Molecular Interactions at Vapor-Liquid Interfaces Binary Mixtures of Simple Fluids

Properties of vapor-liquid equilibria and planar interfaces of binary Lennard-Jones truncated and shifted mixtures were investigated with molecular dynamics simulations, density gradient theory, and conformal solution theory at constant liquid phase composition and temperature. The results elucidate the influence of the liquid phase interactions on the interfacial properties (surface tension, surface excess, interfacial thickness, and enrichment). The studied mixtures differ in the ratios of the dispersion energies of the two components ɛ2/ɛ1 and the binary interaction parameter ξ. By varying ξ and ɛ2/ɛ1, a variety of types of phase behavior is covered by this paper. The dependence of the interfacial properties on the variables ξ and ɛ2/ɛ1 reveals regularities that can be explained by conformal solution theory of the liquid phase. It is thereby shown that the interfacial properties of the mixtures are dominated by the mean liquid phase interactions whereas the vapor phase has only a minor influence.