Evidence of positive co-operativity in the micellar catalysis electron transfer reaction

Abstract

Microenvironments consisting of surfactants are capable of either inhibiting or catalyzing the reaction rate depending on the specific interaction between surfactant and the reactant species. In this context, micellar catalyzed electron transfer reaction between methylene blue (MB) and ascorbic acid (AA) have been studied in presence of three cationic surfactants (TTAB, CTAB & CPC) at different temperatures under pseudo-first order condition. The kinetic data have been analyzed with different models (i.e., pseudo-phase model, Piszkiewicz's model and Raghavan-Srinivasan's model). The micellar binding constant evaluated from the three models are in good agreement with each other. Out of three surfactants, the lower standard enthalpy of activation (∆‡H°) and higher negative standard entropy of activation (∆‡S°) of CPC have been ascribed to the planar structure of the pyridinium head group which provides less steric hindrance to the attacking ascorbate ions. The formation of catalytic sub micellar aggregates between dye and micelle in the reaction medium in a ratio of 1:1 and 1:2 have been well justified by TEM measurements and the result of positive co-operative values. The catalytic activity of surfactants as well as micellar binding constant increases in the following order: CPC>CTAB>TTAB.