Classical Density Functional Theory for Liquid–Fluid Interfaces and Confined Systems: A Functional for the Perturbed-Chain Polar Statistical Associating Fluid Theory Equation of State

A Helmholtz energy functional based on the Perturbed-Chain Polar Statistical Associating Fluid Theory Equation of State is proposed. A weighted density approximation is used to develop two variants of dispersion functionals. We conduct molecular simulations (Monte Carlo) in order to assess both models. The structure of thin liquid adsorbed layers of a few molecular diameters thickness are shown to be particularly meaningful to detect deficiencies of Helmholtz energy functionals and thus discriminate between models. We further compare the model to experimental data of interfacial tensions for pure components and for binary mixtures exhibiting vapor–liquid equilibria and liquid–liquid equilibria. The proposed Helmholtz energy functional is in good agreement with experimental data of pure (organic) substances and mixtures and compares well with molecular simulation data of fluids adsorbing at solid interfaces.