Understanding the self-assembly of amphiphilic molecules in supercooled solvents using molecular simulations

The self-assembly in an aqueous medium is a major scientific area for many practical and biological applications, in which freezing induced self-assembly (FISA) is considered to be a more novel method as compared to solvent evaporation induced self-assembly. FISA is a versatile and green bottom-up method for producing highly ordered and aligned porous materials. The molecular details of these processes are largely unknown because of the very small length scale involved (nanometres). We have attempted to use seeding methodology to capture the nucleation behaviour of water in the presence of amphiphilic molecules. The understanding has been previously utilized for stationary fluids and sheared flows. However, the underneath algorithm is quite valuable for the current project as well. Crystal nucleation is a rare event that can occur on time scales of seconds, far beyond the reach of the brute-force molecular dynamics framework. Seeding overcomes this barrier by the insertion of ice seed in a pre-equilibrated solution. If the ice seed is larger than the critical nucleus, it will grow with time evolution, leading to the formation of different ice crystals and self-assembly of amphiphilic molecules into 3D or 2D structures. Our work provides preliminary molecular insights into the factors affecting the nucleating behaviour of water in the presence of amphiphilic molecules, which play a pivotal role in designing artificial ice recrystallization inhibition (IRI) agents, for use in cryobiology and drug delivery.

  • Abstract: We investigate the concentration-dependent phase diagram of pluronic L64 in aqueous media at 300 and 320 K using coarse-grained (CG) molecular dynamics (MD) simulations. The CG model is derived by adapting the Martini model for nonbonded interactions along with the Boltzmann inversion (BI) of bonded interactions from all-atom (AA) simulations. Our derived CG model successfully captures the experimentally observed micellar-, hexagonal-, lamellar-, and polymer-rich solution phase. The end-to-end distance reveals the conformational change from an open-chain structure in the micellar phase to a folded-chain structure in the lamellar phase, increasing the orientational order. An increase in temperature leads to expulsion of water molecules from the L64 moiety, suggesting an increase in L64 hydrophobicity. Thermodynamic analysis using the two-phase thermodynamics (2PT) method suggests the entropy of the system decreases with increasing L64 concentration and the decrease in free energy (F) with temperature is mainly driven by the entropic factor (TS). Further, the increase in aggregation number at lower concentrations and self-assembly at very high concentrations is energetically driven, whereas the change from the micellar phase to the lamellar phase with increasing L64 concentration is entropically driven. Our model provides molecular insights into L64 phases which can be further explored to design functionality-based suprastructures for drug delivery and tissue engineering applications.

    Read More ...