Theory of hydration forces between surfaces

A statistical thermodynamic theory is presented for the surface forces arising from the interference of hydration layers at planar surfaces. The theory is a generalization of a previously presented lattice fluid theory of water. In this theory the orientation dependent interactions between water molecules are taken into account. Several model surfaces that have different effects upon adjoining water, are examined. If the predominant effect of the surface is to influence the local density of the adjoining liquid but not the local orientation distribution of the water molecules, then the ensuing force between two such surfaces is attractive. This is encountered in surfaces with both a low and a high affinity for water. However, if the main effect of a surface is to influence the orientational distribution in adjoining water layers, the interaction between two such surfaces is repulsive. This repulsive force is associated with the disruption of hydrogen bonding in the hydration layer. For not too small distances between the surfaces, the interactions decay exponentially with distance. Both mechanisms operate simultaneously if the surfaces influence both the local density and the orientational distribution. Then the ensuing interaction can be a nonmonotonous function of surface separation. These results are consistent with experiments and with a generalization of Marcelja and Radic's phenomenological approach. Moreover, the phenomenological order parameters can now be identified in molecular terms. The order parameter associated with repulsion between identical surfaces is a proton-donor/proton-acceptor mismatch. The one associated with attraction is the local excess density.