MOLECULAR-DYNAMICS STUDY OF THE DEPENDENCE OF WATER SOLVATION FREE-ENERGY ON SOLUTE CURVATURE AND SURFACE-AREA

We have investigated changes in water properties when deforming an ini tially spherical cavity into an oblate ellipsoid of equal volume in li quid water. The purely hydrophobic cavity has an initial thermal radiu s of 6.45 Angstrom and is flattened out to an oblate ellipse with a th ickness corresponding to one layer of methane molecules. The water-sol ute interactions are modeled by a repulsive, single-site Gay-Berne pot ential that preserves the volume of the solute; water-water interactio ns are modeled using a pairwise additive potential. The Gibbs free ene rgy change, Delta G, of the aqueous solution was calculated using ther modynamic perturbation theory. Comparison with the process of radially expanding a sphere shows that the free energy change cannot consisten tly be interpreted as being solely proportional to an exposed solute a rea but contains terms involving the curvature of the solute. As the m eanings of exposed surface area and molecular curvature are not well-d efined concepts on the microscopic length scale, these terms have to b e defined to yield a consistent interpretation of the free energy data . The microscopic origin of the curvature dependence of the free energ y is traced back to changes in water-water interactions in the region immediately surrounding the solute. The process of deforming the liqui d around the hydrophobic pocket was found to be dominated by entropic contributions. The free energy values do not contain any contributions arising from the deformation of the hydrophobic solute itself nor any attractive solute-solvent term and thus cannot be compared directly w ith hydrocarbon-water surface tension data.