Effects of pre-micelles of anionic surfactant SDS on the electron transfer reaction between methylene blue and ascorbic acid

Abstract

The effect of pre-micellar cluster of the anionic surfactant sodium dodecyl sulphate (SDS) on the electron transfer reaction between methylene blue (MB) and ascorbic acid (AA) in dilute acid medium has been investigated in the temperature range 298–308K. The reaction is first order each with respect to MB as well as AA. The reaction involves two parallel paths - one uncatalyzed and the other H+-catalyzed path resulting in the rate law, kobs=(k0+k1 [H+]) [AA] [MB]. Iodide ion has been found to have a specific accelerating effect on the reaction rate. The reaction appears to take place between undissociated AA molecule and MB+/HMB2+ cation. Anionic surfactant SDS shows an inhibiting effect on the reaction rate in the pre-micellar region, ultimately leading to a limiting value. The inhibiting effect of SDS has been explained in terms of Piszkiewicz's co-operativity model. The co-operativity index (n) value varies from 1.45 to 1.76 in the studied temperature range. The values of KS (the dissociation constant of the pre-micelle), n and surfactant concentration lead to the fact that the electrostatic binding in the pre-micelle is reasonably strong and most of the MB molecules remain in the pre-micelle cluster before the electron transfer with AA takes place. The formation of pre-micelle is exothermic in nature and thus favored at lower temperature.