Hydrophobic Effect and Hydrophobic Interaction

Abstract

Fundamental problems on the hydrophobic effect and the hydrophobic interaction are reviewed. It is remarked that the solvation thermodynamics characteristic of hydrophobic hydration is actually not so uncommon when the constant-volume solvation process is considered while it is indeed atypical in the constant-pressure process, and what makes water special as a solvent of hydrophobic species is the smallness of its thermal pressure coefficient. What we know so far about the hydrophobic interaction are also described. Recent theoretical studies demonstrate that the osmotic second virial coefficient of methane in water is positive at low temperatures, decreases monotonically with temperature, and is large and negative at high temperatures, thus urging caution in assuming the hydrophobic interaction to be always attractive. A mean-field approximation for the solvation free energy in liquid mixtures is reviewed, which gives simple physical pictures on microscopic mechanisms of the temperature, pressure, and salt concentration dependences of the solvation free energy and the hydrophobic interaction.