The Molecular Origin of the Large Entropies of Hydrophobic Hydration

Abstract

A rigorous statistical mechanical expansion for the entropy in terms of multiparticle correlation functions is used to identify the major contributions to the entropy of hydration of simple, hydrophobic solutes. With a factorization assumption for the solute-water correlation function we have been able to separate the contributions to the entropy of hydration due to translational and orientational solute-water correlations. The required correlation functions are obtained by Monte Carlo simulation. This approach is applied to infinitely dilute solutions of methane, inert gases, and model Lennard-Jones particles in water in order to examine the dependence of the solute size and the solute-water pair interaction energy on the hydration entropy. Solute-water orientational correlations are found to decay linearly with intermolecular distance and virtually vanish in the second hydration shell. The orientational and translational contributions are comparable in magnitude. The primary factor determining the solute-water correlation entropy is found to be solute size, with the pair interaction energy playing a secondary, yet significant role. The dependence of the orientational entropy on the curvature of the solute surface is discussed in connection with enthalpy-entropy compensation phenomena. We find that the large entropies and heat capacities of hydrophobic hydration are well accounted for by solute-water correlations alone and that large perturbations in water structure are not required to explain hydrophobic behavior at room temperature.KeywordsChem PhysHydration ShellHydrophobic HydrationOrientational DistributionSolute SizeThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.