Solvation and structure for systems with strong Coulomb interactions

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John D. Weeks of the University of Maryland, College Park, is supported by an award from the Theory, Models and Computational Methods program in the Chemistry Division to carry out theoretical research on solvation in dielectric fluids like water and in charged ionic fluids as well. The principal investigator and coworkers have developed a new local molecular field (LMF) theory that relates the structure of a nonuniform system with long-ranged Coulomb interactions in an arbitrary external field to that of a simpler "mimic system" with short ranged interactions but in an effective or renormalized field that accounts for the averaged effects of the long ranged interactions. The external field can represent the interaction of the solvent with a fixed, arbitrarily complicated, and possibly charged solute, and the solvation free energy is determined as the field is gradually turned on. The solvation process in the short ranged mimic system avoids computationally expensive methods used in conventional treatments of Coulomb interactions and a very simple and analytic expression for the difference in solvation free energy between the full and mimic systems has been derived. LMF theory gives exceptionally accurate results for solvation of simple spherical models of hydrophobic and hydrophilic solutes of varying sizes in water, the most important solvent. Ongoing research will focus on theoretical generalizations of these ideas, along with applications to more complex solvation problems of interest in biophysics and materials science.An accurate determination of free energy changes during solvation is required for a quantitative understanding of a vast array of chemical and biophysical processes in solution, ranging from protein folding to drug partitioning across cell membranes. The new solvation theory developed by the investigator and coworkers is simple, physically motivated, and very accurate, so the results of this research should be accessible to experimentalists and theorists with diverse backgrounds and may provide a common language for work in overlapping areas of chemistry, physics, and biology. The research in this proposal contributes directly to the interdisciplinary training of graduate students and postdocs, who gain experience in a unique combination of analytical thinking, numerical solutions of newly derived equations, and computer simulations using both in house programs and state of the art simulation packages.