attractive ranges of potential well and pore diameters; fluid-solid phase coexistence densities and pressure are close to the hard sphere fluids at the same temperature, while the pressure decreases significantly for, respectively. We also report the structural properties of the systems undergoing a phase transition.

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  • Abstract: We report a molecular simulation study on the non-monotonic behavior of critical temperature, Tcp, of a confined Yukawa fluid. Close to the adhesive hard sphere (AHS) range of the surface–fluid interaction, Tcp monotonically increases with increasing surface–fluid interaction range. Subsequently, after a certain threshold value, depending on the surface interaction well depth, Tcp decreases monotonically with further increase in the surface interaction range. On the other hand, critical density and pressure show increasing monotonic trends with the surface interaction range. The crossover from 3D to 2D behavior for colloidal fluid in attractive pores is observed around a slit width of 14 molecular diameters for the studied system in this work.

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2009

  • Abstract: It is known that a horizontally vibrated binary mixture in a tapered and inclined channel segregates axially, with the two species moving to the opposite ends of the channel. In general, the parameters that affect the segregation process include the forcing frequency and its amplitude, the constituents’ mass and size, and the taper and inclination of the channel. The ultimate goal here is to locate those parameters that are most significant to the segregation process, thereby providing control variables for practical applications. However, owing to the complexity of the problem, as a first step to better understand the physics behind this phenomenon, we undertake three dimensional molecular dynamics simulations of a horizontally vibrated mono-disperse granular particles in a tapered and inclined channel. Though at this stage, the immediately addressed problem is of more relevance to the granular material industry, it is envisaged that tools developed to understand this process will ultimately have wide applicability to granular systems, occurring in both natural contexts and in geotechnical engineering.

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  • Abstract: Free energy simulation method is applied to calculate the virial coefficients of square-well (SW) fluids of variable well-width and square-well based dimer forming associating fluids. In this approach, Monte Carlo sampling is performed on a number of molecules equal to the order of integral, and configurations are weighted according to the absolute value of the integrand. An umbrella-sampling average yields the value of the cluster integral in reference to a known integral. By using this technique, we determine the virial coefficients up to B6 for SW fluid with variable potential range from to and model associating fluids with different association strengths16.0 and 22.0. These calculated values for SW fluids are in good agreement with the literature. We examine these coefficients in the context of the virial equation of state (VEOS) of SW fluids. VEOS up to B4 or up to B6 describes the PVT behavior along the saturated vapor line better than the series that includes B5. We used these coefficients to find the critical properties of SW fluids and compared with the literature values. Boyle temperature is also determined and is found to increase with the increase in the well-extent and associating strength. We also report Joule Thomson inversion curve for Lennard Jones fluid and SW fluids using different truncated VEOS and compared with that predicted from established EOS.

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  • Abstract: The vapour–liquid phase coexistence and surface tension of hard-core Yukawa fluids with short attraction range, and 10.0, are reported using grand-canonical transition-matrix Monte Carlo (GC-TMMC) with the histogram reweighting method. Surface tension is calculated using finite-size scaling approach of Binder. We also compare GC-TMMC results with the available literature data for Yukawa fluids with and 4.0. Critical properties obtained from rectilinear diameter approach and least square-technique are also reported. GC-TMMC results are found to be more precise than the previous reported values. We also present the corresponding state of surface tension for extremely short-range attractive Yukawa fluids.

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  • Abstract: Configurational-bias grand-canonical transition-matrix Monte Carlo simulations are conducted to investigate various thermophysical properties, such as phase coexistence, critical properties, density and orientation profiles of liquid and vapor phases, and vapor liquid surface tension of methane, ethane, propane, n butane, and n octane in bulk and slit pores of graphite and mica surfaces. An exponential-6 model is used for the investigation of normal alkanes with a cutoff radius, 15 . It is found that the surface tension of the bulk n-alkane, based on a truncated exp-6 model, agrees reasonably well with the experimental data. Critical properties are reported by means of the rectilinear diameter approach and least-squares technique. The shift in the critical temperature under confinements follows more than two linear regimes with an inverse in the slit width, as the slit width approaches the two-dimensional limit. This is contrary to what has been previously reported in the literature. The behavior of the critical temperature shift is sensitive to the nature of the surface. The critical density, on the other hand, fluctuates with a decrease in the slit width. The shift in the critical vapor pressure continuously increases with a decrease in the slit width toward the two dimensional value and becomes constant for pore sizes less than 5 for the fluid studied in this work. Corresponding state plots suggest that the deviation of the saturation vapor pressure from the bulk saturation pressure under confinement is positive for large pores and negative for smaller pores. Vapor liquid surface tension values for n alkanes, in both types of slit pores, are computed via the finite size scaling method of Binder and compared with their bulk values. Our investigation reveals that under slit pore confinement the vapor liquid surface tension decreases many fold.

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  • Abstract: Equation of state and structure of hard-sphere fluids confined in a cylindrical hard pore were investigated at the vicinity of fluid-solid transition via molecular dynamics simulation. By constructing artificial closed-packed structures in a cylindrical pore, we explicitly capture the fluid-solid phase transition and coexistence for the pore diameters from 2.17 to 15. There exist some midpore sizes, where the phase coexistence might not exist or not clearly be observable. We found that the axial pressure including coexistence follows oscillatory behavior in different pore sizes; while the pressure tends to decrease toward the bulk value with increasing pore size, the dependence of the varying pressure on the pore size is nonmonotonic due to the substantial change of the alignment of the molecules. The freezing and melting densities corresponding to various pore sizes, which are always found to be lower than those of the bulk system, were accurately obtained with respect to the axial pressure

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  • Abstract: Vapor-liquid phase equilibria of square-well fluids of variable interaction range and 3.0 in hard slit pores are studied by means of grand-canonical transition-matrix Monte Carlo simulation. Critical density under confinement shows an oscillatory behavior as slit width, reduced from 12 to 1. Two linear regimes are found for the shift in the critical temperature with the inverse in the slit width. The first regime is seen for with linear increase in the slope of shift in the critical temperature against inverse slit width with increasing interaction range. Subsequent decrease in has little consequence on the critical temperature and it remains almost constant. Vapor-liquid surface tensions of SW fluids of variable well extent in a planar slit pore of variable slit width are also reported. GC TMMC results are compared with that from slab based canonical Monte Carlo and molecular dynamics techniques and found to be in good agreement. Although, vapor-liquid surface tension under confinement is found to be lower than the bulk surface tension, the behavior of surface tension as a function of temperature is invariant with the variable pore size. Interfacial width, calculated using a hyperbolic function increases with decreasing slit width at a given temperature, which is contrary to what is being observed recently for cylindrical pores. Inverse scaled interfacial width , however, linearly increases with increase in the scaled temperature

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  • Abstract: We used Mayer sampling technique—a method based on free energy perturbation, to calculate virial coefficients B2–B6 for hard-core Yukawa fluids with interaction range parameter, Lembda = 1.8, 3.0, 4.0, 5.0, 8.0, 9.0, and 10. We used these coefficients in the virial equation of state (VEOS) to obtain compressibility factor, Z, at super critical temperatures and found to be in excellent agreement with the predictions of equations of state based on mean spherical approximation (EOS-MSA) for Yukawa fluids up to a reduced density, of 0.5. We inspected virial coefficients in representing the PVT behavior along the saturated vapor line of Yukawa fluids. VEOS4, VEOS5 and VEOS6 describe the PVT behavior along the saturated vapor line reasonably good for Lembda = 1.8, 3.0 and 4.0; and at higher Lembda, VEOS4 and VEOS5 represent the PVT behavior better than the series that includes B6. We also report critical properties of Yukawa fluids based on VEOS and compared with the literature values.

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  • Abstract: Molecular simulation methodologies are employed to study the first-order transition of variable square-well (SW) fluids on a wide range of weak attractive surfaces. Surface phase diagram of SW fluids of attractive well diameter on a smooth, structureless surface modelled by a SW potential is reported via grand-canonical transition-matrix Monte Carlo (GC-TMMC) and histogram reweighting techniques. Fluids with show quasi-2D vapour–liquid phase transition; on the other hand, prewetting transition is visible for a SW fluid with larger well-extent. The prewetting line, its length, and closeness to the bulk saturation curve are found to depend strongly on the nature of the fluid–fluid and fluid–wall interaction potentials. Boundary tension of surface coexistence films is calculated by two methods. First, the finite size scaling approach of Binder is used to evaluate the boundary tension via GC-TMMC. Second, the results of the boundary tension are verified by virtue of its relation to the pressure tensor components, which are calculated using a NVT-Monte Carlo approach. The results from the two methods are in good agreement. Boundary tension is found to increase with the increase in the wall–fluid interaction range for the quasi-2D system; conversely, boundary tension for thin–thick film, at prewetting transition, decreases with the increase in the wall–fluid interaction range.

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2008

  • Abstract: Prewetting transition is studied for the square-well fluid of attractive-well diameter in the presence of a homogeneous surface modeled by the square-well potential of attractive well. We investigate surface phase coexistence of thin-thick film transition using grand-canonical transition matrix Monte Carlo (GC-TMMC) and histogram reweighting techniques. Molecular dynamics (MD) and are utilized to predict the properties of the fluid for various surface fluid affinities. Occurrences of prewetting transition with the variation of surface affinity are observed for a domain of reduced temperature from. We have used MD and GC-TMMC+finite size scaling (FSS) simulations to calculate the boundary tension as a function of temperature as well as surface affinity. Boundary tensions via MD and GC-TMMC+FSS methods are in good agreement. The boundary tension increases with the decrease of wall-fluid affinity. Prewetting critical properties are calculated using rectilinear diameter approach and scaling analysis. We found that critical temperature and density increase with the decrease of wall-fluid affinity

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  • Abstract: We have prepared a cross-linked polystyrene anion exchange composite membrane for the electrolysis of sodium chloride to produce sodium hydroxide by selective removal of chloride ions. The composite membrane is homogeneously modified by gas phase nitration, followed by amination using hydrazine hydrate, and further reaction with dichloroethane and triethylamine to introduce quaternary ammonium charges on it. We showed that the membrane is specific to the transport of chloride ions through its pores. The performance of the membrane has been evaluated in terms of current efficiency and power consumption, and the effect of various parameters like current density, initial salt concentration, and circulation rate is studied. The maximum current efficiency obtained is 96.5% and the corresponding power consumption is 0.1216 kWh/mol at 5.2 N initial salt concentration and current density of 254 A/m2. © 2008 American Institute of Chemical Engineers AIChE J, 2008

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  • Abstract: We determine and compare the thermodynamic properties of mono- and divacancies in the face-centered-cubic and hexagonal-close-packed hard-sphere crystals via a modified grand canonical ensemble. Widom-type particle insertion was employed to estimate the free energy of formation of mono- and divacancies, and the results are supported by an alternative approach, which quantifies the entropy gain of the neighbor particles. In hcp crystal, we found a strong anisotropy in the orientational distribution of vacancies and observe an eightfold increase in the number of divacancies in the hexagonal plane compared to the one in the out of plane at highest density of interest. This phenomenon is induced by the different arrangement and behavior of the shared nearest neighbor particles, which are located at the same distance from each vacant site in divacancy. The effect of divacancies on the free energy is to reduce that of the hcp crystal relative to the fcc by around at melting

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  • Abstract: Grand-canonical transition-matrix Monte Carlo is employed to analyse the effects of range of interaction, packing fraction and molecular association on phase coexistence properties of square-well (SW) based fluids in disordered pores. The nature of the phase equilibria were studied inside a repulsive disordered porous media with packing fractions, Three values of the SW attractive well range parameter were studied and 2.0. Coexistence number probability distribution reflects the signature of the disordered structure of the porous matrix. Yet, no multiple fluid–fluid transition was observed. The effect of strength of molecular association on coexistence densities, density profile, saturation pressure, and monomer fraction for the SW based dimerizing fluids inside a repulsive disordered media is reported. Association is found to increase as the packing fraction of the matrix increase. Critical properties of these confined fluids are calculated via a rectilinear diameter approach. Fractional shift in the critical temperature linearly decreases with the increase in the attractive well width for non-associating fluids. The rate of decrease in the critical temperature shift increases with the increase in packing fraction. Associating sites are found to suppress the shift in the critical temperature.

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  • Abstract: A modified Monte Carlo method combined with quenched molecular dynamics simulation is used to determine mixing energetics and concentration profiles at interface for systems containing mono-and bilayers of adatoms adsorbed on FCC (100) crystal surface. The systems under consideration are constructed via Lennard–Jones potential at temperatures near 0 K. For systems with monolayer of adatoms, intermixing at the interface becomes preferable with increasing magnitude of the potential well-depth ratio of adatom to substrate atom. The increasing tendency of intermixing is linearly enhanced when the adatom becomes smaller than the substrate atom, otherwise, the intermixing trend is non-linear and weaker. For systems with bilayers of adatoms, complex development of concentration profile is observed along with increasing magnitude of the potential well-depth ratio and atomic size difference between adatom and substrate atom. This behaviour is related to the interplay between contributions of asymmetric bond interaction and relaxation to minimise the total energy of the system.

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2007

  • Abstract: Phase equilibria of a square-well fluid in planar slit pores with varying slit width are investigated by applying the grand-canonical transition-matrix Monte Carlo (GC-TMMC) with the histogram-reweighting method. The wall-fluid interaction strength was varied from repulsive to attractive such that it is greater than the fluid-fluid interaction strength. The nature of the phase coexistence envelope is in agreement with that given in literature. The surface tension of the vapor-liquid interface is calculated via molecular dynamics simulations. GC-TMMC with finite size scaling is also used to calculate the surface tension. The results from molecular dynamics and GC-TMMC methods are in very good mutual agreement. The vapor-liquid surface tension, under confinement, was found to be lower than the bulk surface tension. However, with the increase of the slit width the surface tension increases. For the case of a square-well fluid in an attractive planar slit pore, the vapor-liquid surface tension exhibits a maximum with respect to wall-fluid interaction energy. We also report estimates of critical properties of confined fluids via the rectilinear diameter approach.

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  • Abstract: The interfacial properties as reflected in the interfacial tension values and the density profile of Morse fluids has been studied. The parameter range is chosen to coincide with that describing the behaviour of solid metals. The interfacial tension has been found to follow Guggenheim and MacLeod s relations. However, the constants, while independent of temperature for each metal, are not the universal values predicted with the exception of Macleods exponent p. The density profile illustrates the change in densities across the interface dividing the coexisting vapour and liquid phases. The correlation length is also found to follow the universal relation with temperature, but again the constants, while independent of temperature, are dependent on the type of metal. The value of constant v is found to be different for all five metals considered and is found to differ from the three dimensional sing model value of which is also predicted by applying the Lennard Jones model.

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  • Abstract: The Morse potential energy function (PEF) is considered regarding the characterization of interaction forces of particles with tuning parameters. Phase coexistence of Morse fluids is predicted for different steepness and range of the PEF parameters using the grand-canonical transition matrix Monte Carlo (GC-TMMC) method, with quantification of the parameter S, which is the product of a constant with a unit of reciprocal length and the equilibrium distance between two molecules. We found that a lower limit of S exists bounded by infinite critical temperature. The critical properties of the vapor-liquid equilibrium curves are estimated using a rectilinear diameter method and a scaling law approach. A Clausius-Clayperon type relation of S and critical temperature is derived in this work. Vapor-liquid surface tension of Morse fluids by finite size scaling and GC-TMMC is also reported. Surface tensions are found to be higher at lower S.

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  • Abstract: We use the Mayer sampling method, with both direct and overlap sampling, to calculate and compare classical virial coefficients up to B 6 for various water models .The precision of the computed values ranges from 0.1 percent for B 2 to an average of 25 for B 6. When expressed in a form scaled by the critical properties, the values of the coefficients for SPC water are observed to greatly exceed the magnitude of corresponding coefficients for the simple Lennard Jones model. We examine the coefficients in the context of the equation of state and the Joule Thomson coefficient. Comparisons of these properties are made both to established molecular simulation data for each respective model and to real water. For all models, the virial series up to B 5 describes the equation of state along the saturated vapor line better than the series that includes B 6. At supercritical temperatures, however, the sixth-order series often describes pressure volume temperature behavior better than the fifth-order series.

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2006

  • Abstract: Grand-canonical transition-matrix Monte Carlo is combined with configurational-bias and expanded ensemble Monte Carlo techniques to obtain saturated densities and vapor pressures of select n-alkanes. Surface tension values for butane, hexane, and octane are also computed via the finite-size scaling method of Binder. The exponential-6 model of Errington and Panagiotopoulos is used to describe the molecular interactions. The effect of the number of configurational-bias trial conformations on the efficiency of phase equilibra calculations is studied. We find that a broad range of trial conformation numbers give reasonable performance, with the optimal value increasing with decreasing temperature for a fixed chain length. Phase coexistence properties are in good agreement with literature values and are obtained with very reasonable computing resources. Similar to other recently developed n-alkane force fields, the exponential-6 model overestimates the surface tension relative to experimental values. Statistical uncertainties for coexistence properties obtained with the current approach are relatively small compared to existing methods.

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  • Abstract: Phase coexistence of Morse fluids is predicted for parameters in the range describing the behavior of metals using the grand-canonical transition matrix Monte Carlo method. The critical properties of the vapor–liquid equilibrium curves for three fcc metals, Al, Cu, and Au, and two bcc alkali metals, Na and K, are estimated and the critical temperature values are found to be in good agreement with the experimental data for the fcc metals considered but overestimated for the bcc metals. For Na, it was found that the critical density and vapor pressure as a function of temperature (below the critical temperature) estimates to be approximately concurrent with experimental results.

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2005

  • Abstract: We examine a model system to study the effect of pressure on the surface tension of a vapor liquid interface. The system is a two component mixture of spheres interacting with the square well and hard sphere potentials and with unlike interactions ranging from hard sphere to strongly attractive square well. The bulk phase and interfacial properties are measured by molecular dynamics simulation for coexisting vapor liquid phases for various mixture compositions, pressures, and temperatures. The variation of the surface tension with pressure compares well to values given by surface excess formulas derived from thermodynamic considerations. We find that surface tension increases with pressure only for the case of an inert solute and that the presence of attractions strongly promotes a decrease of surface tension with pressure. An examination of density and composition profiles is made to explain these effects in terms of surface adsorption arguments.

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2004

  • Abstract: Free-energy simulation methods are applied toward the calculation of cluster integrals that appear in diagrammatic methods of statistical mechanics. In this approach, Monte Carlo sampling is performed on a number of molecules equal to the order of the integral, and configurations are weighted according to the absolute value of the integrand. An umbrella-sampling average yields the value of the cluster integral in reference to a known integral. Virial coefficients, up to the sixth for the Lennard-Jones model and the fifth for the SPCE model of water, are calculated as a demonstration.

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  • Abstract: Grand-canonical transition-matrix Monte Carlo simulation is applied to analyze the effect of molecular association on the vapor–liquid coexistence and interfacial behavior of square-well based dimerizing fluids. Finite-size scaling techniques are implemented in conjunction with histogram reweighting to determine the infinite-system surface tension from a series of finite-size simulations. The effect of strength of association and size of association site on coexistence densities, pressure, surface tension, and monomer fraction is presented. Some qualitative features of the dependence of monomer fraction and surface tension on association strength are found to disagree with behavior expected from previous studies using the statistical associating fluid theory (SAFT). Comparison with experimental data shows that molecular models must incorporate an explicit association interaction in order to describe the surface-tension behavior of a real dimerizing fluid (acetic acid).

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  • Abstract: Vapor-liquid interfacial properties of square-well associating fluids are studied via transition-matrix Monte Carlo simulation. Results for one-site and two-site association models are presented. Coexistence properties, surface tension, cluster distribution, density profile, and orientation profile are presented. Molecular association affects the interfacial properties and cluster fractions more than it affects the bulk densities. We observe that the surface tension exhibits a maximum with respect to association strength. This behavior is in agreement with the recent study of Peery and Evans [J. Chem. Phys. 114, 2387 (2001)] for one site system using a square-gradient approach

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2003

  • Abstract: We consider the accuracy of several methods for combining forward and reverse free-energy perturbation averages for two systems. The practice of direct averaging of these measurements is argued as not reliable. Instead, methods are considered of the form, where A is the free energy, is the reciprocal temperature, is the difference in configurational energy, is a weighting function, and the angle brackets indicate an ensemble average performed on the system indicated by the subscript. Choices are considered in which ; the latter being Bennett’s method where C is a parameter that can be selected arbitrarily, and may be used to optimize the precision of the calculation. We examine the methods in several applications: calculation of the pressure of a square-well fluid by perturbing the volume, the chemical potential of a high-density Lennard-Jones system, and the chemical potential of a model for water. We find that the approaches based on Bennett’s method weighting are very effective at ensuring an accurate result one in which the systematic error arising from inadequate sampling is less than the estimated confidence limits, and that even the selection offers marked improvement over comparable methods. We suggest that Bennett’s method is underappreciated, and the benefits it offers for improved precision and especially accuracy are substantial, and therefore it should be more widely used.

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  • Abstract: Vapor–liquid interfacial tension of square-well (SW) fluids is calculated using three different methods viz., molecular dynamics (MD) with collision-based virial evaluation, Monte Carlo with virial computed by volume perturbation, and Binder’s density-distribution method in conjunction with grand-canonical transition-matrix Monte Carlo (GC-TMMC). Three values of the SW attractive well range parameter were studied: Lembda=1.5, 1.75, and 2.0, respectively. The results from MD and GC-TMMC methods are in very good mutual agreement, while the volume-perturbation method yields data of unacceptable quality. The results are compared with predictions from the statistical associating fluid theory (SAFT), and SAFT is shown to give a good estimate for the systems studied. Liquid and vapor coexistence densities and saturation pressure are determined from analysis of GC-TMMC data and the results are found to agree very well with the established literature data

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2000

  • Abstract: Dynamics, stability, morphology, and dewetting of a thin film under the influence of a long-range van der Waals attraction combined with a short-range repulsion are studied based on numerical solutions of the nonlinear two-dimensional (2-D) thin film equation. Area and connectivity measures are used to analyze the morphology and the distinct pathways of evolution of the surface instability. The initial disturbance resolves into an undulating structure of uneven 'hills and valleys'. Thereafter, the morphology depends on the mean film thickness relative to the minimum of the force curve. Relatively thin films to the left of the minimum transform directly into an array of droplets via the fragmentation of ridges. At long times, the droplets merge due to ripening. In contrast, relatively thick films are dewetted by the formation and growth of isolated, circular holes. Coalescence of holes eventually leads to the formation of ridges and drops. Films of intermediate thickness display a rich combination of different morphologies. Thus, the morphology and the sequence of evolution depend crucially on the form of the potential and the film thickness relative to the location of the minimum in the force vs. thickness curve. Different types of patterns can, therefore, even co-exist on a heterogeneous surface.

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