Micellar Effect on the Kinetics of Ethanol Oxidation by Potassium Permanganate in Acidic Medium

Abstract

        The catalytic effect of sodium dodecyl sulphate (SDS) micelles on the oxidation of alcohols by potassium permanganate in an acidic medium was investigated using a double beam Unicam-1800 Schmadzu UV/Visible- spectrophotometer at λmax 525 nm under pseudo-first-order kinetics. The kinetic study followed a first-order dependence each on [KMnO₄], [Ethanol] and fractional order to [H⁺]. The reaction was catalyzed by SDS at low concentration values and invariant at higher [SDS]. The catalytic effect may be attributed to the solubilization and incorporation of the reactants into the stern layer of the micelles and the inhibition at higher levels of SDS is attributed to the repulsion between the protonated oxidant accumulated on the surface of the micelle and the surfactant counter ions. The results are discussed in terms of the Pseudo-Phase Model as proposed by Menger-Portnoy and Piszkiewicz's positive cooperativity model. The activation parameters were obtained from Erying’s equation as ∆H^# 4.16 kJ mol^-1, ∆S^# -0.198kJK^-1 mol^-1 and ∆G^# 63.16 kJmol^-1. The negative ∆S^# suggests an ordered transition state. The micellar rate constant, km and the binding constant Ks, were 7.45 x 10^-3 s^-1 and 40.35 mol dm^-3 respectively. The binding constant indicates the existence of surfactant-substrate interaction and the value of n>1 suggests positive cooperativity. The study showed that electrostatic/hydrophobic interactions play a significant role in the micellar-influenced reaction and further affirmed the fact that all chemical reactions involving protonation are catalyzed by anionic surfactants.