Kinetics of oxidation of vanillic acid by colloidal MnO2: correlation of micellar catalysis to the micellar properties of surfactants and mixed surfactants

The kinetics of oxidation of vanillic acid by colloidal MnO2 in an acidic medium was investigated and used as a probe to study the micellar catalysis and its correlation to micellar properties of surfactants. Three surfactants were employed; cationic cetyltrimethylammonium bromide (CTAB), anionic sodium dodecylsulfate (SDS), and non-ionic Tween-20 (TW-20), and some compositions of the two mixed systems CTAB-TW-20 and SDS-TW-20. The micellar properties were studied using the conductivity method. The reaction rate was enhanced in the presence of the three surfactants and the mixed systems. The catalytic profiles of the pure and mixed surfactant systems could be used to estimate the critical micelle concentrations of surfactant catalytic systems. Nevertheless, the kinetic measurements could not detect the variations in the micellar structures that occur at post-micellar compositions. Overall, the mixed systems studied showed a negative deviation of ideality with synergetic interaction. Therefore, Rubingh equations were used to calculate the micellar mole fraction and interaction parameters in the mixed surfactant systems.