Scaling of hydrophobic solvation free energies

We have calculated the free energy of solvation for hard sphere solutes, as large as 20 A in diameter, in two simple-point-charge models of water. The se results were obtained using umbrella sampling of ensembles with fixed, a mbient temperature and pressure. For the same water models, we have also ca lculated the surface tension of a liquid-vapor interface at room temperatur e. Analogous calculations were carried out for three thermodynamic states o f the Lennard-Jones (LJ) fluid near liquid-vapor coexistence. For both wate r and the LJ fluid at the conditions we have simulated. extrapolation of ou r results suggests that the planar interface between coexisting liquid and vapor phases has the same surface tension as the planar limit of hard spher e solvation. We expect this correspondence to be a general result for fluid s at thermodynamic states close to phase coexistence, as measured by the di fference in chemical potential between bulk liquid and vapor phases, and fa r from the critical point. The solvation free energies we have computed for water and the LJ fluid cross over at microscopic solute sizes from a depen dence on solute volume to an approximate dependence on solute surface area, as predicted by Lum et al.