Micellization of mixtures of amphiphilic drugs and cationic surfactants: a detailed study.

The micellization behaviors of two amphiphilic drugs ((amitriptyline hydrochloride (AMT) and imipramine hydrochloride (IMP)) in presence of cationic surfactants (conventional as well as gemini) have been investigated conductometrically at four mole fractions and four temperatures. The critical micelle concentration (cmc) values come out to be lower than cmc(id) values (cmc(id) is the cmc value at ideal mixing state) indicating attractive interactions between the two components in mixed micelles. Micellar mole fractions of surfactants (X(1) and X(1)(M)), calculated by Rubingh and Motomura models, are always greater than X(1)(id) (micellar mole fraction at ideal mixing). The rigid structure of drugs decreases their contribution in mixed micelles as compared to that predicted by X(1)(id) values. Although α(1) (mole fraction of surfactant) is higher for DTAB than that of 12-4-12, the contribution of 12-4-12 is almost equal to that of DTAB. The interaction parameter (β) is negative at all temperatures and at all compositions indicating attractive interactions. Activity coefficients (f(1) and f(2)) are always less than unity suggesting nonideality in the systems. Thermodynamic parameters suggest dehydration of hydrophobic part of the drug at or above certain temperature which is different for the two drugs.