Kinetics of alkaline fading of methyl violet in micellar solutions of surfactants: Comparing Piszkiewicz's, Berezin's, and pseudophase ion‐exchange models

Abstract

The micellar effect of surfactants of various types on the rate of the reaction between methyl violet and hydroxide ion is studied. The absorption spectra show that the cation of methyl violet is bound by micelles of all types at proper concentrations of surfactants. The observed rate constant in micellar systems containing nonionic Brij‐35, zwitterionic 3‐(dimethyldodecylammonio)‐propanesulfonate, cationic cetyltrimethylammonium bromide and hydroxide surfactants is higher, whereas in solutions of the anionic surfactant sodium dodecylsulfate is lower than that one in the surfactant‐free system. Piszkiewicz's, Berezin's, and pseudophase ion‐exchange models of the kinetic micellar effect are used for the treatment of the dependences of the above‐mentioned constants on the surfactant concentration. The values of the corresponding kinetic parameters are compared and discussed. The influence of nonionic, zwitterionic, and anionic micelles on the reaction rate is discussed on the basis of medium and concentration kinetic effects. The character of the cationic micelles effect is somewhat paradoxical. Although the observed pseudo–first‐order reaction rate constant substantially increases in the presence of such micelles, the second order‐rate constant in these micelles is lower than the corresponding value in surfactant‐free aqueous solution. As a possible explanation, the decrease in the reactivity of the HO– ions is proposed, owing to their electrostatic association with the cationic headgroups (“diverting effect”).

Quantitative analysis of micellar effect on the reaction rate of cationic triphenylmethine dyes with water according to Berezin’s model

“…It should be noted that this approach is quite applied [14,15]. Its validity is to some extent confirmed based on particle size data obtained by the dynamic light scattering, where it was found that at the presence of the triphenylmethine dye the micelle formation occurs earlier than in a dye-free system [1,2].…”

“…An analogy should be drawn with the difference between k plateau /k w and k m /k w values for the Dye + + HOinteraction, where k plateau /k w = 1.33, 1.42, and 14.69, and k m /k w = 1.33, 1.52, and 2.10 in the micellar solutions of DMDAPS, Brij-35, and CTAB, respectively [2]. Hence the values of k plateau /k w and k m /k w are substantially differ only for CTAB system due to the enhanced concentration of the HOions on the surface of cationic micelles (see equation ( 2)), which was confirmed by the high value of the distribution coefficient of HOin this system.…”

“…The obtained P dye values agree with the theoretical ideas about dyes binding degree according to hydrophobic interactions. The values of the distribution coefficient of the cationic triphenylmethine dyes obtained by equation ( 3) can be placed in the following order: P BG (10300) > P MG (2100) > P methyl violet (870 [2]) for the Brij-35 micellar system, i.e. in the absence of electrostatic interactions between the dye and the micelles.…”

“…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]. In a previous paper, we began to consider the applicability of Piszkiewicz's kinetic micellar model on the reaction between an ion and a neutral molecule.…”

Quantitative analysis of micellar effect on the reaction rate of alkaline fading of phenolphthalein

“…Among the surfactant systems considered, the PIE model is applicable only to the treatment of the influence of CTAB micelles. In terms of PIE model for such reactions, in the case of cationic micelles the counterions (Br -) compete with reactive hydroxide ions in the Stern layer [4,5] value is equal to 15, as determined in our recent paper [24], has been used in calculations. Equation (10) of the dependence of the observed pseudo-first order rate constant of bimolecular reaction on surfactant concentration is written in terms of the local concentration of micellar-bound HO - …”