The effect of mixed sodium dodecyl sulfate-polyethylene glycol systems on kinetic of oxidation of o-Cresol by cerium(IV) in H2SO4 medium

The kinetics of the oxidation of o-Cresol by cerium(IV) have been investigated spectrophotometrically at 298K in the absence and presence of sodium dodecyl sulfate (SDS) and polyethylene glycols (PEGs) in sulfuric acid medium to examine the catalytic behavior of SDS, PEG (600, 1500, and 4000), and the mixed systems of PEG 600, 1500, and 4000 with SDS. The reaction was first order with respect to [cerium(IV)] and fractional order with respect to [o-Cresol] and [H2SO4]. However, the effect of SDS catalysis was explained via Piszkiewicz's cooperativity model to calculate the cooperative index (n) and dissociation constants (k(D)) of substrate-surfactant aggregates, while the catalytic activity of PEGs was also examined by use of the Benesi-Hildebrand model. In general, reaction rate enhancements have been observed in the oxidation of o-Cresol by cerium(IV) in the presence of SDS and PEG (600, 1500). On the other hand, the catalytic profiles of the various mixed PEG-SDS systems have shown a sudden increase in the observed rate constant (k(obs)) values even before the actual critical micelle concentration (CMC) of SDS. This premicellar catalysis can be interpreted on the basis of the formation of the active PEG-SDS aggregates at various [SDS] and [PEG]. A plausible mechanism of the oxidation reaction has been suggested; in addition, the major final product was salicylic acid, which is separated by column chromatography and tested by melting point, ultraviolet-visible, and FTIR spectra to compare with the original sample of salicylic acid.