The surface tension and density profile of simple liquids

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Abstract
The thermodynamic perturbation theory of Weeks, Chandler and Andersen (1971) is used to derive a physically transparent expression for the excess free energy of a simple liquid in coexistence with its vapour. The expression is minimised with respect to a single parameter specifying the width of the density profile, to give the surface tension directly. The theory gives useful insight into the thermodynamic and microscopic factors which determine the equilibrium density profile and the magnitude and temperature dependence of the surface tension. Calculated density profiles for argon and results for the surface tension of argon, nitrogen and methane, modelled by a Lennard-Jones 12-6 potential, are presented. Near the triple point, these are in reasonable agreement with experiment and with the results of more sophisticated theories.