Chapter 6 : Solution Thermodynamics and Principles of Phase Equilibria
Section 19 : Activity Coefficient Models
Regular Solution (RS) Model
This activity coefficient model is derived for solutions that show moderate deviations from ideal solution behaviour, and for which are both zero. The model was proposed by Scatchard and Hildebrand (J. H. Hildebrand & R. L Scott. The Solubility of Non-electrolytes, Dover, 1964) and constitutes one of few models that are derived from theory. Known as the Regular Soution Model, the molar excess Gibbs free energy function is given by:
(6.184)
(6.185)
(6.186)
where, = volume fraction = ; Vi = molar volume of pure ith species, and
Further: = solubility parameter for ith species, which is given by:
are both zero. The model was proposed by Scatchard and Hildebrand (J. H. Hildebrand & R. L Scott. The Solubility of Non-electrolytes, Dover, 1964) and constitutes one of few models that are derived from theory. Known as the Regular Soution Model, the molar excess Gibbs free energy function is given by:
= volume fraction = 
; Vi = molar volume of pure ith species, and 
= solubility parameter for ith species, which is given by: