Mixed micellization of octaoxyethylene monododecyl ether and n -alkyltrimethylammonium bromides C. Carnero Ruiz *, J. Aguiar Grupo de Fluidos Estructurados y Sistemas Anfifı ´licos, Departamento de Fı ´sica Aplicada II, Escuela Universitaria Polite ´cnica, Universidad de Ma ´laga, Campus de El Ejido, Malaga 29013, Spain Received 29 January 2003; accepted 10 April 2003 Abstract The micellization process of binary mixtures formed by octaoxyethylene monododecyl ether (C 12 E 8 ) and three different n -alkyltrimethylammonium bromides, including dodecyltrimethylammonium bromide (DTAB), tetradecyl- trimethylammonium bromide (TTAB), and cetyltrimethylammonium bromide (CTAB), was examined by using the fluorescence probing method. The critical micelle concentration values were determined by the pyrene 1:3 ratio method. The experimental data were analyzed on the light of various mixing thermodynamic models within the framework of the pseudophase separation model. In all the cases, a negative deviation from the ideal behavior was found. However, the interaction parameter (b 12 ), as reported by the regular solution theory, was found to be dependent on the micellar composition. It was established that, in addition to the electrostatic interactions between the headgroups of the surfactants, secondary effects of steric character due to the different length of the alkyl chain of the n - alkyltrimethylammonium bromides must be taken into account to justify the experimental data. The mixing thermodynamic functions for the C 12 E 8  /CTAB system were determined. The change in the micellization entropy was interpreted in connection with the hydration status of the mixed micelles. The increase observed in the micellar micropolarity with the content of the ionic component, which was consistent with the corresponding change in the micellization entropy, was attributed to the formation of more open micelles with a more hydrated structure. Data obtained in this study suggest that in the case of the C 12 E 8  /DTAB and C 12 E 8  /TTAB systems, with considerable differences in the critical micelle concentration values, pure non-ionic micelles are formed in the range of low proportion of the ionic component. # 2003 Elsevier B.V. All rights reserved. Keywords: Mixed micelles; Fluorescence probing study; Octaoxyethylene monododecyl ether; n -Alkyltrimethylammonium bromides; Pseudophase separation model 1. Introduction In recent years, the study of the physicochemical properties of solutions formed by mixtures of surfactants has become a topic of increasing interest in the area of self-assembly of amphiphiles. * Corresponding author. E-mail address: ccarnero@uma.es (C. Carnero Ruiz). Colloids and Surfaces A: Physicochem. Eng. Aspects 224 (2003) 221  /230 www.elsevier.com/locate/colsurfa 0927-7757/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-7757(03)00152-3