Transient spectra and decay kinetics of semithionine radicals in sodium dodecylsulphate micellar media

Abstract

The transient spectra and decay kinetics of semithionine species produced by the photoreduction of thionine by ferrous ions were studied in sodium dodecylsulphate (SDDS) micellar media. The results were compared with data obtained for a homogeneous aqueous medium. It was found that, whereas in the homogeneous aqueous medium there is evidence for the interaction of the semithionine species with ferrous ions, particularly at higher pH values and high ferrous ion concentrations, this is not the case in the micellar medium. Further, the decay of the semithionine species which is a kinetically second-order dismutation process in the homogeneous aqueous medium becomes pseudo first order in the SDDS micellar medium and is attributable to re-oxidation of semithionine by ferric ions adsorbed on the same micelle surface. From a comparison of the spectra in the two media at different pH values it is inferred that the species is more likely to be the doubly protonated semithionine TH. ̇ 3 2+ form in the micelle in the pH range 0 – 2.5. Addition of high concentrations of electrolytes such as NaCl had no effect on the spectrum of the species in the micellar medium, but the decay reverted to second order. This has been interpreted in terms of displacement of the ferric ions from the micelle surface into the bulk aqueous phase such that dismutation becomes the only pathway available for semithionine decay. The dismutation rate was found to decrease with increases in the concentration of the micellized surfactant in qualitative agreement with the pseudophase model of micellar kinetics.