Theoretical Treatment of Activity Coefficients

General.— The activity coefficients at infinite dilution have been analysed in many different ways, in attempts to understand the interactions of solute with solvent. The earliest efforts based on empirical relations for homologous series of solutes interpreted the results in terms of group interactions, structural effects, polarity, and electron-donor and -acceptor capacities. These ideas have served as foundations for the more sophisticated theoretical interpretations. 

The fundamental approximation used in most of the theoretical treatments considers the activity coefficient to be separable into two parts  

The symbol 3 refers to the volume fraction of the long-chain molecules and r to the ratio of sites occupied by the long- and short-chain molecules. For the infinitely dilute condition that applies in g.l.c., equation (19) becomes 

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