Experimental and thermodynamic analysis of mixed micelles formed by dodecylethylmethacrylatedimethylammonium bromide and tetradecyltrimethylammonium bromide

The micellization process of a surfactant system formed by the mixture of two cationic surfactants, a conventional one, tetradecyltrimethylammonium bromide (TTAB), and a polymerizable surfactant, methylammonium dodecylethylmethacrylate (PS) bromide, was studied, using a battery of methods such as conductivity, ion selective electrodes and UV-vis spectroscopy. Critical micelles concentrations (CMCs) were obtained using traditional methods and two newer procedures proposed by Carpena and Al-Soufi. The critical micelle concentrations (CMC) of the mixtures were higher than ideal, suggesting that there is repulsion between the two components in the mixing process. We applied four thermodynamic models: Rubingh-Holland, Motomura, Letellier, and EOMMM (equation-oriented mixed micellization model) to analyze the data. This analysis showed that the PSTTAB system exhibits a highly asymmetric behavior, the PS molecules exhibit affinity for the TTAB micelles, but the TTAB molecules show repulsion for the PS micelles. Finally, the mixed micelles are richer in TTAB than the total composition of the system until the mole fraction of PS in the mixture is approximately 0.75, showing an azeotrope at this point.