The origins of hydration forces: Monte Carlo simulations and density functional theory

The force between two infinite planar surfaces, interacting with the intervening solvent via a short-range exponentially decaying attraction and a soft repulsion, has been calculated using Monte Carlo simulations and density functional theory. In the former ease, the intervening liquid is chosen to mimic water, whereas the density functional calculations has been applied to a nonpolar liquid. It was found that the most important feature of the surface-solvent potential, in terms of how it governs the solvation farce, was its range. When the decay length was larger than about half a molecular diameter, a strong repulsion was found at short separations (<4 molecular diameters). The effects of using an orienting surface-solvent potential was also examined in the simulations and found to be of secondary importance. Some important conclusions may be drawn from this study. The first is that there is repulsive solvation force contribution due to the solvent, the range and amplitude of which strongly depends on the nature of the surface-solvent interaction. Second, repulsive solvation forces would also be found in nonpolar solvents, were it possible to construct sufficiently solvophilic surfaces.