Kinetics of alkaline fading of malachite green and crystal violet in critical solutions

The reaction kinetics for alkaline fading of malachite green (MG) in the critical binary solution 2-butoxyethanol + water and of crystal violet (CV) in the critical microemulsion water/bis(2-ethylhexyl) sodium sulfosuccinate (AOT)/n-decane with the molar ratio of water to AOT being 30 have been studied by the three-wavelength UV-spectrophotometry at initial reaction stage and various temperatures. It was found that the reaction rates kobs for both above reactions were well described by the Arrhenius equation in the temperature ranges far away from the critical points. The critical slowing down effects were observed in the near-critical regions and the corresponding critical slowing down exponents were determined to be 0.146 ± 0.014 and 0.035 ± 0.006 for the two reactions, respectively. The former exhibited both the dynamic and thermodynamic critical slowing down effects; while the latter showed no thermodynamic singularity. It was attributed to the particular experimental design in this work that made the dimer/monomer droplet equilibrium in the critical microemulsion system no longer affect the reaction kinetics.