Mixed micellization of antidepressant drug amitriptyline hydrochloride with cationic surfactants

The surface tension measurements have been carried out on aqueous solutions of antidepressant drug amitriptyline hydrochloride with six cationic surfactants (decyl-, dodecyl-, tetradecyl-, cetyltrimethylammonium bromide, cetylpyridinium bromide and cetylpyridinium chloride) at different mole fractions to study the surface and micellar properties at the interface as well as in the micelles. The properties studied include critical micelle concentration (cmc), Gibbs surface excess (Γmax), minimum head group area at the air/water interface (Amin), free energy of micellization (ΔGm∘), and standard Gibbs energy of adsorption (ΔGads∘), while synergistic behavior of drug-surfactant binary mixtures was analysed using Clint's and Rubingh's models. Deviations of cmc from cmc* and X1m from X1ideal indicate the synergistic behavior (i.e., non-ideal behavior) for the drug-surfactant binary mixtures. The values of interaction parameters β and activity coefficients f1 and f2 (for both, in mixed monolayer as well as in mixed micelles) also indicate the synergistic behavior. The excess free energy (ΔGex) for the drug-surfactant binary mixtures explains stability of mixed micelles in comparison to micelles of pure drug and variation with alkyl chain length of surfactants.