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The micellization behavior of two binary mixtures of an anionic sodium dodecylbenzene sulfonate with a nonionic surfactant octylphenol polyethoxylated ether (including OP-10 or OP-7) was investigated in aqueous solution at 298.15 K using the tensiometry, and the effect of hydrophilic chain of nonionic surfactant was discussed. In the framework of pseudophase separation model and based on the regular solution theory, the related micellization parameters and thermodynamic parameters were estimated by some models including Clint’s model, Rubingh’s model, etc. The two mixtures exist some differences in the micellization behavior. The synergistic effect, the antagonism, nonideal mixing and ideal mixing are observed in the two mixtures. The dependence of the mixed critical micelle concentration, the composition in mixed micelle, the interaction parameter between molecules on the composition in aqueous solution is explained by the electrostatic interaction, the steric effect and the shell structure of mixed micelle, etc. Thermodynamic parameters are adopted to account for the difference in energy and verify the micellization behavior. The two binary mixtures show different contributions of energy to the process of micellization. These findings help with understanding the nature on the interaction between two surfactants and the effect of the length of hydrophilic chain of nonionic surfactant so as to obtain some information in theoretical and practical investigations.

The micellization behavior and the molecular interaction of quasi-binary mixture constituted by an amphoteric sodium N-acyl amino sulfonate based on castor oil (AAS-CO) with a cationic stearyltrimethyl ammonium bromide (STAB) in aqueous solution at 298.15 K were investigated using both the tensiometry and the conductometry and the two techniques were adopted to determine the mixed critical micelle concentration (cmc). Based on the regular solution theory and in the framework of the pseudophase separation model, Rubingh’s model and Clint’s model were adopted to estimate the mole fractions and the activity coefficients in mixed micelle, the interaction parameters between quasi-one AAS-CO and STAB. Also, thermodynamic parameters were obtained by thermodynamic models. Synergistic interaction between surfactants and non-ideal mixing were observed in the entire range of composition in this investigation. The addition of quasi-one AAS-CO into aqueous solution can induce the alteration of interaction behavior between surfactants, which can be explained rationally by the shielding effect, the electrostatic repulsion and the steric hindrance. Thermodynamic parameters imply that it appears an enthalpically spontaneous behavior in the process of micellization and there exists a largest negative deviation of free energy change of micellization from the ideal case at the optimum composition in which a maximum synergism happens. These findings will help with understanding the molecular interaction between quasi-one component and other one in surfactant mixtures.