STATISTICAL MECHANICAL STUDIES OF MOLECULAR SOLVATION (HYDROPHOBIC HYDRATION, LIQUID THEORY, MOLECULAR DYNAMICS)

Statistical Mechanical investigations of liquids are undertaken using molecular dynamics (MD) simulation and a more analytic, integral equation theory. A detailed geometrical and dynamical analysis of the solvent in an aqueous solution containing two nonpolar atomic solutes is presented first. The results, obtained from a MD simulation, clearly illustrate the "clathrate-like" structure promotion induced by apolar solutes. Of particular interest, the relatively lengthy simulation displays a rich variety of cage-like solvation shell structures encompassing the solute particles. The dynamic structure of the solvent reveals that all measures of molecular mobility are decidedly retarded for the solvation shell molecules. These retarded mobilities, along with the enhanced intermolecular geometric structure, correlate with significantly longer intermolecular hydrogen bond lifetimes for shell molecules. An analysis of the librational frequency distributions reveals a correlation between librational oscillations and the degree of intermolecular association, high frequency oscillations correlating with highly associated molecules. These librational frequency distributions are employed in an investigation of the solvent isotope effect on the thermodynamics of hydrophobic hydration. Results indicate that intermolecular vibrational contributions are significant, however, additional structural features are also important.; A more analytic theory is employed in the investigation of internal rotation of molecular solutes in liquids. The theory used is the so-called extended RISM theory. Of primary focus is a determination of the solvent mediated free energy for internal rotation of both polar and nonpolar solutes in highly associated and nonassociated fluids. In particular, we compute rotational free energies for n-butane and 1,2-dichloroethane in both CCl(,4) and water model fluids. Also, the liquid state distributions of conformers for the neat liquids n-butane and 1,2-dichloroethane are determined. The theoretical results are compared to simulation where possible. Although a qualitative assessment of solvent effects is faithfully reproduced, extended RISM theory consistently overestimated the magnitude these effects. Such a consistent overestimation, however, leads to very accurate predictions of relative differences among the solutions studied.