Alkaline Fading of Malachite Green in β-Cyclodextrins

Abstract

The basic hydrolysis of Malachite Green (MG) in the presence of β-Cyclodextrin (β-CD) has been studied using UV-Vis spectroscopic techniques and at 20 °C. β-CD was found to catalyze the basic hydrolysis. Indeed, this basic hydrolysis is catalyzed by the interaction cyclodextrin hydroxyl group, in its deprotonated form with the carbocation in the host-guest complex. The proposed model has been successfully applied to a reaction catalyzed by CD. It considers two simultaneous pathways in the aqueous medium involving free hydroxyl ions and the substrate-CD complex. The model allows us to obtain the kinetic parameters including the bimolecular rate constant between MG and HO− in bulk water (kw = 1.47 ± 0.01 mol−1s−1), the rate constant between MG and the deprotonated hydroxyl group of β-CD inside the host-guest complex (kCD = 0.25 ± 0.03 s−1) and the binding constant of MG inside the β-CD (KS = 2500 ± 50). This behavior is like the hydrolysis of Cristal Violet (CV) in the same reaction media.