RELATIONSHIP BETWEEN THE SOLUBILITY PARAMETER AND THE SURFACE FREE-ENERGY OF SOME SOLIDS

Solubility parameters for a number of drugs and related organic solids were determined from their dispersion and polar surface free energy data by means of an equation found to be valid for solvents. The relation fits quite well all solids investigated with an excellent correlation coefficient. The calculated solubility parameters, O, were checked with the ones estimated from molecular groups and fragment constants according to Fedors method, F. A similar O of 14.6 was obtained for o-hydroxybenzoic and p-hgdroxybensoic acids. The deviations from F shown in ethyl-p-aminobenzoate, benzoic acid, and aspirin were attributed to the imperfection of their vapor. The solubility profile of hydrocortisone acetate was obtained in dioxane-water mixtures, and the solubility parameter of the drug was determined from the peak solubility in this binary solvent system. The experimental results for some drugs were compared with their solubility parameters calculated from the surface free energy data. Excellent agreement was found for hydrocortisone acetate, whereas griseofulvin and p-hydroxybenzoic agreed fairly. The "chameleonic" behavior of benzoic acid results in different experimental Solubility parameter, and the percentage deviation from O ranged from 3.4-20.8. A deviation of 11.8% was obtained for ethyl-p-aminobenzoate and was attributed to the influence of its crystal structure which would account for its low E value measured. The relation suggested was also applied to the dicarboxylic acids. The odd-even alteration seen in their melting points is not reflected on their solubility parameters. The high O calculated for these acids were related to their elongated shape. © 1990 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.