Interaction between Zwitterionic Surfactants and Amphiphilic Drug: A Tensiometric Study

Abstract The physicochemical properties, viz, critical micelle concentration (cmc), surface excess concentration ( Γ 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, β m and β σ (calculated using Rubingh's and Rosen's models), suggest attractive interactions among the components. The Krafft temperature measurements also indicate strong attractive interactions. Γ 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 σ , are slightly smaller than X 1 m ). The standard Gibbs energy of micellization (Δ G mic º) and adsorption (Δ G ad º) as well as excess energy of mixing (Δ 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.