Journal of Colloid and Interface Science 235, 201–209 (2001) doi:10.1006/jcis.2000.7364, available online at http://www.idealibrary.com on Effect of Temperature on Micelle Formation in Aqueous NaBr Solutions of Octyltrimethylammonium Bromide Ryszard Zieli´ nski Department of Technology and Environmental Protection, Faculty of Commodity Science, Pozna´ n University of Economics, Aleja Niepodlegl o´ sci 10, 60-967 Pozna´ n, Poland E-mail: zielinsk@novci1.ae.poznan.pl. Received September 24, 1997; accepted December 4, 2000 Association processes in aqueous solutions of octyltrimethylam- monium bromide, C 8 TAB, have been studied in aqueous NaBr so- lutions at temperatures from 20 to 55 ◦ C. The values of the critical micelle concentration, CMC, were determined from the intersec- tions of two straight line portions of the plots of the relationship between adiabatic compressibility of the solutions and surfactant concentration. The value of the CMC thus determined exhibits min- ima at a certain temperature, T min . The value of T min shifts toward a lower temperature with increasing NaBr concentration. Based on the most probable micelle size model, the values of thermodynamic functions of micelle formation have been estimated at various tem- peratures. C  2001 Academic Press Key Words: amphiphiles; critical micelle concentration; micelles; octyltrimethylammonium bromide;thermodynamics. 1. INTRODUCTION In aqueous solutions of ionic surfactants their molecules can exist in two different forms depending on the surfactant concen- tration. At concentrations lower than the critical micelle concen- tration, CMC, they can exist in the monomeric form, while at concentrations exceeding the value of the CMC, they can form micellar aggregates being in equilibrium with the monomers. Association of ionic surfactants in aqueous solutions is in- duced by hydrophobic interactions between alkyl chains of the molecules, balanced by hydration and/or electrostatic interac- tions (1, 2). Among alkyltrimethylammonium bromides (C N TAB) the ef- fect of temperature on micelle formation in aqueous solutions have been investigated for the octyl (3, 4), nonyl (5, 6), decyl (4–11), dodecyl (3, 4, 7–10, 12–15), tetradecyl (4, 9, 10, 18–20), and hexadecyl (3, 9, 10) derivatives. There have been only few publications (10, 21–33) on the effect of added salt on the micelle formation in aqueous salt solutions of alkyltrimethylammonium bromides. In previous work (4, 22, 34) we reported results of our study on the effect of the length of the alkyl chain (22, 34) and of the temperature (4) on the adiabatic compressibility of aqueous solutions of alkyltrimethylammonium bromides (C N TAB) and described the effect of the alkyl chain length of the surfactant and of the temperature on the value of the CMC. We have shown that the plots of the CMC against temperature exhibit a minimum at a certain temperature, T min , and the location of the T min value shifts toward lower temperatures with an increase in the number of carbon atoms in the alkyl chain of the surfactant. In the present work we measure the density and ultrasound velocity in aqueous solutions of octyltrimethylammonium bro- mide (C 8 TAB) in the temperature range from 20 to 55 ◦ C at 5 ◦ C intervals to elucidate information on the effect of NaBr con- centration on the nature of the solute–solvent interactions in the solutions. We discuss the experimental data in terms of the effect of the added salt concentration on the values of thermodynamic functions for the process of the micelle formation derived from the values of the CMC based on the most probable micelle size model proposed by Emerson and Holtzer (35). 2. EXPERIMENTAL 2.1. Materials Special grade samples of octyltrimethylammonium bromide (C 8 TAB) (Tokyo Kasei Kogyo Co., Ltd.) and sodium bromide (NaBr) (Wako Chemicals Co., Ltd.) were used as supplied except for being dried in vacuo at temperature below 40 ◦ C for at least 48 h before use. Purity of the surfactant sample was determined by means of pyrolysis gas chromatography. The composition of the sample based on weight percentage of C N TAB was as fol- lows: C 6 TAB, 0.11; C 8 TAB, 99.89. All solutions were prepared by weighing, using distilled and degassed water. 2.2. Measurements All measurements were carried out in the temperature range from 20 to 55 ◦ C at 5 ◦ C intervals. Each temperature was adjusted within ±0.01 ◦ C. Measurements of the density of the solutions were performed using the Ostwald-type pycnometers of 20-cm 3 capacity. The pycnometers were calibrated at each temperature using distilled and degassed water based on generally accepted 201 0021–9797/01 $35.00 Copyright C  2001 by Academic Press All rights of reproduction in any form reserved.