The sphere—rod transition of micelles and the two-step micellization of dodecyltrimethylammonium bromide in aqueous NaBr solutions

Abstract Light scattering was measured on aqueous NaBr solutions of dodecyltrimethylammonium bromide in the presence of 1.00 to 4.00 M NaBr, and these results are discussed together with the previous data at lower NaBr concentrations. When the NaBr concentration is lower than 1.8 M , spherical micelles alone are formed. At 2.00 and 3.00 M NaBr, the first and second critical micelle concentrations (CMC) exist, but at 4.00 M NaBr they coincide with each other. The primary micelle formed above the first CMC has the molecular weight about 23,500–24,400 in 1.00–4.00 M NaBr and is spherical. The secondary micelle formed above the second CMC has the molecular weight about 29,000–57,500 in 2.00–4.00 M NaBr, and its shape would be rodlike. Combined with the previous data at lower NaBr concentrations, it is found that a linear Corrin-Harkins relation holds for the first CMC up to 3.00 M NaBr, and that a linear double logarithmic relation between molecular weight and ionic strength holds for the spherical micelles. Another linear double logarithmic relation between molecular weight and ionic strength holds for rodlike micelles. The sphere-rod conversion of ionic micelles appears as a sharp transition with respect to the salt concentration, while it occurs as a chemical equilibrium with respect to the surfactant concentration. The sphere-rod transition of dodecyltrimethylammonium micelles induced by the change in counterion species from Cl − to Br − means that such a transition of ionic micelles is caused by the counterion binding, which reduces the electrostatic repulsion among charged head groups.