THE ENTROPY OF HYDRATION OF SIMPLE HYDROPHOBIC SOLUTES

Infinite-dilution partial molar entropies of solvation of simple, mona tomic solutes in water are defined in terms of the entropy associated with (1) solute insertion at constant volume and at a fixed position i n the solvent, and (2) expansion or contraction of the pure solvent to maintain constant pressure. A statistical mechanical expansion for th e entropy of solution in terms of multiparticle correlation functions is applied to this definition to identify three intrinsic contribution s to the hydration entropy - solute-solvent pair correlations, rearran gement of solvent in the vicinity of the solute molecule, and expansio n or contraction of the pure solvent - which we evaluate for the inert gases in water at 25 degrees 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 e xclusively by solute-water pair correlations. The solute-water pair co rrelation entropy also prevails as the primary factor determining entr opies of hydration for the larger solutes; however, solvent reorganiza tion now makes a small, negative contribution to the entropy.