Abstract
Several approaches quantitatively describe the effect of surfactant micellar solution on the reaction rate. The most used among them are Piszkiewicz’s, Berezin’s, and Pseudophase Ion-Exchange (PIE) models. The last-named was developed by Bunton and Romsted. Piszkiewicz’s model is based on representations of the micellization according to the mass action law with the formation of a catalytic micelle, which consists of some surfactant molecules and a substrate. In our previously paper, this model was used to explain the kinetic micellar effect on the reaction of cationic triphenylmethine dyes with water once again showed the main disadvantages of this approach. Berezin’s model is based on another model of micelle formation viz. the pseudophase model, and the binding of reagents by micelles is considered as the distribution of a substance between two phases. In this work, we aim to consider the applicability of Berezin’s approach for the interaction of malachite green and brilliant green cations with water molecule as a nucleophile in aqueous systems of nonionic, anionic, cationic, and zwitterionic surfactants. On the whole, Berezin's model performed well when applied to the description of the micellar effect on the reaction of similar dye with the hydroxide ion. However, it was revealed that this model does not take into account the change in the local concentration of the HO– ions due to a compression of the double electric layer upon addition of reacting ions to the system, as well as the constant of association of the HO– ions with cationic head groups of surfactant. In this case, when water is used as a nucleophile, the question of the degree of nucleophile binding can be solved differently. The PIE model is also based on a pseudophase model of micellization, but a substrate binding by micelles is considered as an association in a stoichiometric ratio of 1:1, and a nucleophile concentration is expressed in a local concentration based on the neutralization degree of micelles. Given the latter, its approach cannot be applied to the kinetic micellar influence on the reaction of cationic triphenylmethine dyes with water.
Highlights
The models of the kinetic micellar effect are usually tested using the reactions of nucleophilic addition of hydroxide ions or alkaline hydrolysis [1,2,3,4,5,6]
The application of Berezin’s model gives satisfactory results for describing the effect of nonionic, anionic, cationic, and zwitterionic micelles in the case of interaction of the dyes with water as a nucleophile: i. shows a satisfactory correspondence between the experimental points and the points calculated by Berezin’s model equation, which is characterized by a high R2 value; ii. the correlation between the obtained Pdye values by Berezin’s model equation and the dyes logP
Values corresponds to the fact that the less hydrophilic substrates have a greater affinity to the weakly polar micellar pseudophase. iii. the obtained km values by Berezin’s model equation correspond to the concept of the effect of weakly polar micellar pseudophase according to the Hughes-Ingold rules
Summary
The models of the kinetic micellar effect are usually tested using the reactions of nucleophilic addition of hydroxide ions or alkaline hydrolysis [1,2,3,4,5,6]. We began to consider the applicability of Piszkiewicz’s kinetic micellar model on the reaction between an ion and a neutral molecule. As an example of this reaction type, we used the fading of cationic triphenylmethine dyes in an acidic medium due to the interaction with water molecule as a nucleophile [7]. The premicellar surfactants effect was explained by the high affinity of the dye and surfactant monomers, which leads to micelle formation before its critical micelle concentration, CMC, value, or by the dye interaction with premicellar surfactant aggregates
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