Effect of surfactants on the oxidation of oxalic acid by soluble colloidal MnO2

Kinetics of the oxidation of oxalic acid by water-soluble colloidal MnO2 has been studied spectrophotometrically in the absence and presence of non-ionic octylphenoxypolyethoxyethanol (Triton X-100, TX-100) and anionic sodium dodecyl sulfate (SDS) surfactant micelles. Anionic SDS micelles were found ineffective whereas those of TX-100 increased the rate that reached a plateau at higher [TX-100]. The reaction followed first- and fractional-order kinetics with respect to [MnO2] and [H+], respectively, both in the absence and presence of TX-100. The activation parameters (Ea, ΔH#, and ΔS#) were evaluated and the effect of added manganese(II) sulphate is also reported in both aqueous and aqueous–micellar media. On the basis of experimental findings, a probable mechanism is proposed. The reaction proceeds through the adsorption of oxalic acid and hydrogen ions on the surface of the colloidal MnO2. Freundlich isotherm is used to explain the adsorption of oxalic acid on colloidal MnO2.