The modified quasi-chemical model: Part II. Multicomponent solutions

Further improvements to the modified quasi-chemical model in the pair appro ximation for short-range ordering (SRO) in liquids are extended to multicom ponent solutions. The energy of pair formation may be expanded in terms of the pair fractions or in terms of the component fractions, and coordination numbers are permitted to vary with composition. The model permits complete freedom of choice to treat any ternary subsystem with a symmetric or an as ymmetric model. An improved general functional form for "ternary terms" in the excess Gibbs energy expression is proposed. These terms are related to the effect of a third component upon the binary pair interaction energies. It is shown how binary subsystems that have been optimized with the quasi-c hemical model can be combined in the same multicomponent Gibbs energy equat ion with binary subsystems that have been optimized with a random-mixing Br agg-Williams model and a polynomial expression for the excess Gibbs energy. This is of much practical importance in the development of large databases for multicomponent solutions. The model also applies to SRO in solid solut ions as a special case, when the number of lattice sites and coordination n umbers are constant.