Mixed-micelle formation in the binary mixtures of dodecyltrimethylammonium bromide (DTABr) and cetyltrimethylammonium bromide (CTABr) surfactants in water-ethanolamine mixed solvent systems has been studied by conductometric method in the temperature range of 298.1 to 313.1？K at 5？K intervals. It was observed that the presence of ethanolamine forced the formation of mixed micelle to lower total surfactant concentration than in water only. The synergistic interaction was quantitatively investigated using the theoretical models of Clint and Rubingh. The interaction parameter β12 was negative at all the mole fractions of DTABr in the surfactant mixtures indicating a strong synergistic interaction, with the presence of ethanolamine in the solvent system resulting in a more enhanced synergism in micelle formation than in water only. The free energy of micellization values was more negative in water-ethanolamine mixed solvent system than in pure water indicating more spontaneity in mixed micelle formation in the presence of ethanolamine than in pure water. 1. Introduction Mixed micelles are widely recognized and employed in numerous fields ranging from biological systems to technical applications such as in detergency, cosmetic, pharmaceuticals, enhanced oil recovery, and as flotation agents. This is because they provide a direct and convenient approach to improve physical stability as well as better chemical and surface active properties over the individual surfactant; hence little quantity is required for any application [1–4]. The physicochemical properties of these systems are dependent on environmental conditions such as the presence of additives, with different additives behaving differently in mixed surfactant systems. In a given medium (usually, water or binary mixtures of either two nonaqueous solvents or nonaqueous solvent with water), amphiphilic molecules self-aggregate together to minimize the unfavorable interaction with the solvent (medium) and form different types of aggregates (depending on their properties and the other physicochemical conditions which are experienced by them). Hydrophobic or, more generally, solvophobic interactions play an important role in raising the above situation and, therefore, several studies have been made by altering the medium (solvent) properties either by the incorporation of additives [5–10] or by mixing with other solvents [11–19]. The effects of different kinds of additives including glycols, hydrazine, adn formamide on the micellization of single surfactants have been widely studied [20–23]. These solvents
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