Acoustic study on the kinetics for the dissociation-recombination reaction between micelle and counterion in sodium dodecyl sulfate solution

Abstract

Abstract The ultrasonic absorption of sodium dodecyl sulfate (SDS) in aqueous solution was measured at 30°C. in the range of frequency between 70 kc. and 95 Mc. and of concentration from 6.5 to 100 mM. The excess absorption was observed only in the solutions the concentration of which was higher than the critical micellar concentration. The excess absorption of sound and the relaxation frequencies obtained from the absorption curves show a discontinuity with the variation of the concentration of SDS in the neighborhood of 50 mM. Other properties like viscosity, conductivity, and velocity of sound also exhibit such a change near the same concentration. It is concluded that a change in the properties of the micelles of SDS occurs in the neighborhood of this concentration. The mechanism of the observed ultrasonic absorption is considered and attributed to the reaction M″ ⇌ M′ + 2Na+, where M′ and M″ are two types of micelles. The rate constants of the forward and backward reactions are found to be 1.5 × 105 sec.−1 and 8.4 × 109 mole−2 sec.−1, respectively, and the partial molal volume change between the initial and final states in the reaction is 29.4 cm.3 mole−1.