Solubility in binary solvent systems III: predictive expressions based on molecular surface areas.

The nearly ideal binary solvent model, which has led to successful predictive equations for the partial molar Gibbs free energy of the solute in binary solvent mixtures, was extended to include molecular surface areas as weighting factors. Two additional expressions were derived and compared to previously developed equations (based on molar volumes as weighting factors) for their ability to predict anthracene and naphthalene solubilities in mixed solvents from measurements in the pure solvents. The most successful equation in terms of goodness of fit involved a surface fraction average of the excess Gibbs free energy relative to Raoult's law and predicted experimental solubilities in 25 systems with an average deviation of 1.7% and a maximum deviation of 7.5%. Two expressions approximating weighting factors with molar volumes provided accurate predictions in many of the systems studied but failed in their ability to predict anthracene solubilities in solvent mixtures containing benzene.