Lattice model of mixtures

The following discussion is the traditional way of examining the condition of stability. It was originally discussed in the context of the lattice model of mixtures (see below). 

Lattice model of solutions. In a lattice model of mixtures (Guggenheim 1952) the size of particles are assumed to be nearly the same. Hence A and B can occupy the same lattice sites. It is well known that in this case the deviations from SI solutions are expressed in the form (see Appendix M)... 

A special and simple case occurs when Aab is independent of composition (in the theory of lattice models of mixtures, a similar condition is assumed) hence, the integral in (4.119) can be evaluated immediately and we get... 

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