DEACYLATION BY HYDROXIDE ION IN CATIONIC MICELLES - REACTIVITY AT MICELLE-WATER INTERFACES

Reaction of OH- with phenyl benzoate was examined in aqueous solutions of cetyltrialkylbromides (alkyl - Me, Et, n-Pr, n-Bu, CTABr, CTEABr, CTPABr, and CTBABr, respectively) and tetradecylquinuclidium bromide (TDQBr). With up to 0.1 M OH-observed rate constants increase modestly to maxima, but in 0.5 M OH-, reaction in inhibited under all conditions. These results are fitted quantitatively by a pseudophase model that describes ion-micelle interactions in terms of equations of the form of the Langmuir isotherm. Second-order rate constants at the micellar surfaces are lower than in water by factors of 23-55, depending on the surfactant. Similar second-order rate constants were obtained for reaction in solutions of CTAOH. At high [OH-] this low reactivity in the micellar pseudophase overcomes the rate-enhancing effect of concentration of OH- at the micellar surface. The classical pseudophase ion-exchange model fits the data qualitatively but not quantitatively, based on usually accepted values of the ion-exchange parameter and fractional micellar ionization, alpha.