Specific interactions of anions and pre-micelles in the alkaline fading of crystal violet carbocation

Abstract

Abstract The specific interactions of inorganic anions (Cl−, Br−, I− and SO42−) and pre-micelles in the alkaline fading of crystal violet carbocation (CV+) have been studied in the temperature range 303–318 K. The pseudo-first order rate constant decreases with increasing the concentration of anions and ultimately reaches a limiting value. Cl− and Br− ions appear to exert simple salt effects on the reaction rate, while larger ion I− and bivalent ion SO42− exert some specific electrostatic interaction effects on the CV carbocation. This salt effect on the reaction rate has been rationalized in terms of Olson–Simonson (O–S) model of ion pair formation. The same reaction has been studied in the presence of two anionic surfactants, namely, sodium dodecyl sulfate (SDS) and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) at concentrations well below their CMC. A sharp decrease in the rate constant with increasing surfactant concentration suggests a pre-micellar aggregation between the CV+ ion and SDS/AOT monomers. The results have been explained using O–S model as well as Piszkiewicz cooperativity model. The single hydrocarbon tail in SDS binds favorably to the CV+ in the pre-micelle whereas in AOT the presence of two hydrocarbon tails makes the binding less favorable due to some steric crowding. The cooperativity index (n) values in the pre-micelle cluster suggest the formation of a 1:1 and 1:2 dye-surfactant ion pair. In AOT pre-micelle the nucleophile HO− gets easier access to the reaction center leading to a lower energy of activation and higher rate of reaction than those in case of SDS.