Regular Solution Theory Method

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 nearly the case for most solutions of nonpolar compounds, provided that the molecules do not differ greatly in size. The attractive intermolecular forces then primarily determine the excess Gibbs energy. Scatchard and Hildebrand made a simple assumption relating mixture interactions to those in pure fluids; the result is a simple theory in which the activity coefficients are a function of pure-component properties only. The important property is the solubility parameter, which is related to the energy required to vaporize a liquid component to an ideal gas state.

This method requires the following pure component properties:

• Vapor pressure

• Liquid molar volume

• Solubility parameter