Local Composition Structured Fluid Model for the Excess Gibbs Energy of Liquid Mixtures

Abstract

An n-fluid local composition model is derived for the excess free energy of multicomponent liquid mixtures. Taking into account that differences in fluid structure affect the intermolecular interactions leads to the following expression for a binary mixture:The parameters A ij ) ln γ j ∞ and R ij can be expressed in terms of molecular properties, and the model should therefore have general validity. For most systems tested, the approximation R ij ) 1/R ji is valid. This leads to a three-parameter version that, with the exception of the alcoholalkane systems, shows greater flexibility than the nonrandom two-liquid equation in correlating vapor-liquid equilibrium (VLE) behavior. As is the case for other local composition models, multicomponent behavior can be predicted from knowledge of the binary VLE alone. Because they can be shown to be special forms, the local composition structured fluid model also provides a proper framework for the extension of the Porter, Van Laar, Margules, and Scatchard-Hildebrand equations to multicomponent systems.