The hydrophobic effect and the influence of solute-solvent attractions

We have studied the effect of weak solute-solvent attractions on the solvation of nonpolar molecules in water at ambient conditions using an extension and improved parameterization of the theory of solvation due to Lum, Chandler, and Weeks [J. Phys. Chem. B 1999, 103, 4570]. With a reasonable strength of alkane-water interactions, an accurate prediction of the alkane-water interfacial tension is obtained. As previously established for solutes with no attractive interactions with water, the free energy of solvation scales with volume for small solutes and with surface area for large solutes. The crossover to the latter regime occurs on a molecular length scale. It is associated with the formation of a liquid-vaporlike interface, a drying interface, between the large hydrophobic solute and liquid water. In the absence of attractions, this interface typically lies more than one solvent molecular diameter away from the hard sphere surface. With the addition of attractive interactions between water and the hard sphere, the average separation of the interface and solute surface is decreased. For attractive force strengths typical of alkane-water interactions, we show that the drying interface adjacent to a large hydrophobic solute remains largely intact, but is moved into contact with the solute surface. This effect results from the "soft modes" characterizing fluctuations of liquid-vapor interfaces. We show that attractive interactions are of almost no consequence to the temperature dependence of the solvation free energies relevant to protein folding.