Interaction between Zwitterionic Surfactants and Amphiphilic Drug: A Tensiometric Study

The physicochemical properties, viz, critical micelle concentration (cmc), surface excess concentration (Gamma(max)), minimum area per head group (A(min)) of zwitterionic surfactants (designated as n(-)-2-m(+); n = 8, 10, 12 and m = 12, 14, 16) and their mixtures with amphiphilic antidepressant drug amitriptyline hydrochloride (AMT) were determined by using surface tension measurements. The cmc and ideal cmc (cmc(id)) values along with interaction parameters, beta(m) and beta(sigma) (calculated using Rubingh's and Rosen's models), suggest attractive interactions among the components. The Krafft temperature measurements also indicate strong attractive interactions. Gamma(max) (or A(min)) increases (or decreases) with the addition of gemini surfactant; the values being closer to that of the drug. These values and micellar mole fraction (X-1(m)-calculated from Rubingh's model and X-1(Moto)-calculated from Motomura's model) indicate larger contribution of gemini surfactants in mixed micelles and smaller contribution at air/solution interface (as mole fraction values at interface, X-1(sigma), are slightly smaller than X-1(m)). The standard Gibbs energy of micellization (Delta G(mic)degrees) and adsorption (Delta G(ad)degrees) as well as excess energy of mixing (Delta G(ex)(m)) are all negative. All these results suggest higher stability of the mixed systems. UV absorbance results also suggest that the mixed micelles are stable for several days.