The structure and surface tension of the liquid-vapour interface near the upper critical end point of a binary mixture of Lennard-Jones fluids

Abstract

We present the results of calculations of the density profiles, relative adsorption and surface tension of the liquid-vapour and liquid-liquid interfaces of a binary mixture of Lennard-Jones fluids for a second path through the critical end point. This path consists of the triple point line in the three phase region and a path of constant critical composition and varying temperature in the two phase region. In the three phase region the mixture exhibits a first order Cahn wetting transition at a temperature T _w which is a few degrees below the critical end point, T _c. For temperatures T<T _w the density profiles for the two liquid-vapour interfaces, αβ and αγ, are qualitatively different and the relative adsorption Г_1,2 is positive at the αγ interface and negative for the αβ interface. At T = T _w, where the intermediate liquid phase, β, completely wets the αγ interface, Г_1,2 changes discontinuously for αγ but varies continuously for the αβ and the liquid-liquid (βγ) interfaces. The temperature derivative of the surface tension of the αγ interface is discontinuous at the wetting transition. At the critical end point the αγ and αβ surface tensions merge smoothly into the α, βγ surface tension in agreement with Widom's predictions. By analysing our results in terms of number-concentration variables we show that concentration fluctuations are important in determining the behaviour of the liquid-vapour surface tension as well as the liquid-liquid surface tension. We find that T _c - T _w depends rather sensitively on the parameters and mixing rule which characterize the intermolecular potentials.