Extended solubility approach: solubility parameters for crystalline solid compounds.

A method is suggested to obtain solubility parameters for crystalline solid compounds involving a quadratic equation based on the original Scatchard-Hildebrand solubility expression. The geometric mean delta 1 delta 2, of the Hildebrand approach is replaced by w12 = K delta 1 delta 2, and log alpha 2/(V2 phi 2(2)/2.3RT) is regressed against delta 1 in a second-degree power series for parabens and benzoic acid in a series of normal alcohols. The method provides reasonable solubility parameters for the solid solutes and affords a convenient calculation of the solubility of drugs in a homologous series of solvents.