General Equations of Excess Functions for Nonideal Binary Mixtures

9 General Equations of Excess Functions for Nonideal Binary Mixtures 

So far, we have seen that deviation from ideal behavior may affect one or more thermodynamic magnitudes (e.g., enthalpy, entropy, volume). In some cases, we are able to associate macroscopic interactions with real (microscopic) interactions of the various ions in the mixture (for instance, coulombic and repulsive interactions in the quasi-chemical approximation). In practice, it may happen that none of the models discussed above is able to explain, with reasonable approximation, the macroscopic behavior of mixtures, as experimentally observed. In such cases (or whenever the numeric value of the energy term for a given substance is more important than actual comprehension of the mixing process), we adopt general (and more flexible) equations for the excess functions. 

Let us consider a generic mixture of components 1 and 2 in the binary compo- 

Guggenheim s polynomial expansion (equation 3.171 Guggenheim, 1937) and the two Redlich-Kister equations (3.172 and 3.173 Redlich and Kister, 1948) are of general applicability for any type of mixture  

If Aq = A = A2 = 0, the excess Gibbs free energy of mixing is zero throughout the compositional field and the mixture is idea/. 

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