DENSITY-FUNCTIONAL THEORY - STRUCTURE AND INTERFACIAL PROPERTIES OF BINARY-MIXTURES

Based on Density Functional Theory (DFT) of inhomogeneous fluid mixtur es we model the liquid vapour interface of some binary mixtures. Attra ctive contributions to the Helmholtz free energy are treated by a mean -field approximation (MF). The local density approximation (LDA) is ap plied to the repulsive reference system described by a hard sphere equ ation of state. A cut-off and shifted Lennard-Jones-12-6 potential div ided according to the WCA-prescription is used as interaction pair pot ential. Binary Lennard-Jones parameter are introduced and fitted to ex perimental liquid-vapour compositions to describe interactions between different species. The calculations give good results in the case of simple fluid mixtures like Ar-N-2 and CO-N-2. Similar results are obta ined for the nonpolar mixture n-heptane-toluene. The predicted surface tensions gamma agree very well with experimental data. Computed parti al density profiles clearly show the enrichment of the component with the lower surface tension in the interphase. The relative Gibbs surfac e excess concentration is calculated from partial density profiles and compared with experimental values. The influence of activity coeffici ents on Gibbs adsorption is discussed.