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The long term goal of this project is develop a microscopic mechanism of interactions which drive topological transitions in various microemulsion phases. As a first step in that direction, we used computationally extensive all-atom molecular dynamics (MD) simulations, with Replica Exchange (RE), to investigate a non-ionic ternary mixture of water/octane/C9E3 [nonyltri(ethylene oxide)] with an oil thickness of the nanoscale size and compared our results with water transport models derived from experiments. We observed the dynamics of the ternary system within a wide range of temperatures ( 7 - 88 oC) and showed, that surfactants efficiently solvate water molecules and carry out passive water transport through the oil slab. We also found that a “polar solvation cage” created by surfactant molecules is highly dynamic, thus, there is no specificity in hydrogen bond formation with respect to any particular surfactant oxygens. However, majority of water oxygens form a hydrogen bond with surfactant head terminal hydrogen.

One of the fascinating observations is that the major amount of water solubilized by surfactants is situated on a border between surfactant and oil layers and is not homogeneously distributed in the “surfactant-oil” slab. We also detected that with temperature increasing the larger aggregates are free to travel through the oil layer, which raises overall water presence in the oil slab. Despite the fact that “water-surfactant” aggregates vary in topology and sizes, our cluster analysis indicated that the majority of these aggregates do not show dense water core formation, thus, suggesting the “hydrated surfactants” transport mechanism. We will soon submit a manuscript with the above-described results to Langmuir.

In related works, we developed a novel, computationally efficient technique of developing coarse-grained (CG) force fields from atomistic simulations. Since microemulsion domains can be quite large in size, coarse-grained simulations are needed to simulate long-term processes occurring in volumes over tens on nanometers in linear dimensions. However, very few approach accurate approaches exist for deriving realistic coarse-grained potentials. To achieve this goal, we used the ideas of renormalization group theory to build an optimization scheme for the CG force field. This novel approach is designed to accurately reproduce correlations among various CG molecular degrees of freedom. We applied our Molecular Renormalization Group Coarse-Graining (MRG-CG) technique to coarse grain polymers and mobile ions. While in many simplified models both water and salt are treated as continuous media, it is often desirable to describe mobile ions in an explicit manner. In a recent work, we have derived an effective interaction potential for monovalent ions by systematically CG the all-atom NaCl and KCl aqueous solutions at several different ionic concentrations. Our approach is based on explicitly accounting for cross-correlations among various observables that constitute the compact basis set of the CG Hamiltonian. Compactness of the Hamiltonian allows us to accurately reproduce many-body effects, in contrast with many existing algorithms. The resulting Hamiltonian produced ionic distributions that are virtually identical to those obtained in atomistic simulations with explicit water, capturing short-range hydration effects. Our coarse-grained model of monovalent electrolyte solutions allows the incorporation of ions into complex coarse-grained simulations, for example to describe microemulsions with ionic surfactants.

In terms of human resource development, the PRF was used to support the work of a graduate student in my group. After planned submission of our manuscript to Langmuir, she will work on developing coarse-grained models for various surfactants using the MRG-CG technique.