Surface segregation and surface tension at the liquid–vapour interface of a binary mixture of Lennard-Jones fluids

Abstract

We have calculated the surface segregation C(z) and the surface tension for the planar liquid–vapour interface of a binary mixture of Lennard-Jones fluids at various concentrations and temperatures. Calculations are based on the square-gradient approximation for the Helmholtz free-energy functional. For parameters appropriate to argon and krypton “argon” is preferentially adsorbed at all concentrations and the adsorption is particularly pronounced for small “argon” concentrations and at low temperatures. The magnitude and concentration dependence of the surface tension γ is only weakly dependent on C(z); γ depends rather on a contribution γ_NN which involves the total density profile N(z). By approximating γ_NN we derive the Bhatia–March relation which relates the surface tension to the isothermal compressibility of the bulk liquid mixture. We also derive a formula for (∂γ/∂x)_{x= 0}, where x is the concentration of one of the species, which should provide insight into the factors which determine adsorption in dilute mixtures.