Kinetics and mechanism of the reduction of colloidal MnO2 by glycyl-leucine in the absence and presence of surfactants

Abstract

The kinetics of the reduction of water-soluble colloidal manganese dioxide by glycyl-leucine (Gly-Leu) has been investigated in the presence of perchloric acid both in aqueous as well as micellar media at 35°C. The study was carried out as functions of [MnO2], [Gly-Leu] and [HClO4]. The first-order-rate is observed with respect to [MnO2], whereas fractional-order-rates are determined in both [Gly-Leu] and [HClO4]. Addition of sodium pyrophosphate and sodium fluoride enhanced the rate of the reaction. Further, the use of surfactant micelles is highlighted as, in favourable cases, the micelles help the redox reactions by bringing the reactants into a close proximity due to hydrogen bonding. While the ionic surfactants SDS and CTAB have not shown any effect on the reaction rate, the nonionic surfactant TX-100 has catalytic effect which is explained in terms of the mathematical model proposed by Tuncay et al. (1999). The Arrhenius and Eyring equations are valid for the reaction over the range of temperatures used and different activation parameters (Ea, ΔH#, ΔS# and ΔG#) have been evaluated. Kinetic studies show that the redox reaction between MnO2 and Gly-Leu proceeds through a mechanism combining one- and two-electron pathways: Mn(IV)→Mn(III)→Mn(II) and Mn(IV)→Mn(II). On the basis of the observed results, a possible mechanism has been proposed and discussed.