Acoustic study of the kinetics of the dissociation-recombination reaction between micelle and counterion in solutions of sodium salts of fatty acids—sodium caprylate and sodium caprate

Abstract

The ultrasonic absorption has been measured in aqueous solutions of sodium caprylate and sodium caprate at 30°C over the frequency range of 5–95 mc. As in the case of sodium dodecyl sulfate, the excess absorption was observed only in solutions in which the concentration was higher than the critical micellar concentration (CMC), both in the presence and absence of salt. The mechanism for these features was attributed to the reaction M″→M′ + 3 N a + for sodium caprylate and M″→M′ + 2.5 N a + for sodium caprate, where M′ and M″ are two types of micelle. The rate constants of the forward and backward reactions were 3.8 × 10 7 sec −1, 3.3 × 10 9 mole −3 sec −1 for sodium caprylate and 1.1 × 10 7 sec −1, 2.8 × 10 9 mole −2.5 sec −1 for sodium caprate, respectively. Some kinetic characteristics including the free energy, enthalpy, and entropy were calculated.