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

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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 point are in agreement with the predictions of Widom's one density, mean field theory and with experiment. The role of concentration fluctuations in determining the liquid-vapour surface tension along the whole of the critical isotherm is discussed.

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