Grand canonical Monte Carlo simulation of hydration forces between nonorienting and orienting structureless walls

The hydration forces between structureless model walls were calculated using the grand canonical Monte Carlo technique. Several wall-water interaction potentials were tried, including both orientation independent and strongly directional potentials which reflected the preference of water for tetrahedral hydrogen bonding coordination. Primary attention was given to large wall-to-wall separations (4 nm and more), where the oscillations of the hydration force due to layering effects decayed. The hydration force was found to be highly sensitive to the presence of orientation-dependent terms in the wall-water interaction potential. Strongly directional potentials led to hydrophobic attraction of the walls even when the wall-water interaction was substantially stronger than the water-water interaction. The reason had to do with the orientational ordering induced by the walls in the adjacent water layers. (C) 2002 American Institute of Physics.