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 interface of a binary mixture of Lennard-Jones fluids on a path of constant temperature and varying composition which passes through the consolute point for liquid-liquid separation but lies entirely in the two phase region. Our calculations are based on a square gradient approximation to the free energy functional of the inhomogeneous fluid. For bulk liquid concentrations x 1≲0·5 the density profile of species 1, the more volatile component, exhibits a maximum in the interface indicating pronounced surface segregation. The relative adsorption Г1,2 is positive for x 1≲0·66 and negative for larger x 1, i.e. the system exhibits an aneotrope. Our analysis suggests that it may be more useful to focus attention on the ratio Г1,2/x 1 than Г1,2 itself. We comment on recent attempts to measure Г1,2 for such interfaces. Our results for the surface tension in the vicinity of the critical end ...