The entropy of hydration of simple hydrophobic solutes

Infinite-dilution partial molar entropies of solvation of simple, monatomic solutes in water are defined in terms of the entropy associated with (1) solute insertion at constant volume and at a fixed position in the solvent, and (2) expansion or contraction of the pure solvent to maintain constant pressure. A statistical mechanical expansion for the entropy of solution in terms of multiparticle correlation functions is applied to this definition to identify three intrinsic contributions to the hydration entropy - solute-solvent pair correlations, rearrangement of solvent in the vicinity of the solute molecule, and expansion or contraction of the pure solvent - which we evaluate for the inert gases in water at 25°C. For the smaller solutes, we find that the solvent reorganization and solvent expansion contributions offset one another such that the entropy of hydration is determined almost exclusively by solute-water pair correlations. The solute-water pair correlation entropy also prevails as the primary factor determining entropies of hydration for the larger solutes; however, solvent reorganization now makes a small, negative contribution to the entropy.