Interaction of the chloropentaamminecobalt(III) cation with mixed micelles of anionic and non-ionic surfactants: a kinetic study

Abstract

Alkaline hydrolysis rates of the cholropentaamminecobalt(III) cation have been studied at 25.0 °C and an ionic strength of 0.05 mol dm^–3 in aqueous mixed micelles composed of anionic sodium dodecyl sulfate (SDS) and non-ionic n-dodecylpenta(oxyethylene glycol) monoether (C₁₂E₅) surfactants over a wide range of total surfactant concentration (C_t) and SDS mole fraction (α). The critical micelle concentrations (c.m.c.s) of mixtures of the two surfactants over the entire α range have also have been determined and found to follow the regular solution model for non-ideal mixing. The kinetic results have shown that interactions of the cationic cobalt complex with the mixed micelles cause a decrease of the reaction rate with increasing C_t(at constant α) and α(at constant C_t). The estimated binding constant of the complex decreases markedly as α decreases as a result of the reduction of the charge density at the mixed micellar surface. The present findings are compared with those obtained previously concerning the effect of urea and its alkyl derivatives upon the rate of the same hydrolysis reaction in SDS micellar solutions.