The mixed micellar properties of double-chained surface active ionic liquid (1, 3-didecyl-2-methylimidazolium chloride or TEGO) with two conventional cationic surfactants, dodecyltrimethylammonium bromide (DOTAB) and cetyltrimethylammonium bromide (CTAB) were investigated by using surface tension and conductivity experiments in aqueous solution at 303.15 K. TEGO is a special surface active agent with two specific critical micelle concentrations (CMC). To consider the effect of hydrophobic groups in synergism between components (surfactant and TEGO) in the mixed micelle, two cationic surfactants were selected that differed only in the length of the hydrocarbon chain (DOTAB: C12 and CTAB: C16). The experimental critical micelle concentration (CMCexp), degree of micellar dissociation (g), the ideal critical micelle concentration (CMCid), micellar mole fractions (X1m\documentclass[12pt]{minimal}
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\begin{document}$${\rm{X}}_2^m$$\end{document}) and the interaction parameter (βm) were determined by using Rubingh’s model. The achieved βm of the studied system is negative in the whole compositions denoting the synergistic interaction between components and their values increase with increasing chain lengths of surfactant from DOTAB to CTAB. The activity coefficients (f1m\documentclass[12pt]{minimal}
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\begin{document}$${\rm{f}}_2^m$$\end{document}) are always less than unity in all mole fractions signifying non-ideality in the mixtures. Thermodynamic functions for mixed systems were estimated. The standard Gibbs energy of micellization (ΔGmic0\documentclass[12pt]{minimal}
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\begin{document}$$\Delta {\rm{G}}_{mic}^0$$\end{document}) associated with transfer of surfactant monomers from the bulk phase to micelle phase was evaluated according to Zana’s model in different situations and discussed with logical points in the new approach. The results show that the effect of hydrophobicity can regulate the synergism between cationic surfactants in the same electrical charges of head groups.
