Interfacial tension of nonassociating pure substances and binary mixtures by density functional theory combined with Peng-Robinson equation of state

We develop a density functional theory and investigate the interfacial tension of several pure substances N-2, CO2, H2S, normal alkanes from C-1 to nC(10), and binary mixtures C-1/C-3, C-1/nC(5), C-1/nC(7), C-1/nC(10), CO2/nC(4), N-2/nC(5), N-2/nC(6), N-2/nC(8), N-2/nC(10), nC(6)/nC(7), nC(6)/nC(8), and nC(6)/nC(10). The theory is combined with the semiempirical Peng-Robinson equation of state (PR-EOS). The weighted density approximation (WDA) is adopted to extend the bulk excess Helmholtz free energy to the inhomogeneous interface. Besides, a supplementary term, quadratic density expansion (QDE), is introduced to account for the long-range characteristic of intermolecular dispersion attractions, which cannot be accurately described by the WDA. In the bulk limit, the QDE vanishes and the theory is reduced to the PR-EOS. For pure substances, the potential expansion parameter is the only adjustable parameter in the QDE and determined by using a single measured interfacial tension at the lowest temperature examined. Then without any parameter adjustment, we faithfully predict the interfacial tensions of pure substances and mixtures over a wide range of conditions.