Interaction of tetracaine hydrochloride with sodium deoxycholate in aqueous micellar phase and at the surface

Assessing drug–surfactant interactions is important in pharmacology and for developing better pharmaceutical formulations. In this paper, we have investigated the interaction between tetracaine hydrochloride (THC) with sodium deoxycholate (SDC). The characteristics of this drug–surfactant system in solution, micellar phase, and at the surface were assessed using the mixing protocol and thermodynamic approach employed in the study of mixed surfactant systems. From the surface tension data, critical micelle concentration of the mixed system was determined and it showed synergism in the region where the solution is clear. The mole fractions of the drug and surfactant, and the interaction parameter were evaluated in the micellar phase and at the solution surface. The hydrodynamic diameters of the drug–surfactant aggregates were determined using dynamic light scattering measurements and the aggregates formed in the middle composition region are very large in size. The SDC aggregates are able to entrap water-soluble THC. From the transmission electron microscopy images, the mixed micelles appear to have spherical and prolate ellipsoidal shapes. It has been shown that activity coefficients and interaction parameter corresponding to equimolar mixed micelle can be calculated directly without resorting to iteration and these values can serve as reference values for verifying the correctness of the iterated values of mole fractions, activity coefficients and interaction parameter in the mixed micelle.