The effect of surfactants on the dissociation constants of phenothiazine derivatives Alkalimetric determination of diethazine and chlorpromazine

The effect of a cationic, an anionic and a non-ionic surfactant on the acid-base equilibria of the phenothiazine derivatives, diethazine hydrochloride and chlorpromazine hydrochloride, has been studied. It has been found that the presence of cationic and non-ionic surfactants strongly enhances the dissociation of the two derivatives, whereas the anionic surfactant decreases the dissociation constant. These effects are in agreement with a theory based on a pseudophase, ion-exchange model of micelles. From the dissociation-constant values as a function of the surfactant concentration, the binding constants for diethazine and chlorpromazine with the surfactants Septonex and sodium dodecylsulphate have been calculated. The ability of cationic surfactants to solubilize the free bases of the phenothiazine derivatives and to increase their dissociation constants has been utilized to develop a new method for alkalimetric determination of the derivatives in a micellar medium. The method has been applied to determination of the content of the active component in pharmaceutical preparations.