Journal of Colloid and Interface Science 294 (2006) 449–457 www.elsevier.com/locate/jcis Interactions in binary mixed systems involving a sugar-based surfactant and different n-alkyltrimethylammonium bromides J.M. Hierrezuelo, J. Aguiar, C. Carnero Ruiz ∗ Grupo de Fluidos Estructurados y Sistemas Anfifílicos, Departamento de Física Aplicada II, Escuela Universitaria Politécnica, Universidad de Málaga, Campus de El Ejido, 29013 Málaga, Spain Received 27 May 2005; accepted 14 July 2005 Available online 25 August 2005 Abstract In this paper, mixtures of sugar-based decanoyl-N -methylglucamide with three different n-alkyltrimethylammonium bromides (n = 12 (DTAB), 14 (TTAB), and 16 (CTAB)) have been studied using conductance and fluorescence spectroscopic techniques. The critical micelle concentration values of pure and mixed systems were determined by both the conductance and the pyrene 1:3 ratio methods. The experimental results were interpreted using thermodynamic mixing approaches based on the pseudophase separation model. These analyses allowed us to determine the interaction parameters and the composition of the mixed micelles through the whole composition range. Since all the ionic surfactants used in this study have the same headgroup, the differences observed between the three mixed systems were attributed to the lengths of their hydrocarbon chains. It was established that, besides interactions of electrostatic character, additional short-range interactions must be considered. By using the static quenching method, the mean micellar aggregation numbers of mixed micelles were obtained. In the cases of the mixed systems with DTAB and TTAB it was observed that the aggregation number is initially reduced with the participation of the ionic component, remaining almost constant and close to the aggregation number of the pure ionic micelle. However, in the systems involving CTAB it is observed that the size of micelles initially increases and then decreases slightly for mixtures with a high content of the ionic component. The hydrophobic index pyrene 1:3 ratio was used to examine possible changes in the micellar micropolarity; however, no definitive conclusions could be derived from these experiments. In order to study the evolution of the local viscosity of the mixed micelles upon addition of the ionic surfactant, fluorescence polarization measurements were carried out with two different probes, fluorescein and coumarin 6. It was found that the participation of the ionic component in the mixed micelle induces the formation of less ordered structure than that of pure nonionic micelles. An attempt was made to correlate these effects with the interaction parameters obtained from the theoretical mixing model and, consequently, with the alkyl chain length of the ionic components.  2005 Elsevier Inc. All rights reserved. Keywords: Mixed micelles; MEGA-10; n-Alkyltrimethylammonium bromides 1. Introduction The increasing interest in studies of systems constituted by surfactant mixtures is motivated by the fact that they show synergistic interactions in solution. These interactions can be manifested as enhanced surface activity, spreading, wetting, foaming, detergency, and many other phenomena [1]. In fact, it is well known that in most uses and industrial applications mixed surfactant systems are more efficient than pure surfac- * Corresponding author. E-mail address: ccarnero@uma.es (C. Carnero Ruiz). tants [1–5]. In addition, these mixed systems are also interesting from a theoretical point of view. This is because it is important to get thermodynamic models at mixing with both interpreta- tive and predictive abilities. The most relevant advances in this area achieved until 2001 have been reviewed by Hines [5]; how- ever, new treatments have emerged in the past few years. In this respect, it is noteworthy to mention the phenomenologi- cal procedure recently reported by Maeda [6] for analyzing the stability and the synergism of ionic/nonionic mixed micelles in salt solutions. Mixed surfactant systems constituted by common surfac- tants, such as linear alkyl sulfonates, alkyl sulfates, alcohol ethoxylates, and alkyltrimethylammonium salts have been ex- 0021-9797/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.07.024