Kinetic and mechanistic studies on the oxidation of d-glucose by MnO2 nanoparticles. Effect of microheterogeneous environments of CTAB, Triton X-100 and Tween 20

Abstract

The kinetics of electron transfer reaction between D-glucose and MnO2 nanoparticles has been studied in weakly acid medium. In the colloidal solution of MnO2, ultrafine nanoparticles of MnO2 remain supported on two-dimentional gum acacia sheets in the form of crumbled paper ball assuming the form of spherical particulates of size 60–200nm as detected by TEM. The nanoparticles are found to be amorphous. Apart from the inhibition, due to adsorption of reaction products on the surface of the nanoparticles, the reaction is first order with respect to MnO2, but complex order with respect to glucose as well as H+. Glucose is oxidized to δ-lactone through a one-step two-electron transfer process and the latter is rapidly hydrolyzed to gluconic acid. The oxidation of glucose in absence of surfactant is entropy-controlled. In the presence of surfactants of cetyl trimethyl ammonium bromide (CTAB), Triton X-100 (TX-100) and Tween 20, the reaction rate passes through a maximum around the CMC. Both the reactants are distributed between the aqueous and the micellar pseudophases and then react, following Berezin’s model. Ion-dipole interaction and H-bonding appear to play an important role in the binding between glucose and the surfactant molecules while protonated MnO2 remains at the periphery of the Stern layer within the micelle. The binding of glucose with the surfactants is controlled by the entropy changes associated with the glucose-water interaction, micelle-water interaction and glucose-micelle interaction.