Dye-surfactant interaction in aqueous premicellar and micellar environments in the alkaline fading of di-positive methyl green carbocation

Abstract

The alkaline fading of di-positive methyl green carbocation (MeG2+) has been studied in different aqueous premicellar and micellar environments of anionic, cationic as well as non-ionic surfactants. Mono- and di-negative anions like halides and sulphate have been found to show an inhibiting effect on the fading reaction owing to the formation of dye-anion ion-pair adduct before hydrolysis. The binding strength of the dye-anion ion-pair adduct is in the order: SO42- > I- > Br- > Cl- which rationalizes the same order of inhibition of the rate. A substantial change in the electronic spectra of MeG2+ in the presence of anionic surfactants, sodium dodecyl sulfate (SDS) and dioctyl sodium sulfosuccinate (AOT), in the pre-CMC region indicates a change in the equilibrium distribution of the two stereo isomers of the dye owing to the formation of MeG2+-surfactant ion-pair and ion-pair micelle. The premicellar rate inhibition of the fading reaction by SDS and AOT occurs due to the formation of 1:1 MeG2+-surfactant ion-pair (where ≥ 85 % of MeG2+ remain associated) through electrostatic interaction before hydrolysis. The cationic surfactant cetyltrimethylammonium bromide (CTAB) shows a retarding effect in the post-micellar region as the HO- ion experiences both electrostatic repulsion and steric hindrance to approach the carbocation centre of the dye, embedded in the micelle. The polar hydroxyl group and the polyoxyethylene groups present in the moiety of the non-ionic surfactant Brij35 assist to increase the local concentration of HO- and MeG2+ in the palisade layer resulting in a catalyzing effect on the fading reaction in the post CMC region. Both pre- and post-micellar rate effects have been analyzed quantitatively by different established kinetic models. Detailed calculations have been made to find out different enthalpy and entropy of activation as well as of binding in order to establish the nature of interactions between the dye MeG2+ and monomeric surfactant(s) /micelles as well as solvent molecules. Results of spectrophotometric and tensiometric investigations strongly support the existence of different molecular interactions in different stages of the hydrolysis reaction in the presence of surfactants.