Interfacial properties of selected binary mixtures containing n-alkanes

Abstract

We report the correlation and prediction of the subcritical vapour–liquid interfacial properties of three asymmetric binary mixtures composed of long n-alkanes in equilibria with a smaller solvent: hexane + decane, carbon dioxide + decane, and ethane + eicosane. The interfacial region is described by the complementary use of the Square Gradient Theory (SGT) and molecular dynamics (MD) simulations that allow the prediction of both the macroscopic and molecular level properties of the binary fluid mixtures. Calculations with SGT rely on the description of the vapour–liquid equilibria (VLE) by means of the Statistical Associated Fluid Theory (SAFT) equation of state. MD simulations are performed in the canonical ensemble using united-atom potentials to probe coexisting phases and the accompanying interface simultaneously. In addition to the phase equilibrium compositions and interfacial tensions, other interfacial properties, such as concentration profiles along the interfacial region, surface activities, and relative Gibbs adsorption isotherms at the interfaces have been obtained from a combination of SGT and MD. The entropy and surface enthalpy change of surface formation are also calculated for the case of ethane + eicosane mixture, where we report three different isothermal conditions. While pure component data are used to fit model parameters, mixture results are predictions. When possible, results are compared to available experimental data and quantitative agreement is observed throughout. The particularly high excess adsorption in the interface of the smaller solvents, CO 2 and ethane, is noted.