Theoretical Studies of Surface Forces and Phase Equilibria

The interaction between large hydrophobic and hydrophilic surfaces in aqueous solution is investigated, using Metropolis Monte Carlo simulations and density functional theory. Special attention is devoted to the solvation force and its dependence on the nature of the solvent-solvent and solvent-surface interactions. In order to facilitate simulations of surface forces in aqueous systems, methods are developed to ensure that equilibrium with a bulk solution of the pure solvent is maintained at all surface-surface separations. The results suggest that the range and strength of the surface forces in simple solvents are mainly governed by the cohesivity of the solvent. In this respect, water is not unique and interactions similar to the attractive "hydrophobic force" and the repulsive "hydration force" are likely to exist in many other solvents. The high cohesivity in water makes these interactions stronger in aqueous solution than in most other systems. However, even though anisotropic properties of the solvent may have a large influence on the net repulsive force between solvophilic surfaces, hydrogen bonding is not a criterium for the existence of such a repulsion. Phase equilibria in heterogeneous systems are also studied, using simulations and density functional theory. Coexistence in confined geometries between a gas and a liquid phase, as well as between various "layered phases" is established. In the latter case, comparisons are made with the behaviour at an isolated surface. Phase diagrams are constructed and methods developed, to avoid phase transitions during the simulations