Solvent Reorganization Energy and Entropy in Hydrophobic Hydration

Abstract

Monte Carlo simulations of methane in water at 25, 65, and 160 °C are used to obtain the water binding energy as a function of distance from the solute, average water pair interaction energies in the first and second solvation shell, the triplet solute−water oxygen−water oxygen correlation function, the water orientational distribution with respect to the solute as a function of distance from the solute, and water−water orientational correlation functions in the first and second solvation shells. The calculated correlation functions are used in conjunction with the inhomogeneous forms of the energy equation and the correlation expansion for the entropy [Lazaridis, J. Phys. Chem. 1998, 102, 3531] to calculate the solvent reorganization energy and entropy. It is shown that these quantities contain two large contributions of opposite sign: a positive contribution from the exclusion of solvent molecules by the solute and a negative contribution from the orientational arrangement of water next to the solute. The large magnitude of the heat capacity of hydration is due to flattening of the water orientational distribution with respect to the solute as the temperature increases, which leads to diminution of the orientational arrangement contribution and concomitant increase in both the enthalpy and the entropy of solvation.