Colloids and Surfaces A: Physicochem. Eng. Aspects 309 (2007) 216–223 Stability of mixed micelles of cetylpyridinium chloride and linear primary alkylamines Luis Garc´ ıa-R´ ıo a,∗ , Jos´ e R. Leis a , Juan C. Mejuto c , Victor Mosquera b , Pedro Rodr´ ıguez-Dafonte a a Departamento de Qu´ ımica F´ ısica, Facultad de Qu´ ımica, Universidad de Santiago, 15782 Santiago, Spain b Grupo de F´ ısica de Coloides y Pol´ ımeros, Departamento de F´ ısica Aplicada y Departamento de F´ ısica de la Materia Condensada, Universidad de Santiago, Spain c Departamento de Qu´ ımica F´ ısica, Facultad de Ciencias, Universidad de Vigo, 32004 Ourense, Spain Received 24 July 2006; received in revised form 3 January 2007; accepted 8 January 2007 Available online 12 January 2007 Abstract The influence of chain length on the composition of micelles of a binary mixture of cetylpyridinium chloride with n-octyl, n-decyl, and n-lauryl amines was established from the variation of the critical micelle concentration (cmc) as a function of the solution composition. A synergistic effect was observed in all instances that were found to be correlated with chain length. Experimental data were compared with theoretical predictions based on the equilibrium between micelles and monomers in solution. The Motomura treatment was used to determine the composition of each compound in the mixed micelles ( X m i ). Mixing non-ideality was expressed in terms of the molecular interaction parameter (β 12 ) as determined using the theory of Holland and Rubingh. Finally, the molecular thermodynamic model for mixed surfactant systems developed by Puvvada and Blankschtein was used to estimate the micellization free energy (G M ) and to evaluate the synergistic phenomenon. © 2007 Elsevier B.V. All rights reserved. Keywords: Micelles; Amines; Cmc; Stability 1. Introduction Micellar systems consisting of mixtures of surfactants of vari- able structure are of a high theoretical and industrial interest [1–3]. Their current extensive use has been promoted by the fact that mixed micelles possess much more favorable practi- cal physicochemical properties than single-component systems. Thus, in the pharmaceutical field, mixed micelles have been found to enhance the absorption of various drugs in the human body [4,5]. Also, a number of cleaning products contain mixtures of anionic and non-ionic surfactants that facilitate dissolution and improve tolerance of water hardness [3]. In cosmetics, the synergistic behavior of mixed surfactants at their cmc enables their use at low concentrations in order to avoid potential skin irritation [6–8]. This synergistic phenomenon can also be highly beneficial for the environment as it allows the amount of surfac- ∗ Corresponding author. Tel.: +34 981 563100; fax: +34 981 595012. E-mail address: qflgr3cn@usc.es (L. Garc´ ıa-R´ ıo). tants released, and hence their impact, to be substantially reduced [9]. In this work, we used conductivity measurements to deter- mine the critical micelle concentration (cmc) of various alkylamine/cetylpyridinium chloride systems. Although mixed micellar systems have been widely studied [10–18], mixtures of cationic surfactants have been less frequently examined and substantial deviations from their ideal behavior encountered [16,19–23]. Most of the major studies on mixed cationic sys- tems have so far involved n-alkyltrimethylammonium bromide or n-alkylpyridinium chloride surfactants, and focused on the influence of chain length on specific properties of the mixtures. A few, however, have carefully examined the effect of changes in the head group on such properties [22,23]. In this work, we used binary mixtures of n-alkylamines as surfactants in order to increase the structural differences between the mixture compo- nents and hence deviations from linearity. Also, the ability to use various n-alkylamines in the mixed system afforded greater flex- ibility in examining the effect of chain length on the properties of the surfactant mixtures (Scheme 1). The ability of alkylamines to 0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.01.006