Micellar Effect on Chromium(VI) Oxidation of D-Glucose in the Presence and Absence of Picolinic Acid in Aqueous Media: A Kinetic Study

Abstract

The kinetics and mechanism of Cr(VI) oxidation of D-glucose in the presence and absence of picolinic acid (PA) in aqueous acid media have been carried out under the conditions, [D-glucose]_T ⋟ [Cr(VI)]_T at different temperatures. Under the kinetic conditions, HCrO₄^- has been found kinetically active in the absence of PA while in the ΡΑ-catalysed path Cr(VI)-PA complex has been established as the active oxidant. In the ΡΑ-catalysed path, Cr(VI)-PA complex receives a nucleophilic attack by the substrate to form a ternary complex which subsequently experiences a redox decomposition (through 2e transfer) leading to lactone (oxidised product) and Cr(IV)-PA complex. Then Cr(IV)-PA complex participates further in the oxidation of D-glucose and ultimately is converted into Cr(III)-PA complex. In the uncatalysed path, Cr(VI)-substrate ester experiences an acid catalysed redox decomposition (2e transfer) at the rate determining step. The uncatalysed path shows a second-order dependence on [H⁺], Both the paths show first-order dependence on [D-glucose]_T and [Cr(VI)]_T. The ΡΑ-catalysed path is first-order in [PA]_T. These observations remain unaltered in the presence of externally added surfactants. Effect of cationic surfactant (i.e. cetylpyridinium chloride, CPC) and anionic surfactant (i.e. sodium dodecyl sulfate, SDS) on both the uncatalysed and ΡΑ-catalysed path has been studied. CPC inhibits both the uncatalysed and ΡΑ-catalysed path while SDS catalyses the reactions. The observed micellar effects have been explained by considering the hydrophobic and electrostatic interaction between the surfactants and reactants. Applicability of different kinetic models, e.g. pseudo-phase ion exchange (PIE) model, Menger-Portnoy model, Piszkiewicz cooperative model, has been tested to explain the observed micellar effects. Effect of [surfactant]_T on the activation parameters has been explored to rationalise the micellar effect.