Interfacial properties of binary mixtures of simple fluids and their relation to the phase diagram

Abstract

Interfacial properties of binary fluid mixtures were studied using both molecular dynamics (MD) simulations and density gradient theory (DGT). The focus of the study is on the relation of the interfacial properties to the phase diagram of the mixture. Two binary Lennard-Jones mixtures were investigated in a wide range of states: a highly asymmetric mixture (type III), which exhibits vapour–liquid equilibria (VL₁E and VL₂E), liquid–liquid equilibria (L₁L₂E), a three-phase equilibrium (VL₁L₂E), and supercritical fluid–fluid equilibria (F₁F₂E), and, as a reference, an ideal mixture (type I). The studied interfacial properties are: the surface tension, the relative adsorption, the width of the interfacial region, and the enrichment of the low-boiling component, on which we set a focus. Enrichment was observed at VL₁ interfaces; and, to a small extent, also at L₁L₂ interfaces; but not at the supercritical F₁F₂ interfaces. The large enrichment found at VL₁ interfaces of the type III mixture can be interpreted as a wetting transition: approaching the VL₁L₂E three-phase line from the VL₁ side, the enrichment gets stronger and can be interpreted as precursor of the second liquid phase L₂. However, the actual existence of a three-phase line in the phase diagram is no prerequisite for an enrichment. The enrichment is found to be highly temperature-dependent and increases with decreasing temperature.