On the choice of "geometric" thermodynamic models

A number of "geometric" models have been proposed for estimating the thermo dynamic properties of a ternary solution from optimized data for its binary subsystems, Among the most common of these are the Kohler, Muggianu, Kohle r/Toop, and Muggianu/Toop models. The latter two are "asymmetric" models in that one component is singled out and treated differently, whereas the fir st two models are "symmetric." it is shown that the use of a symmetric mode l when an asymmetric model is more appropriate can often give rise to large errors. Equations are proposed for extending the symmetric/asymmetric dich otomy inter N-component systems (N = 3), while still permitting the flexibi lity to choose either a symmetric or;an asymmetric model for any ternary su bsystem. An improved general functional form for "ternary terms" in the exc ess Gibbs energy expression is also proposed. These terms are related to th e effect of a third component upon the binary pair interaction energies. Al l the above considerations also apply when short-range ordering is taken in to account by using the modified quasichemical model. Finally, some argumen ts in favor of the Kohler model over the Muggianu model are presented.