Journal of Colloid and Interface Science 316 (2007) 787–795 www.elsevier.com/locate/jcis Role of the solvophobic effect on micellization María Luisa Moyá ∗ , Amalia Rodríguez, María del Mar Graciani, Gaspar Fernández Departamento de Química Física, Universidad de Sevilla, C/Profesor García González 2, 41012 Sevilla, Spain Received 31 May 2007; accepted 19 July 2007 Available online 26 July 2007 Abstract Experimental data of amphiphiles aggregation phenomena in water–organic solvent mixtures were considered with the idea of investigating the role of the solvophobic effect on micellization. Changes in the critical micelle concentration, in the micellar ionization degree (for ionic surfactants) and in the aggregation number accompanying variations in the composition of the bulk phase of the micellar solutions were examined with the scope of understanding which properties of the water–organic solvent mixtures are important in the micellization process. Results point out that the cohesive energy density, measured either through the Hildebrand–Hansen solubility parameter or the Gordon parameter, seems to play an important role in determining the contribution of the solvophobic effect on the Gibbs energy of micellization in water–organic solvents mixtures.  2007 Elsevier Inc. All rights reserved. Keywords: Micellization; Solvophobic effect; Water–organic solvent mixtures; Polarity; Cohesive energy density; Solubility parameter 1. Introduction Surfactants are widely used in both industry and everyday life, and the properties of surfactant aqueous solutions have received considerable attention. Formation of micelles and its dependence on environmental factors (temperature, additives, etc.), thermodynamics of formation, counterion binding, ag- gregation numbers, catalyzing functions, etc., are important physicochemical aspects that need detailed and intensive at- tention for both fundamental understanding and application prospects. Changing solvent quality provides the opportunity to study the role of the so-called solvophobic effect [1] and the increasing use of surfactants in applications which require wa- ter free or water poor media [2] makes this type of research more interesting. Most of the investigations carried out focused mainly in two aspects: the requirement from a solvent for am- phiphilic assembly and what the structural properties of the aggregates formed in these media are. The solvents which bring about self-association of conventional amphiphiles have high cohesive energies and dielectric constants and considerable hy- drogen bonding ability [3]. Evans et al. [4] suggested that the * Corresponding author. E-mail address: moya@us.es (M.L. Moyá). hydrogen bonding ability is a prerequisite for micellization to happen. The gradual replacement of water with other polar sol- vents allows one to explore a wide bulk phase polarity range and its influence on micellization. In the present work, changes in the cmc, in the micellar ionization degree, in the aggrega- tion number and in the polarity of the interfacial region caused by the presence of various weight percentages of glycerol, GLY, in water–glycerol dodecyl-, tetradecyl- and hexadecyltrimethyl- ammonium bromide micellar solutions, with wt% GLY up to 50%, have been reported. Experimental data obtained previ- ously by our research group and other groups were also con- sidered in order to understand which properties of the bulk phase of the micellar solutions determine the contribution of the solvophobic effect on the Gibbs energy of micellization. The scope of this work is to find a way of fitting together all the experimental data obtained in different binary aqueous mix- tures for a given surfactant. The authors want to stress that in this manuscript only organic solvents which are mainly located in the bulk phase of micellar solutions will be considered. Sol- vents which incorporate to some degree into the micelles caused changes in the characteristics of the aggregates not only because of variations in the bulk phase properties (solvophobic effect), but also because of their incorporation into the micellar aggre- gates. The contributions of the two effects cannot be separated. 0021-9797/$ – see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.07.035