Mechanism on the interaction between amimo sulfonate amphoteric surfactant and sodium dodecyl benzene sulfonate in aqueous solution

The micellar properties of binary mixtures of sodium dodecyl diamino sulfonate (C12AS) and sodium dodecyl benzene sulfonate (SDBS) in aqueous solution at the pH of 6.5 have been characterized employing both conductometric techniques and isothermal titration microcalorimetry (ITC). The critical micelle concentration (cmc) values for the individual surfactants and their corresponding mixtures were determined from both the conductivity measurements and ITC. The interaction parameters between the surfactants, the activity coefficients and the molar fractions of components in the mixed micelles, and the thermodynamic parameters calculated by various approaches, like Rubingh models, were evaluated. The results show that the synergistic effect between C12AS and SDBS in all systems plays an vital role in the reduction of the overall cmc value in aqueous solution at 25 °C, and that the steric effect of the head group, the lower charge density, and the capability of accepting proton on the molecule for C12AS affect the formation of mixed micelle. Thermodynamic data show that the micellization for binary mixture of C12AS and SDBS can occur spontaneously, and is both enthalpy and entropy driven process.