LAUSR

Abstract The micellization behavior of the mixed system of amphoteric sodium 3-(N-dodecyl ethylenediamino)-2-hydropropyl sulfonate (C12AS) and cationic octadecyltrimethyl ammonium bromide (OTAB) in water/isopropanol (abbr. IPA, 20 g·L−1) solution at 40 °C was investigated using both the tensiometry and the conductometry. According to the regular solution theory, pseudophase separation model and other thermodynamic models (including the Rubingh’s model, the Rodenas’s model, Lange’s model, etc.), some parameters were estimated to achieve a full understanding of their aggregation behaviors. For all the mixtures of C12AS/OTAB, the mixed critical micelle concentration and the composition of C12AS in mixed micelle have deviations from their ideal cases, indicating a non-ideal mixing. The presence of cosolvent IPA produces the changes in the composition in mixed micelle and the optimum mixing ratio which can obtain a maximum synergism. The interaction parameter indicates an attractive interaction between two surfactants. Thermodynamic parameters show that the mixed micellization is an entropically spontaneous process and the presence of IPA seemingly reduces the entropy contribution to the formation of mixed micelle. The above behaviors can be explained rationally by the hydrophobic effect, the steric effect and the attractive interaction between two surfactants. These findings are useful for optimizing the composition of mixed surfactant systems and understanding the interaction behavior between surfactant molecules to further achieve more effective and economical formulations. Graphical Abstract