Roshchyna K. Micellar effects in kinetics of the reactions of nucleophilic addition to the carbonium ions of triphenylmethane dyes

The reaction rate constants for series of triphenylmethane dyes with hydroxide ion in micellar solutions of different surfactants were determined by the spectrophotometric method. Moreover, the influence of different surfactant types on the reaction rate was explored. As the result of hydrophobic effects and electrostatic interactions the cationic triphenylmethane dyes Crystal Violet, Malachite Green and Brilliant Green were found to bind to all types of surfactant micelles. In case of the cationic surfactants the increase of ОН- ions concentration in the micellar Stern layer results in additional increase of the reaction rate in comparison with non-ionic surfactants. In solutions of the anionic surfactants an association of cations of the dyes with anions of the surfactants takes place. This leads to a dramatic decrease in the rate constant even in a pre-micellar region of surfactant concentration. Due to their hydrophobicity, the anionic dyes Bromophenol Blue (BPB), Acid Fuchsin (FA) and Phenolphthalein bind to all types of micelles, except of anionic ones. This leads to the decrease in the reaction rate in micellar media which contain non-ionic, cationic and zwitterionic surfactants. For FA and BPB significant rate constant decrease occurs in the solutions of cetyltrimethylammonium bromide. This effect can be observed even in pre-micellar range and might be caused by the formation of the ionic associates between the dyes anions and the cations of the surfactants.