Effects of non-ionic and anionic micellar aggregates on the kinetics of the iridium(IV) oxidation of monosubstituted malonic acids

Abstract

The kinetics of the electron-transfer reaction between the hexachloroiridate(IV) ion and phenylmalonic acid have been studied in the absence and in the presence of non-ionic (Triton X-100, Triton X-102, Triton X-165, C₁₀E₆, C₁₂E₆, C₁₄E₆, C₁₂E₁₀, C₁₂E₂₃) and anionic (sodium dodecyl sulfate, sodium tetradecyl sulfate) micelle-forming surfactants over a wide acidity and surfactant concentration range at 25.0 °C and ionic strength 0.05 mol dm^–3. A number of kinetic measurements using benzylmalonic acid as reducing agent have also been performed. The redox reactions exhibit a first-order dependence on both the reductant and oxidant concentrations. The reaction rate in the absence of surfactants decreases as the hydrogen-ion concentration increases. In the presence of non-ionic micelles the oxidation process is retarded by an increase in either the acidity or the surfactant concentration. The inhibitory effects of these micellar aggregates upon the reaction rate depend on the hydrophobic micellar environment and the number of the ethoxy groups of the non-ionic surfactants. In the presence of anionic micelles a retarding effect has been observed only with sodium tetradecyl sulfate for the benzyl derivative. Binding constants of the neutral reductant species have been estimated by kinetic measurements. A mechanism for the electron-transfer reactions examined is suggested.