Modelling of the surface tension of pure components with the gradient theory of fluid interfaces: a simple and accurate expression for the influence parameters

In this work the gradient theory of fluid interfaces is used to compute the surface tension of substances of industrial interest (hydrocarbons, gases and refrigerants) once an expression has been derived for their influence parameters. The vapour–liquid equilibria are first determined with a volume-corrected Peng–Robinson equation of state (PR-EOS). The volume corrections are accurately described by a correlation similar to the one suggested by Soreide, but with new parameters regressed on experimental data of the fluids considered here. The influence parameters are computed for each fluid outside the critical region. The results support the assumption that the density-dependence of the influence parameter can be neglected while a temperature-dependence needs to be conserved. A simple correlation is derived to account for this temperature-dependence. For hydrocarbons and gases, the parameters of the temperature-dependence are correlated with the acentric factor; for refrigerants, they are kept constant. When the gradient theory is applied with the expression presented here for the influence parameter and combined with the volume-corrected PR-EOS, the overall average absolute deviations of the calculated surface tensions is 2.2% for hydrocarbons and gases, 4% for refrigerants.