Electron-Transfer Kinetics of 4,4'-Bis(alkoxycarbonyl)-2,2'-bipyridine-Iron Complexes in Micellar Solution

The kinetics of electron-transfer reactions of a series of iron complexes Fe(EX)/sub 3//sup 2 +/ (EX = 4,4'-bis(alkoxycarbonyl)-2,2'-bipyridine, with alkoxy = methoxy, ethoxy, propoxy, butoxy) have been studied in homogeneous and in micellar solution. In homogeneous solution the rates of electron-transfer reactions with aquocerium(IV) ions are similar for the different complexes. In the presence of anionic micelles of sodium dodecyl sulfate the observed rate constants are enhanced as compared to those in homogeneous solution. At high surfactant concentrations, the rate enhancement is similar for the different Fe(EX)/sub 3//sup 2 +/ complexes investigated, whereas in the region of the critical micelle concentration relevant differences occur. By the use of a theoretical description of the diffuse double layer previously developed, the electrostatic enrichment of counterions at the micellar surface turns out to be the dominating contribution to the overall rate enhancement. If the overall enhancement is divided by the electrostatic part, additional contributions due to alterations of the second-order rate constant in the micellar environment are found. At low surfactant concentrations the results suggest a formation of micelles of altered size and shape, which are able to solubilize more than one iron complex. Among these iron complexes charge delocalization effects might occur and further contribute to the rate enhancement.