Mixed micelle study of cetyltrimethylammonium bromide and 1-(2-hexadecyloxy-2-oxoethyl)-1-methylazepanium bromide in water and organic-water solvent mixture at different temperatures

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Binary mixed micelle formation of cationic-cationic surfactants are commonly studied using cationic surfactants consisting different alkyl chain lengths in order to determine the role of hydrophobic interactions in mixed micelle formation. In the present binary mixed micelle study, cationic surfactants—cetyltrimethylammonium bromide (CTAB) and 1-(2-hexadecyloxy-2-oxoethyl)-1-methylazepanium bromide (HDOMAB) are systematically investigated in water, 10% and 20% ethylene glycol as well as 1,4-dioxane at different temperature (298.15–313.15) using conductivity technique. Experimentally obtained cmc values were lower than cmc* values at lower mole fraction of CTAB, indicating synergistic interaction. However, as the mole fraction of CTAB increased, cmc values became higher than cmc* values, revealing transition from synergistic to antagonistic micellization behaviour. This composition-dependent transition was supported by the interaction parameter ( β ), activity coefficient ( f₁Rub and f₂Rub ) and the excess Gibbs free energy obtain from Rubingh model ( Δ GₑₓRub ). Additionally Motomura and Rodenas model were also used. Further, thermodynamic analysis revealed that the negative Gibbs free energy and enthalpy values indicates the micellization is spontaneous and exothermic process, while the positive entropy values suggests that the process of micelle formation is driven by both enthalpic and entropic contribution. Overall, the results demonstrate that a slight difference in the hydrophobicity of surfactants due to the presence of ester functionality can significantly influence the mixed micelle interaction and produces a composition-dependent transition from synergistic to antagonistic behaviour. The obtained results provides insight into non-ideal mixed micelle formation and show that the surfactant composition and solvent environment are important factors that control interfacial properties and thermodynamic of binary cationic surfactant system.