Kinetics of oxidation of aspirin by Ce(IV) in surfactant, polymer, and mixed surfactant-polymer media

The catalytic effects of the two surfactants, sodium dodecyl sulfate (SDS) and dimethyldodecylammonium oxide (DDAO), and the two polymers polyethylene glycol (PEG) and polyvinylpyrrolidone (PVD), on the kinetics of oxidation of Aspirin by Ce(IV) were investigated. Mixed systems of the two surfactants and SDS-PEG/PVD were also examined as catalysts of the reaction. Both surfactants catalyzed the reaction, but the DDAO was found to work only above its critical micelle concentration (CMC), whereas below it a precipitate appeared and retarded the measurements. The catalytic profiles of both surfactants showed maxima, which enabled the application of Piszkiewicz’s model to calculate the cooperative index (n) and dissociation constants (kD) of premicellar substrate-surfactant aggregates. Besides, less values of dissociation constant and a higher value of n were found for DDAO compared to those of SDS. The results proved higher catalytic activity of DDAO because the SDS counter ions may impose some restriction on Ce(IV) diffusion toward the substrate solubilized within the micellar core. Both polymers, PEG and PVD, were found to catalyze the reaction, and their catalytic data was modeled using the Benesi-Hildebrand equation. Interestingly, the catalytic profiles of mixed SDS-DDAO and SDS-PEG/PVD reflected the detailed surfactant-surfactant and surfactant-polymer interactions reported in the literature. The reaction’s product was identified from UV–VIS spectra assisted by the calculations of reaction stoichiometry and 2,4-dnitrophenylhydrazone test. A mechanism of the reaction that coped with the experimental data was suggested.