APPARENT MOLAR ENTHALPIES OF AMPHIPHILIC DRUGS IN AQUEOUS-SOLUTION

Apparent molar enthalpies have been determined as a function of concentration by heat conduction calorimetry for aqueous solutions of the amphiphilic drugs chlorpromazine hydrochloride, promethazine hydrochloride, promazine hydrochloride, and imipramine hydrochloride. The concentration dependence of the apparent molar enthalpy could be quantitatively described using a mass-action model of association based on the Guggenheim equations for the activity coefficients of mixed electrolytes. Micellar properties predicted by the application of this theory to calorimetric data were in good agreement with experimental values from light scattering techniques, but the derived values of the monomer-counterion interaction parameters were strongly negative indicative of possible premicellar association.