Effects of alkyl-substituted ureas on the binding of the chloropentaamminecobalt(III) cation to sodium dodecyl sulfate micelles: a kinetic investigation

Abstract

The effects of the addition of 1.0 mol dm^–3 methylurea, 1,1′-dimethylurea, 1,1′.3.3′-tetramethylurea and butylurea on the rate of alkaline hydrolysis of the chloropentaamminecobalt(III) cation have been studied at 25.0 °C in water and in aqueous sodium dodecyl sulfate (SDS) solutions over a wide range of surfactant concentration. Beyond the critical micelle concentration the reaction rate decreases with increasing SDS concentration as a result of binding of the cationic complex to the negatively charged micellar aggregates, but the rate-retarding effects of SDS are less significant than those found previously in the presence of unsubstituted urea and without additives. The added ureas cause a decrease in the binding constant of the cobalt complex and an increase in the rate constant for the pathway involving the cationic complex solubilized in the micellar pseudo-phase. The changes in both these quantities depend on the hydrophobic character of the additives and are due to reduction of the micelle surface charge density. It is likely that a direct mechanism operates in which the additives penetrate the micellar surface.