ANALYSIS OF THE RELATIONSHIP BETWEEN TERNARY MIXTURES AND THEIR BINARY SUBSYSTEMS AS REPRESENTED BY THE UNIQUAC AND NRTL MODELS

The second derivative of the Gibbs free energy of mixing, G(11), was c alculated for the binary mixtures water + alcohol and solvent + alcoho l (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 co rresponding ternary equations to liquid-liquid equilibrium data for th e ternary systems water + alcohol + solvent. Apart from the usual resu lt that the binary interaction parameters of the UNIQUAC and NRTL equa tions, and the corresponding thermodynamic descriptions of binaries in terms of G(11), depend heavily on the system from which they have bee n obtained (which should be borne in mind when using the binary parame ters in calculations for multicomponent systems), the chief conclusion of this work is that the UNIQUAC and NRTL models do not imply any con sistent relationship between the slopes of the ternary tie-lines and t he characteristics of the G(11) of the corresponding homogeneous binar y mixtures.