Molecular simulation study of fundamental effects of molecular association on properties of fluid interfaces

Fluids with molecular association such as hydrogen bonding often exhibit a variety of anomalous properties. An extreme example is hydrogen fluoride (HF), in which properties are dominated by the formation of clusters in the fluid and vapor phases. Interfacial properties can be strongly impacted by association phenomena. We examine the influence of molecular association on liquid-vapor interfacial properties and surface tension in particular.; We examine simple molecular models that are selected to exhibit the basic features of molecular association while being easily characterized. The effect of strength of association and size of association site on coexistence densities, pressure, surface tension, and monomer fraction is presented. Some qualitative features of the dependence of monomer fraction and surface tension on association strength are found to disagree with behavior expected from previous studies using the Statistical Associating Fluid Theory.; Interfacial properties are more sensitive to the changes in the size of the site and strength of association than the bulk properties. We observe that the surface tension exhibits a maximum with respect to association strength. Comparison with experimental data shows that molecular models must incorporate an explicit association interaction in order to describe the surface-tension behavior of a real associating fluid.; We also present a study of pressure effect on the surface tension of a vapor-liquid interface. The system is a two-component mixture of spheres interacting with the square-well (A-A) and hard-sphere (B-B) potentials, and with unlike (A-B) interactions ranging from hard sphere to strongly attractive square well. The variation of the surface tension with pressure compares well to values given by surface-excess formulas derived from thermodynamic considerations. We find that surface tension increases with pressure only for the case of an inert solute (hard-sphere A-B interactions), and that the presence of A-B attractions strongly promotes a decrease of surface tension with pressure.