Interaction and Micellar Behavior of Binary Mixture of Amino Sulfonate Amphoteric Surfactant with Octadecyltrimethylammonium Bromide in Aqueous Solutions of NaCl

The interaction behavior and the process of mixed micellization for binary mixtures of an amino sulfonate amphoteric surfactant sodium 3-(N-dodecyl ethylenediamino)-2-hydropropyl sulfonate (C12AS) and a cationic surfactant octadecyltrimethylammonium bromide (OTAB) in aqueous solution of NaCl (0.250 M) at 313.15 K were investigated using both the tensiometry and the UV vis spectrometry using pyrene as a probe. The mixed critical micelle concentration (cmc) of the C12AS/OTAB mixtures were determined by two techniques. Based on regular solution theory, pseudophase separation model, Clint's model, Rosen's model, and Rubingh's model, some parameters (including the ideal mixed cmc, the compositions in mixed micelle, the interaction parameters between two surfactants, the activity coefficients in mixed micelle, some thermodynamic parameters, etc.) were evaluated and calculated. The deviation of mixed cmc from its ideal value indicates a nonideal mixing between two surfactants. In comparison with those in aqueous solutions, the presence of NaC1 induces some differences in the dependence of compositions in mixed micelle on its content in bulk solution, and there has a stronger synergism between two surfactants. Thermodynamic parameters shows that the mixed micellization of the C12AS/OTAB mixtures is an entropically spontaneous process, while the addition of NaC1 to the solution can partly suppress the contribution of entropy to the process of micellization. The electrostatic attraction between two surfactants, the steric effect of hydrophilic group of C12AS, and the interaction repulsion between head groups of individual surfactants may play vital roles on the process of micellization. These findings help with probing the effect of additives on the process of micellization and understanding the interaction behavior between surfactants so as to design surfactant formulations.