Analysis of the relationship between ternary mixtures and their binary subsystems as represented by the uniquac and nrtl models

Abstract

The second derivative of the Gibbs free energy of mixing, G11, was calculated for the binary mixtures water + alcohol and solvent + alcohol (alcohol is methanol or ethanol, solvent is n-amyl acetate, iso-amyl acetate, hexyl acetate or 1-octanol) using, for G, binary uniquac and nrtl equations with interaction parameters obtained by fitting the corresponding ternary equations to liquid-liquid equilibrium data for the ternary systems water + alcohol + solvent. Apart from the usual result that the binary interaction parameters of the uniquac and nrtl equations, and the corresponding thermodynamic descriptions of binaries in terms of G11, depend heavily on the system from which they have been obtained (which should be borne in mind when using the binary parameters in calculations for multicomponent systems), the chief conclusion of this work is that the uniquac and nrtl models do not imply any consistent relationship between the slopes of the ternary tie-lines and the characteristics of the G11 of the corresponding homogeneous binary mixtures.