Prediction of internal structure and properties in fluid model interfaces of binary and ternary liquid mixtures

The scope of this study is to demonstrate how density functional theory as part of the statistical mechanics of inhomogeneous fluids can contribute to a better understanding of the internal structure and interfacial properties of vapor-liquid and liquid-liquid interfaces in multicomponent liquid mixtures. Especially fluid mixtures with liquid-liquid phase separation can serve as models for liquid-liquid extraction systems. The partial density profiles in such an interface describe molecular concentration gradients which may act as a driving force for mass transfer across this interface. In the application of the density functional theory to liquid-liquid interfaces, we analyzed the influence of different approximations within the theory on the calculation of interfacial structure and thermodynamic potential properties. Especially the use of a weighed density approximation allows us to take short-range local ordering into account. It results in oscillating partial densities or concentration profiles and it yields a highly structured grand canonical potential density across the interface which demonstrates how energetically inhomogeneous a liquid interface is. For a ternary model system, the interfacial tension is calculated, capillary wave contributions are added and their influence is discussed. (c) 2004 Elsevier B.V. All rights reserved.