Equilibrium and dynamic surface properties of cationic/anionic surfactant mixtures based on bisquaternary ammonium salt

The interaction behavior and the process of mixed micellization for binary mixtures of bisquaternary ammonium salt (BQAS) and sodium dodecyl sulfate (SDS) were investigated using the surface tensionmeter, and the synergistic properties of these mixtures were evaluated. The interaction parameters and thermodynamic parameters of BQAS/SDS mixtures were calculated based on the regular solution theory. Moreover, the dynamic surface tension curves of these mixtures were examined using bubble profile analysis method, and the corresponding kinetic parameters were obtained. Results indicate that the critical micelle concentrations of the BQAS: SDS mixtures with the molar ratios of 3:7, 5:5, and 7:3 are 4.0 × 10−5 mol/L, 5.0 × 10−5 mol/L and 6.0 × 10−5 mol/L, respectively, which are two orders of magnitude lower than that of either component. Despite the mixing ratios vary from 3:7 to 7:3, these mixtures have almost equal surface tensions. Consequently, the component of adsorption layer is dominated by the catanionic surfactant complexes, rather than the mixing ratios in the bulk phase. The interaction parameters βσ and βm of binary systems with the BQAS: SDS molar ratios of 5:5 are −18.11 and −15.56, respectively, indicating the strong synergistic effects in the reduction of surface tension and the formation of micelle. The thermodynamic parameters suggest that the formation of micelles is an exothermic and entropy increase process. Moreover, results of dynamic surface tension indicate that the BQAS/SDS mixtures exhibit a faster adsorption process than either pure component, resulting from the enhanced hydrophobicity of catanionic surfactant complexes. In addition, the initial stage of adsorption of BQAS/SDS mixtures exhibits a mixed kinetic-diffusion mechanism.