Alkaline Fading of Triarylmethyl Carbocations in Self-Assembly Microheterogeneous Media

Abstract

This review reports on the alkaline fading of crystal violet and other related carbocations in the presence of different microheterogeneous media (micelles, microemulsions and vesicles).Scheme 1 2. SELF-ASSEMBLY MICROHETEROGENEOUS MEDIA Micelles, microemulsions and vesicles are the more popular biolayer models [29 -31]. These aggregates, with different hydrophobic and hydrophilic regions, transform them into nano-reactors with the capacity of concentrating reactants in a small volume [32 -35]. MicellesMicellar (see Scheme 2) systems and related association colloids also have the ability to alter chemical reactivity. The chemical reaction rates and equilibria observed in micellar media can differ from those observed in conventional media, owing to solubilisation of the reagents, reduction of their effective concentrations through their segregation in different 'compartments' within the bulk medium, concentration within micelles and changes in the polarity of the different regions of the system, and the physicochemical properties of the medium [36,37]. Hence, a potential use of micellar media is in the control of chemical reactivity [38 -40].Scheme 2 Micellar aggregate.Alkaline fading of triarylmethyl carbocations in self-assembly microheterogeneous media 141

Alkaline Fading of Malachite Green in β-Cyclodextrins

“…This catalysis is consistent with the possibility of a nucleophilic attack by an ionized CD hydroxyl group on the MG+ associated with the CD [24]. This behavior is like that reported in the literature (i.e., cleavage of aryl esters in the presence of CD [29,30] or that observed for CV hydrolysis [31,32]. A direct attack of OHon the MG that is associated with the MG-CD complex should be ruled out, since given the important effect of the dielectric constant on the reaction rate (vide supra), it would imply a greater catalytic effect of the presence of cyclodextrins in the medium.…”

Alkaline Fading of Malachite Green in β-Cyclodextrins

“…This observed catalysis is consistent with the possibility of a nucleophilic attack by an ionized CD hydroxyl group on the MG+ associated with the CD [24]. This behavior is like that reported in the literature (i.e., cleavage of aryl esters in the presence of CD [31,32] or that observed for CV hydrolysis [33,34]. A direct attack of OHon the MG that is associated with the MG-CD complex should be ruled out, since given the important effect of the dielectric constant on the reaction rate (vide supra), it would imply a greater catalytic effect of the presence of cyclodextrins in the medium.…”

Cleavage of Carbofuran and Carbofuran-Derivatives in Micellar Aggregates

“…In every case, an inhibitory effect upon the observed rate constant was measured. This behaviour can be modelled on the basis of the micellar pseudophase model [31] in which, the reaction will take place both in the micellar and in the water pseudophase, with a balanced distribution of carbofuran between both pseudophases. On the other hand, taking into account the electrostatic repulsions [30,35], the hydroxyl ions concentration at the micellar pseudophase must be very low due to Scheme 4 Pseudophase model mechanism in reverse micelles.…”

“…However, in practice, this limit is never reached, and the rate constant continues to grow as the micellar concentration increases, so it is considered as non-existent. This lack of the model leads to a failure in the initial hypothesis of a degree of constant ionisation of the micellar surface, whereas, much of the kinetic result can be explained on the basis of the kinetic pesudophase micellar model [27][28][29][30][31][32][33][34][35][36][37][38]. In some cases, where constraints are too high to use these models, new models arise based on coulomb potential interactions between ions and micelles, such as the Poisson-Boltzman Model (PBE).…”