Micellar Effects on the Reaction S2 O8 2- + Fe(CN)4 (bpy)2-

The reaction Fe(CN)4 (bpy)2- + S2 O8 2- has been studied in aqueous solutions of hexadecyltrimethylammonium chloride (CTAC) and bromide (CTAB), sodium dodecyl sulfate (SDS), dodecyl tricosaoxyethylene glycol ether (Brij35 ), and octylphenol(ethyleneoxide)9.5 ether (Triton X-100). The influence of changes in surfactant concentration on the reaction rate has been investigated. Experimental kinetic data show that the reaction rate is not affected by the presence of SDS, Brij35 , and Triton X-100 in the reaction medium within the surfactant concentration range studied. In CTAC and CTAB solutions the observed first order rate constant is much higher than that obtained in pure water, which can be explained by considering that the two anionic reactants are localized at the positively charged micellar surface. This is also in agreement with the observed decrease in the reaction rate when [cationic surfactant] increases. An increase in the surfactant concentration would provoke a decrease in the local concentration of the reagent in excess, S2 O8 2- ions, at the micellar surface, that is, at the reaction site, which would result in a decrease of the observed pseudo-first-order rate constant. Temperature was maintained at 298.2 K.