Thermodynamic properties of a liquid–vapor interface in a two-component system

We report a complete set of thermodynamic properties of the interface layer between liquid and vapor two-component mixtures, using molecular dynamics. The mixtures consist of particles which have identical masses and diameters and interact with a long-range Lennard-Jones spline potential. The potential depths in dimensionless units for like interactions is 1 (for component 1) and 0.8 (for component 2). The surface excess entropy decreases when the temperature increases, so the surface has a negative excess heat capacity. This is a consequence of the fact that the surface tension decreases to zero at the critical point, proportional to (TC,i−T)2ν(TC,i−T)2ν. The surface entropy decreases also as the excess concentration of component 2 increases, at a given temperature. The surface becomes less able to store energy as the more volatile component accumulates. We show that the more volatile component 2 has a maximum in its excess concentration in the interfacial region when the density of component 1 starts to go down. The vapor is described with the Soave–Redlich–Kwong equation of state, and the liquid activity coefficients are described by the three-suffix Margules equation. The observed densities and surface tension variations are fitted with good accuracy, with the critical exponents β=0.32β=0.32 and 2ν=1.262ν=1.26, respectively. The results for the critical temperature of mixtures can be fitted to a second order polynomial, with significant deviations from Kay's rule.