Direct calculation of interfacial properties of fluids close to the critical region by a molecular-based equation of state

The crossover soft-SAFT equation coupled with the Density Gradient Theory is applied here to the prediction of interfacial tensions of pure fluids and mixtures, including non-polar and polar compounds, far from and near to the critical region. The approach is able to accurately describe the interfacial tensions of pure systems and mixtures as compared with available experimental data, in the whole range of thermodynamic conditions, with the same degree of accuracy of vapor–liquid equilibria previously reported. A correlation for the influence parameter as a function of the molecular weight is obtained for the n-alkanes and 1-alkanol families, empowering the equation with predictive capabilities for interfacial tensions of compounds of the family for which experimental data is unavailable or difficult to obtain, such as heavy n-alkanes. It is shown how the equation is able to capture the S-shape of the interfacial tension of water, succeeding a stringent test to any equation applied to water. For mixtures, the theory is able to quantitatively predict the mixture behavior of heavy alkanes, with calculations made from pure component parameters and no fitting; good results are also obtained for other mixtures considered, including nitriles systems, while two binary parameters, close to the ideal values, were needed for the description of CO2–butane mixtures. The results presented here show the robustness of using an accurate and versatile equation of state for the bulk combined with a simple and elegant method for calculating interfacial properties, with very modest computational effort.