LAUSR

The micellization behavior of amphoteric sodium 3-(N-dodecyl ethylenediamino)-2-hydroxypropyl sulfonate (C12AS) and nonionic octylphenol polyoxyethylene ether having ten oxyethylene glycol ether (OP-10) was investigated in water–isopropanol (IPA)–sodium chloride (NaCl) solution. The critical micelle concentration (cmc) including the single cmc for the single surfactant and the mixed cmc for the binary mixture of C12AS/OP-10 was determined by both tensiometry and the UV–Vis spectrometry. The mole fraction of component (X1 and $$X_{1}^{{{\text{ideal}}}}$$X1ideal) and activity coefficients (f1 and f2) in mixed micelle, the interaction parameter (β12) between two surfactants, and thermodynamic parameters (ΔGM and (∆Gmic) were estimated according to the pseudophase separation model, the Clint’s model, the Rubingh’s treatment, the Maeda’s treatment, etc. Based on the regular solution theory, cosolvent effects on the interaction between two surfactants were analyzed. It shows that the mole fraction of C12AS in mixed micelle decreases with the concentration of IPA. The addition of IPA in the water/NaCl solution induces a non-monotonous change in the synergistic effect between the two surfactants. Thermodynamic data show that the increase in the concentration of IPA disfavors the spontaneous process of micellization. These phenomena are attributed to the changes in the hydrophobic effect of the hydrophobic group of the surfactant, the electrostatic repulsion between the ionic head groups of C12AS, and the steric effect of the head groups of two surfactants on adding IPA.