AmericanChemicalSociety.com

The goal of work sponsored by the "G" grant from the Petroleum Research Fundwas to use computational techniques to study microscopic processes governingtopological structural transitions in microemulsion systems. We made two-foldprogress with respect to this goal: 1) We carried out one of thecomputationally most demanding simulations of microemulsions reported up todate, both in and out of equilibrium. In the latter case, we investigate indetail transport processes in such systems. 2) We have developed a generalcoarse-graining technique for complex molecules and polymers, providinggroundwork for future microemulsion studies probing large length-scales andlong time-scales. The report below summarizes our achievements.

1. Surfactants Transport Water Molecules Across an Oil Layer by Enveloping Them in a Polar Solvation Cage
Microemulsions have been of great fundamental and industrial interest for manydecades. In addition to the rich morphological behavior, microemulsions have avaluable property to store and transport small and macro- molecules, allowinga wide range of applications, including drug storage and release, oilrecovery, textile cleaning, preparation of various cosmetic products andperfumes, and food industry. Water transport as a particular example of masstransport has great importance in membrane and microemulsion science,especially for biological systems. On the mesoscopic scale, it was suggestedthat water transport could occur through the formation of reverse micelles,spontaneous emulsification, hydrated surfactants, and diffusion of singlemolecules. All these mechanisms have been observed under different conditions,but no unified picture has been created so far. Consequently, most of theavailable knowledge on molecular transport in microemulsions was obtainedmainly from the macroscopic scale measurements, which do not provide directatomistic insight.

We used microsecond timescale atomistic simulations to study the relaxationdynamics of the microemulsion water/octane/$C_9E_3$ system. In order todetermine what transport mechanism occurs under the conditions of surfactantexcess we studied the system under wide range of temperatures 7 - 88 C andshowed that surfactant acts as an effective solvent for water and carries outpassive water transport through oil. Interestingly, most of surfactantsolubilized water is situated between surfactant and oil layers and is nothomogeneously distributed in the surfactant-oil slab. With raising thetemperature, the larger aggregates are allowed to travel through the oil layerincreasing overall water presence in oil. Also, our cluster analysis indicatesthat most complexes do not form dense water core, supporting the ``hydratedsurfactants'' transport mechanism.

One of the fascinating observations in this work is that the major amount ofwater solubilized by surfactant is situated on a border between surfactant andoil layers and is not homogeneously distributed in the surfactant-oil slab. Wealso detected that with increasing the temperature the larger aggregatesdetach from the interface and travel through the oil layer, raising overallwater presence in oil. Despite the fact that water-surfactant complexes varyin topology and sizes, our cluster analysis showed that the majority of thesecomplexes do not show dense water core formation, thus, providing evidence forthe ``hydrated surfactants'' transport mechanism.
We have written a final draft of a manuscript, describing this work, whichwill be shortly submitted to the Journal of Physical Chemistry B.

2a. Molecular Renormalization Group Coarse Graining of DNA.

Our long-termgoal is to build an accurate coarse-grained (CG) model of the chromatin,derived systematically from all-atom simulations of smaller fragments. As afirst step toward achieving this goal, we developed CG model of a linear DNAchain, playing the role of a linker DNA segment in chromatin (Biophys. J,2009). A unique aspect of our work was to accurately derive the CG inter-DNAelectrostatic potential from atomistic simulations, instead of relying on thestandard models of continuum electrostatics which are inadequate at smallseparations. To achieve this goal, we used the ideas of renormalization grouptheory to build an optimization scheme for the CG force field. This novelapproach is designed to accurately reproduce correlations among various CGmolecular degrees of freedom, which, in turn, allowed to correctly describethe dependence of DNA's flexibility on the solution ionic strength (Proc NatlAcad Sci, 2010).

2b. Molecular Renormalization Group Coarse-Graining of Electrolyte Solutions.

Similarly, the aqueous salt buffer provides an important contribution to thestructure and function of biological molecules. While in many simplifiedmodels both water and salt are treated as continuous media, it is oftendesirable to describe mobile ions in an explicit manner. In a recent work, wehave derived an effective interaction potential for monovalent ions bysystematically coarse-graining the all-atom NaCl and KCl aqueous solutions atseveral different ionic concentrations (J. Chem. Phys. B, 2009). Our approachis based on explicitly accounting for cross-correlations among variousobservables that constitute the compact basis set of the coarse- grainedHamiltonian. Compactness of the Hamiltonian allows us to accurately reproducemany-body effects, in contrast with many existing algorithms. The resultingHamiltonian produced ionic distributions that are virtually identical to those obtained in atomistic simulations with explicit water, capturing short-rangehydration effects. Our coarse-grained model of monovalent electrolytesolutions allows the incorporation of ions into complex coarse-grainedbiomolecular simulations, where both electrostatic and short-range hydrationeffects must be taken into account.