Evidence of formation of dye–surfactant ion pair micelle in the anionic surfactant mediated alkaline fading of methyl violet carbocation

Abstract

In the submicellar solution of anionic surfactants sodium dodecyl sulfate (SDS) and dioctyl sodium sulfosuccinate (AOT), the alkaline fading of methyl violet (MV+) carbocation has been studied spectrophotometrically in the temperature range 293–308 K. The interaction behavior of the dye carbocation with the anionic surfactant and the observed submicellar rate retardation is consistent with the ion pair and subsequent ion‐pair micelle formation. The change in the spectral pattern along with a change in the peak intensity and shoulder intensity with increasing surfactant concentrations indicates the shifting of the equilibrium between the helical and distorted helical isomers of MV+ owing to the existence of dye–surfactant ion pair, ion‐pair micelles, and dye‐embedded micelles. This dynamic dye–surfactant ion‐ion interaction has been corroborated by the tensiometric studies. The decrease in the fading rate and the change in the spectral pattern in the presence of anion, especially I–, support the existence of the dye–anion interaction. The premicellar rate inhibition and the corresponding interaction have been accounted for by the use of two kinetic models namely, Olson–Simonson and Piszkiewicz cooperativity models. The cooperativity index value suggested a 1:1 dye–surfactant association in both the surfactants. The ratio of the concentration of the premicellar complex and the free dye cation indicates appreciable binding. The kinetic and thermodynamic parameters associated with the dye–surfactant complex have been explained in terms of dye–surfactant, dye–water, and surfactant–water interactions.

Comparative study of the co-assembly behaviour of 3-chloro-4-hydroxy-phenylazo dyes with DTAB

“…Fig. 4 permits the distinction of three surfactant concentration regions commonly referred to when observing the aggregation between oppositely charged dye and surfactant: 32,36,39 Region A. Formation of dye-surfactant ion pairs.…”

“…Region B. Dye-induced micelles. 32,39 The region of dyeinduced micelles was identified as the DTAB concentration region, where the spectrum of the dye is still sensitive to DTAB concentration changes. 32 Part of it lays below the critical micelle concentration (cmc) of pure DTAB, which was determined to be 9 mM in the present buffer system at 25 1C.…”

“…Ghosh et al referred to the aggregates in this region as dye-embedded micelles. 39 In view of the fact that the dyes and DTAB carry opposite charges, the location of dyes close to the positively charged DTAB head groups is likely. Therefore, the term ''dye-embedded micelles'' may not be the most accurate description of aggregates in this case.…”