Catalytic effect of CTAB reverse micelles on the reduction of methylene blue by ascorbic acid

The kinetics of the reduction of methylene blue (MB+) by ascorbic acid (HA(-)) has been studied in the presence of CTAB/CHCl3/hexane/water reverse micelles. In the absence of acid, the reaction obeys simple first order kinetics with respect to both the reactants. The second order rate constant in the presence of reverse micelles, k(w) (0.0360 mol(-1) dm(3) s(-1)) was found to be about sixty times greater than that of in aqueous medium, k(o) (5.33 x 10(-4) mol(-1) dm(3) s(-1)) under identical experimental conditions. The significant increase is attributed to (i) the concentration effect produced in the reverse micelles and (ii) lower micro polarity of the reverse micelles, which facilitates the reaction between oppositely charged ions. The effect of W (= [H2O]/[CTAB]) at constant [CTAB] and variation of [CTAB]. at fixed W has been studied. The kinetics at W<6 is explained on the basis of special properties of the entrapped water while the kinetics at W>10 is explained on the basis of ionic strength. The effect of [CTAB] on rate is also explained in terms of the binding of the reactants at the micellar interface as shown by the absorption spectra of methylene blue and ascorbic acid.