Kinetics of the complex reaction of ascorbic acid with hydrogen peroxide catalyzed by CTAB

Kinetics of the reversible-consecutive reaction of ascorbic acid H(2)A with hydrogen peroxide was studied in phosphate buffer (mu = 0.1 mol center dot L-1) at pH 7 and 25 degrees C with conduction calorimeter. The apparent rate constants of this reaction, k(1), k(-1) and k(2), were measured at different concentrations of cetyltrimethylammonium bromide(CTAB). The results indicated that the apparent rate constants of the reaction, k(1), k(-1) and k(2), were remarkably affected by the cationic surfactant CTAB. ki initially increased, up to a maximum, and then fell off with increasing CTAB micellar concentration, while ki was hardly affected by CTAB of low concentrations( < 2 mmol center dot L-1) and increased when the CTAB concentration was more than 5 mmol center dot L-1; with the increasing concentration of CTAB, k(2) decreased before cmc, then increased up to a maximum, and at last fell off. CTAB micelles of low concentration had almost no influence on the activity of intermediate A(-.), but high concentrated micelles could promote the dismutation of A(-.) and inhibit its oxidation reaction. Electrostatic effect, hydrophobic effect, and local concentration effect of CTAB premicelles and micelles were the main factors influencing this reversible-consecutive reaction.