Determination of interfacial tension of binary mixtures from perturbative approaches

Abstract

We determine the interfacial properties of mixtures of spherical Lennard-Jones molecules from direct simulation of the vapour–liquid interface. We consider mixtures with same molecular size but different dispersive energy parameter values. We use the extensions of the improved version of the inhomogeneous long-range corrections of Janecek, presented recently by MacDowell and Blas and Martínez-Ruiz et al., to deal with the interaction energy and microscopic components of the pressure tensor. We have performed Monte Carlo simulations in the canonical ensemble to obtain the interfacial properties of mixtures of Lennard-Jones molecules with a cut-off distance rc = 3σ in combination with the inhomogeneous long-range corrections. The pressure tensor is obtained using the mechanical (virial) and thermodynamic route. The vapour–liquid interfacial tension is also evaluated using three different procedures, the Irving–Kirkwood method, the difference between the macroscopic components of the pressure tensor, and the test-area methodology. This allows to check the validity of the recent extensions presented to deal with the contributions due to long-range corrections for intermolecular energy and pressure tensor in the case of binary mixtures. In addition to the pressure tensor and the surface tension, we also obtain density profiles, coexistence densities, and interfacial thickness as functions of pressure, at a given temperature. According to our results, the main effect of increasing the ratio between the dispersive energy parameters of the mixture, ε22/ε11, is to sharpen the vapour–liquid interface and to increase the width of the biphasic coexistence region. Particularly interesting is the presence of a relative maximum in the density profiles of the less volatile component at the interface. This maximum is related with adsorption or accumulation of these molecules at the interface, a direct consequence of stronger attractive interactions between these molecules in comparison with the rest of intermolecular interactions. In addition to that, the interfacial thickness decreases, the width of the tangential microscopic component of the pressure tensor profile increases, and the surface tension increases as ε22/ε11 is larger.