Dilip Asthagiri, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, Hank Ashbaugh, Department of Chemical and Biomolecular Engineering, Tulane University, 6823 St. Charles Avenue, New Orleans, LA 70118, Michael E. Paulaitis, Ohio State University, 125 Koffolt Laboratories, 140 West 19'th Avenue, Columbus, OH 43210, and Lawrence R. Pratt, T-12, Los Alamos National Lab, Los Alamos, NM 87545.
A quasi-chemical theory implemented on the basis of molecular simulation is derived and tested for hydrophobic hydration of CF4(aq). The theory formulated here subsumes a van der Waals treatment of hydration, and identifies contributions to the hydration free energy of CF4(aq) that naturally arise from chemical contributions defined by quasi-chemical theory and fluctuation contributions defined in the context of random phase approximation. The resulting gaussian statistical thermodynamic model avoids consideration of drying-then-rewetting problems, and is physically reliable in these applications as judged by the size of the fluctuation contribution. The specific results here confirm that unfavorable-binding-energy tails reflect few-body close solute-solvent encounters. The solvent near-neighbors are pushed by the medium into unfavorable interactions with the solute, in contrast to the alternative view that a pre-formed interface is pulled by the solute-solvent attractive interactions. The poly-atomic model of CF4(aq) studied gives a satisfactory description of the experimental solubilities including the temperature dependence. The proximal distributions evaluated here for poly-atomic solutes accurately reconstruct the observed distributions of water near these (non-spherical) molecules. Extensions of the above non-van der Waals approach to interactions between two solutes will be noted.