Effect of temperature on micelle formation in aqueous solutions of alkyltrimethylammonium bromides

The ultrasound velocity in aqueous solutions of alkyltrimethylammonium bromides having octyl, decyl, dodecyl, and tetradecyl chains has been measured as a function of concentration at different temperatures from 15 to 55°C. It increases linearly with increasing surfactant concentration, suddenly changes the slope of the line at the critical micelle concentration (CMC), and then varies linearly with increasing concentration. The value of the CMC thus determined gives minima at 47, 28, 20, and ca. 15°C for the octyl, decyl, dodecyl, and tetradecyl derivatives, respectively. Since the temperature change of the CMC is very small, the standard free energy of micelle formation decreases monotonically with increasing temperature, thus giving a positive standard entropy of micell formation and indicating the hydrophobic effect in the micelle formation. At temperatures above that corresponding to the minimum CMC, micelle formation is exothermic, thus promoted both entropically and energetically. At temperatures below that corresponding to the minimum CMC, it is endothermic. The standard free energy of micelle formation is lowered by 1.81 kJ per mole CH 2 group at 25°C.