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Three cationic surfactants labeled as APS-8 ((Z)-4-(((1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)imino)methyl)-N,N-dimethyl-N-octylbenzenaminium bromide), APS-12 ((E)-4-(((1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)imino)methyl)-N-dodecyl-N,N-dimethylbenzenaminiumbromide), and APS-16 ((E)-4-(((1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)imino)methyl)-N-dodecyl-N,N-dimethylbenzenaminium bromide) were opportunely synthesized in order to study the effect of the hydrocarbon chain length on the process of micellization in mixed surfactant systems. The critical micelle concentration ( CMC ) values of binary mixtures containing APS-x (x = 8, 12, 16) and a conventional cationic surfactant (cetyltrimethylammonium bromide, CTAB) were retrieved by using surface tension measurements. The behavior of the mixed systems has been analyzed in the light of Rubingh’s regular solution theory. In particular, two parameters describing respectively the interactions at the air/water interface and in the micellar phase were obtained for each system. Results show that micellization and adsorption properties of the mixed systems depend on the hydrophobic chain length and on the interaction of APS-x with CTAB. Furthermore, applying the regular solution theory (RST) to the experimental data allowed obtaining the interaction parameter of the mixed micelles (β) and the air–water interface (β σ ). Results indicate an attractive interaction between the micelles and reveal a synergistic effect between the two components of the mixtures both in micelles and at the interface. The activity coefficients and the experimental  CMC  values in micelles indicate synergism less than as well as at the interface.

Synergistic interaction in cationic antipyrine/CTAB mixed systems at different phases