Micellization behavior of a catanionic surfactant with high solubility mismatch: Composition, temperature, and salt effects Ricardo M.F. Fernandes, Eduardo F. Marques ⁎, Bruno F.B. Silva, Yujie Wang Centro de investigação em Química, Department of Chemistry, Faculty of Science, University of Porto; Rua do Campo Alegre 687, 4169-007, Portugal abstract article info Article history: Received 28 May 2010 Received in revised form 27 August 2010 Accepted 30 August 2010 Available online 29 September 2010 Keywords: Salt-free catanionic surfactant Mixed micelle Surface tension Critical micelle concentration Conductivity Non-ideal mixing Solubility mismatch While cationic/anionic surfactant mixtures have been reported to form strongly non-ideal mixed aggregates in aqueous solution, the micellization of true catanionic surfactants, i.e. the salt-free ion-paired compounds obtained from the mixtures, has been much less investigated. However, if there is a significant solubility mismatch between the paired chains, these surfactants may show less conventional aggregation features in water, such as a temperature-driven vesicle–micelle transition and the coexistence of two lamellar phases in equilibrium. In this work, we carry out micellization studies for a catanionic surfactant of the type C 16 + C 8 - , cetyltrimethylammonium octylsulfonate (TASo), by surface tension and conductivity measurements. For comparison purposes, the catanionic mixture cetyltrimethylammonium bromide (CTAB)/sodium octylsulfo- nate (SOSo) at different mixing ratios and the neat SOSo have also been studied. For the non-ideal CTAB/SOSo mixture, Rubingh's model has been applied to determine the β interaction parameter and the mixed micelle composition. The cmc of TASo has been investigated as a function of temperature, salt and excess ionic surfactant, displaying some differences with respect to conventional ionic and nonionic surfactants. The trends in the thermodynamic parameters of micellization of the three types of systems are comparatively rationalized. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Surfactant mixtures have a relevant role in a variety of technical applications, like those related with enhanced interfacial performance or control of the rheological properties of materials. The mixed system almost invariably yields enhanced interfacial properties (e.g. de- creased critical micelle concentration, higher surface activity, etc.) with respect to the individual surfactants and to ideal behavior, in what is usually termed as synergism [1,2]. In particular, aqueous mixtures of two oppositely charged surfactants—the so-called catanionic mixtures, pseudo-three compo- nent systems [3,4]—exhibit a solution behavior that deviates considerably from ideality. This is manifested by a strong decrease of the cmc with reference to the ideal mixture cmc, along with multifaceted associative phase behavior, such as formation of large micelles, spontaneous vesicles, solid precipitates and dispersed liquid crystalline particles. This phase behavior can be manipulated inter alia upon changing the cationic/anionic ratio [3,5–8], adding salt [9], or changing temperature [10], total concentration [11] or pH [12]. With respect to the micellization process, a common way of expressing the degree of interaction between two surfactants is the β interaction parameter as calculated from the regular solution theory and Rubingh's non-ideal model [1,13]. A number of studies of different types of surfactant mixtures have been reported in order to evaluate the effect of headgroup chemistry and alkyl chain length on the β parameter [14–21]. Directly related to catanionic mixtures, but with an interesting nuance from a thermodynamic point of view, are the so-called catanionic surfactants. These compounds are built up from stoichio- metric amounts of cationic and anionic surfactants yielding charge neutrality, with complete removal of the inorganic salt [3,4,9,22]. With water, they form simple two-component systems and typically behave as swelling double-chained amphiphiles with high Krafft point [23]. Catanionics with highly asymmetric chain lengths, however, can be water-soluble at room temperature, as is the case of cetyltrimethylammonium octylsulfonate (TASo) [10,11]. Due to the solubility mismatch between the two amphiphilic ions, this com- pound is soluble at 25 °C, self-assembling into micelles first and vesicles at higher concentration [10]. Below 3 wt.% in surfactant, the vesicles transform into elongated micelles upon temperature increase, and beyond 15 wt.%, two lamellar phases form at 25 °C [11]. The driving force for the temperature-induced vesicle–micelle transition is likely the increase in solubility of octylsulfonate, So - , which should increase the aggregate charge and film spontaneous curvature [10]. In this work the main goal is to probe the effect of excess ionic surfactant, salt and temperature on the micellization behavior of TASo, a type of highly asymmetric catanionic surfactant whose micellization has not been investigated before. It has been suggested Journal of Molecular Liquids 157 (2010) 113–118 ⁎ Corresponding author. Tel.: + 351 22 0402 535; fax: + 351 22 0402 559. E-mail address: efmarque@fc.up.pt (E.F. Marques). 0167-7322/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2010.08.014 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq