Hildebrand defined a regular solution as one in which the excess entropy vanishes when the solution is mixed at constant temperature and constant volume. This is true for most systems of non-polar compounds, provided the molecules do not differ greatly in size. The excess Gibbs energy is then primarily determined by the attractive intermolecular forces.
Scatchard and Hildebrand made a simplifying assumption for relating mixture interactions to those of pure fluids. The result was a simple theory in which the liquid activity coefficients are a function of pure component properties only. The correlating property is the solubility parameter, which is related to the energy required to vaporize a liquid component to an ideal gas state.
The liquid activity coefficients are determined with an equation relating the solubility parameters of the components and the mixture in a volumetric fashion.
There are no adjustable parameters in regular solution theory. It is useful for mixtures of non-polar components, but should not be used for highly non-ideal mixtures, especially if they contain polar compounds. Solubility parameters are included in PRO/II for most library components.
Note: The Regular Solution implementation in PRO/II supports non-rigorous liquid-liquid equilibria using WATER DECANT. This is in addition to application in systems utilizing single liquid VL or rigorous two-liquid VLL equilibria.
See "Regular Solution Theory" under "Liquid Activity Coefficient Methods" in the PRO/II Reference Manual, Volume I, Chapter 2, for more information on this method.
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