Hydrophobic Effect, Water Structure, and Heat Capacity Changes

The hydration heat capacity (Δ ) of nine solutes of varying hydrophobicity was studied using a combination of a random network model equation of water and Monte Carlo simulations. Nonpolar solutes cause a concerted decrease in the average length and angle of the water−water hydrogen bonds in the first hydration shell, while polar and ionic solutes have the opposite effect. This is due to changes in the amounts, relative to bulk water, of two populations of hydrogen bonds: one with shorter and more linear bonds and the other with longer and more bent bonds. Heat capacity changes were calculated from these changes in water structure using a random network model equation of state. The calculated changes account for observed differences in Δ for the various solutes. The simulations provide a unified picture of hydrophobic and polar hydration, and both a structural and thermodynamic explanation of the opposite signs of Δ observed for polar and nonpolar solutes.