On the first-order deviations from SI solutions

We have seen in section 6.2 that the first-order deviations from symmetrical ideal solution have the form 

2) holds for all 0 xA 1, then the Gibbs-Duhem relations impose a 

Since at xB = 0 we must have Ha = P% hence Oq = 0. Applying the Gibbs-Duhem relation to (L.4) we, obtain 

The excess Gibbs energy per mole of the mixture with respect to SI is 

Historically, this form of the excess Gibbs energy was suggested as the simplest function which obeys the requirements that g iX,S[ must be zero when either xA — 0 or X/j — 0. This is known as Margules equation, see, e.g., Prausnitz et al (1986). From (L.6), one can obtain both equations (L.3) and (L.4). The latter was derived from theoretical arguments based on lattice model for mixtures (Guggenheim 1952). 

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