The Composition of Mixed Micelles Formed by Dodecyl Trimethyl Ammonium Bromide and Benzethonium Chloride in Water

Abstract

Abstract The composition of the mixed micelles formed by benzethonium chloride and dodecyl trimethyl ammonium bromide in water were experimentally determined by a combination of surfactant ion‐selective electrodes and UV–Vis spectroscopy measurements. Results were compared with the computed compositions from three theories on mixed micelles available in the literature (Regular Solution, Motomura’ and Georgiev's). The basis of the Junquera and Aicart method and the determination of the micelle aggregation number ( N ) by pyrene fluorescence quenching, i.e., the constancy of N and the mixed micelle composition when the total concentration is changed, were not supported by our experimental results. Experimental micelle compositions were statistically equal to those computed by Regular Solution Theory. The mixed micelle composition and the quantity of non‐micellized surfactant molecules change with concentration above the critical micelle concentration, and therefore do not support the assumptions used in the Junquera and Aicart procedure and in the determination of N by pyrene fluorescence quenching. The reason why the latter procedure gives N values similar to those obtained from other techniques is discussed.