Effects of Sodium Dodecyl Sulfate, SDS, Micelles on the Electrochemical Behavior of a Model Arenediazonium Ion. 2. Estimation of the Association Constant of the Electrochemically Generated Aryl Radicals with SDS Micelles

Abstract

The effects of sodium dodecyl sulfate, SDS, on the reductive electrochemistry of p-nitrobenzenediazonium tetrafluoroborate, PNBD, were investigated at two different pHs by employing differential pulse polarography (DPP). At pH = 2, the reduction peaks of the −N2+ and −NO2 groups are completely overlapped. Upon addition of SDS ([SDS] < critical micelle concentration, cmc), their peak potentials, Ep, are shifted in opposite directions, Ep(−N2+) toward more anodic values and Ep(−NO2) toward more cathodic values, and two polarographic peaks are clearly observed, suggesting that PNBD interacts with the sulfate group of SDS leading to the formation of an ion-pair. At the same pH but at [SDS] > cmc, the peak potentials shift in the reverse directions as those below the cmc and the polarographic peaks overlap again, an effect that is interpreted in terms of the incorporation of PNBD to the SDS micellar aggregates. At pH = 5, the reduction peaks of the −N2+ and −NO2 are clearly separated. Addition of SDS shifts their peak potentials in opposite directions, the same as those at pH = 2, up to a maximum (−N2+) or minimum (−NO2) at [SDS] = cmc, after which the directions are reversed getting a constant value. Peak currents, ip, for either −N2+ and −NO2 decrease smoothly reaching a plateau region. Quantitative analyses of the effects of SDS on Ep and on ip at [SDS] > cmc allowed estimations of the association constants of the parent PNBD and of the electrochemically generated aryl radical, Ar•, with SDS micelles. The association constant of the aryl radical is higher than that of PNBD by a factor of ∼2, indicating that the nitrobenzene radicals are preferentially stabilized in SDS micelles. The results obtained, together with those in previous work, suggest that the combination of DPP with arenediazonium ions as probe molecules provides a rapid, easy, and low-cost method to estimate the stability constants of a large number of aryl radicals to biomimetic systems, information that may be valuable for understanding relevant redox reactions in the more complex biological systems.