A CALORIMETRIC STUDY ON THE SELF-ASSOCIATION OF AN AMPHIPHILIC PHENOTHIAZINE DRUG IN AQUEOUS-ELECTROLYTE SOLUTIONS

The association characteristics of an amphiphilic phenothiazine drug, promethazine hydrochloride, in aqueous solutions containing high concentrations of electrolyte have been examined by heat conduction calorimetry. The application of deconvolution techniques has permitted the continuous determination of the concentration dependence of the apparent molar enthalpy extending to regions of high dilution. Theoretical treatments have been derived to describe thermogenesis in solutions of amphiphilic compounds exhibiting a range of association patterns. The calorimetric data for promethazine hydrochloride in the presence of 0.2-0.6 mol dm-3 sodium chloride could be satisfactorily described by an association scheme in which a primary unit of 3-4 monomers is formed below the critical micelle concentration by a continuous self-association process and grows with increasing concentration by the stepwise addition of monomers. Values of the stepwise equilibrium constants and the molar enthalpy changes for the formation of the primary units and for the stepwise addition of monomers have been derived. © 1990 American Chemical Society.