Repulsive hydration forces and attractive hydrophobic forces in a unified picture

The solvation force between two planar surfaces immersed in a model liquid mimicking water is obtained using Monte Carlo computer simulations and free energy functional theory calculations. When the surfaces are hydrophilic, the solvation force is repulsive, as is the net force, provided the solvent-surface interaction is strong, compared with the surface-surface and solvent-solvent interactions. The range of the surface-solvent interaction is shown to have a profound influence on the resulting solvation force, whereas solvent orientational order is only of minor importance. We conclude that a repulsive force would also be found in a less polar solvent, confined between sufficiently solvophilic surfaces. With hydrophobic surfaces, the solvation force is attractive. The net lowering of solvent density between the surfaces increases the attraction beyond the expected van der Waals interaction, This density depression contribution is substantial even at rather large separations and will be present for all fluids confined between solvophobic surfaces. Our calculations suggest that the famous "hydrophobic force" is often dominated by density depression, (C) 1997 Academic Press.