Notes Temperature-Sensitive Critical Micelle Transition of Sodium Octanoate Alfredo Gonza ´ lez-Pe ´rez, Juan M. Ruso, Gerardo Prieto, and Fe ´lix Sarmiento* Group of Biophysics and Interfaces, Department of Applied Physics, Faculty of Physics, University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain Received September 15, 2003. In Final Form: November 19, 2003 1. Introduction The particular geometrical characteristics of surfactants can determine the type of self-assembly structure they take on in solution. Depending on the packing parameter, the surfactant can self-organize into different kinds of structures such as spherical micelles, rodlike micelles, lamellar phases, vesicles, and so forth. 1 Changes in these self-organized structures can be brought on by modifying the properties of the solvent. 2 One interesting transition is the so-called sphere to wormlike transition that results in an elongation of the micelle. 3 This wormlike transition is characterized by its viscoelastic properties and studied by applying the theories of De Gennes, 4 originally devel- oped for polymer solutions and later modified by Cates et al. 5 to study wormlike micelles in solution. In the past few years, many papers have been written on sphere to rodlike transitions in micellar solutions induced by the addition of salts and alcohols, 6-10 but few works describe the micellar transitions produced with the increase in sur- factant concentration. It has been reported that micelles can evolve from spherical to ellipsoidal shape as the hydrocarbon chain length increases for alkyltrimethy- lammonium bromides. 11,12 Recent papers of Woo et al. 13 show the micellar transition structure of micelles for a few compounds and report that the change in shape appears at a factor of 2 for the ratio CMT/CMC (where CMT is defined as the concentration at which the transition of spherical to nonspherical micelles occurs, and CMC is the concentration at which first spherical micelles are formed). More recent works of Gonza ´ lez-Pe ´rez et al. 14 do not confirm this relationship; they found that the transition appears to be strongly dependent on the alkyl chain length for alkyltrimethylammonium bromides and alkyldimethylbenzylammonium chlorides. Aqueous solutions of sodium octanoate have been studied attending to their special properties of self- assembly because they are a limit case of micelle formation; they show a high critical micelle concentration and very low aggregation number. 15 Due to the small aggregation number of sodium octanoate micelles, this compound has been frequently used for molecular dynamics simulation studies and included in the modeling of the solubilized alcohols. 15-17 The primary works on the development of micellar properties of sodium octanoate in aqueous solution have been performed by Ekwall et al. 18-23 In these papers, the authors describe a second break in the molality dependence of many physical magnitudes such as con- ductivity, density, viscosity, and vapor pressure osmom- etry. This second break was interpreted as a critical micelle transition. Recently, the temperature dependence of the CMC was determined by D’Angelo et al. 24 The aim of the present work is to study the temperature dependence of the critical micelle transition of sodium octanoate measuring conductivity, density, and sound velocity in a range of concentrations above the critical micelle concentration. As far as we are aware, the variation of the CMT of this surfactant with temperature has not been previously reported. 2. Experimental Section 2.1. Materials. Sodium octanoate (CAS 1984-06-1) obtained from Lancaster Synthesis (No. 10241, 97%) was used for density and ultrasound velocity measurements, whereas that purchased from Sigma Chemical Co (C-5038, +99%) was used for conduc- tivity determinations. Experiments were carried out using double- distilled water. 2.2. Density and Ultrasound Velocity Measurements. Densities and ultrasound velocities of aqueous solutions of sodium octanoate were continuously, simultaneously, and automatically measured using a commercial density and ultrasound velocity measurement apparatus (Anton Paar DSA 5000 densimeter and a sound velocity analyzer). Both the speed of sound and density are extremely sensitive to temperature, so it was kept constant within (10 -3 K using the Peltier method. Before each series of * Corresponding author. E-mail: fsarmi@usc.es. Tel: +34 981 563 100. Fax: +34 981 520 676. (1) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (2) Rosen, M. J. Surfactant and Interfacial Phenomena, 2nd ed.; John Wiley & Sons: New York, 1989. (3) Hoffmann, H.; Thuning, C.; Schmiedel, P.; Munkert, U.; Ulbricht, W. Tenside, Surfactants, Deterg. 1994, 31, 389. (4) De Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: New York, 1979. (5) Cates, M. E.; Candau, S. J. J. 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