Kinetic evidence for the effects of urea on the properties of aqueous micellar solutions of sodium dodecyl sulfate

Abstract

The kinetics of the alkaline hydrolysis of the chloropentaamminecobalt(III) cation have been studied at 25.0 °C in water and in aqueous micellar solutions of sodium dodecyl sulfate (SDS) in the presence of varying amounts of urea. The rate of the alkaline hydrolysis with and without added urea is strongly retarded by the SDS surfactant above the critical micelle concentration (c.m.c.) as a result of binding of the cobalt complex to the anionic micellar surface. The effects of added urea on the c.m.c. of the surfactant have been demonstrated kinetically. Moreover, urea addition causes an enhancement of the overall hydrolysis rate both in water and, to a larger extent, in the aqueous micellar solutions, and a decrease in the estimated binding constant of the cobalt complex. The observed effects of urea on the hydrolysis rate and on the other parameters derived kinetically have been discussed and related to changes in the properties of the SDS micellar aggregates, such as reduction of the micelle surface charge density on urea addition.