The critical micellar concentrations (cmc's) of sodium dodecyldiaminosulfonate amphoteric surfactant (C12AS) in aqueous solutions containing seven inorganic salts with chloride anions were measured by tensiometry using the Wilhelmy plate method at 25 °C. The mathematical relationship between the cmc of C12AS and the concentration of an inorganic salt in an aqueous solution was established. The effect of an inorganic salt on micellization of C12AS was also discussed. The results show that the dependence of the cmc of C12AS on an inorganic salt within the range of the effective concentration of the inorganic salt investigated in this experiment can be well described by the linear equation. The decrease of the cmc with an inorganic salt may be mainly attributed to the salting-out effect of the hydrophobic moiety of C12AS. Except for the salting-out effect, the causes can be partly from the vital contributions of both the electrostatic interaction between ionic heads of C12AS and the hydration of inorganic ions in the process of micellization. Also, the type of inorganic ion can affect the process of micellization. The data of Gibbs energy changes show that there is a spontaneous process of micellization, and the addition of an inorganic salt in an aqueous solution is favorable thermodynamically to the process of micellization.

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In aqueous solution at a pH of about 6.5, the micellization of binary mixtures constituted by sodium n-dodecyl diamine sulfonate (C12AS) and three octylphenol polyoxyethylene ether (OP-n) with different numbers of oxyethylene glycol ether (n), including OP-4, OP-7, and OP-10, was examined by the UV−vis spectroscopy method. The critical micelle concentration (cmc) values were determined by the UV−vis spectroscopy method and the tensiometric method. The experimental data were analyzed according to various mixing thermodynamic models within the framework of the pseudophase separation model. For all mixed systems, the negative deviation of experimental cmc values from the ideal values calculated from the Clint equation was found, and negative values for the interaction parameter were obtained, suggesting nonideal mixing of two surfactants in aqueous solution and a synergistic effect between surfactants. Among the C12AS/OP-n mixed systems, the stronger hydrophilicity of nonionic surfactant results in an increase of values of minimum cmc or optimum molar fraction, where the maximum synergism between surfactants occurs, and the C12AS/OP-7 system presents the highest degree of synergism between surfactants in aqueous solution, resulting from their suitable molecule structure. The electrostatic interaction between head groups of C12AS and the effects of steric character of head groups of both nonionic surfactant and C12AS together affect the mixed micelle formation. Thermodynamic data show that the micellization for all the binary mixed systems is spontaneous and is an enthalpy-and entropy-driven process, and the C12AS/OP-7 system has the greatest stability of micellization.

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