Activity Coefficients and Composition Analytical Expressions

Using phenomenological arguments, from purely empirical to those based on statistical mechanics, an expression for the molar excess Gibbs 

Development of a model for requires an understanding of the different effects involved in the mixing of molecules and, of course, an analytical representation of these effects. As a starting point we note that, since  

The first effect tends to be the dominating factor in mixtures of molecules that are not very different in size and the early investigators in the field van Laar, Hildebrand, Scatchard, Wohl, etc., assumed that = 

0n the other hand, for mixtures involving molecules of very different size, such as polymer-solvent ones, can be much smaller than S. The assumption of // = 0, leads to the concept of athermal solution and result in the Flory-Huggins expression for polymeric solutions (Prausnitz et al), where = 0 was also assumed. 

We will classify the models for G and, hence, for the activity coefficient, into two types  

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