6.19 Activity Coefficient Models

In this section we present the forms of the various activity coefficient models commonly used for VLE calculations. Such models may be divided into two major groups depending upon their applicability to various types of solutions:

1. Margules/Van Laar/Regular Solution Models

2. Wilson/NRTL (Non-Random Two Liquid)/UNIQUAC (Universal Quasi Chemical) Model.

The models in Group 1 are termed as "Homogeneous Mixture" models, while those in Group 2 are termed "Local Composition" models. The reason for such distinction is as follows. The non-ideal behaviour of liquid solutions derives from two sources: (i) difference in molecular size/shape of constituent (ii) and the difference between the inter-species and intra-species molecular interaction energies. The Group 1 models generally apply to systems in which the inter-species and intra-species molecular interaction energies differ from each other, but in a relatively moderate measure. On the other hand type 2 models are principally useful for describing systems where the constituent molecular species differ on account of both size/shape as well molecular interaction energies. Thus while group I models useful for moderate deviation from ideal solution behaviour, those in group 2 represent strongly non-ideal solutions.