Entropy of hydrophobic hydration: Extension to hydrophobic chains

A statistical mechanical formulation for the entropy in terms of multiparticle correlation functions, used previously to calculate entropies of the hydration of simple hydrophobic solutes, has been generalized to molecular solutes of arbitrary shape by recasting the correlation function expansion as a summation over sites that define the solute molecule. The new formulation for the entropy is applied to a Monte Carlo simulation study of normal alkanes at infinite dilution in water to calculate contributions to the entropy of hydration from water-solute site pair correlations and to examine the role these contributions play in stabilizing different solute conformations. In this implemention, the water-solute site pair correlations are determined only for individual water molecules with their nearest solute site and are defined by water orientational and water oxygen radial distributions around the site, independent of solute orientation relative to the water molecule. We show that these distribution functions give an accurate representation of water structure around the individual n-alkane sites for methane through normal butane, account for the large negative entropies of hydration of these alkanes at 25 degrees C, and predict the stabilization of gauche-butane relative to trans-butane in water on the basis of an entropically favorable (energetically unfavorable) trans --> gauche transition. Contributions to the entropy of hydration arising from solute-induced perturbations in water-water correlations (i.e., water structure enhancement) have also been examined, and we show energy-entropy compensation of these contributions within the framework of the correlation function expansion for the entropy.