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 modestlyto maxima, but in 0.6 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 23-56, 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, a.
Deacylation by Hydroxide Ion in Cationic Micelles. Reactivity at Micelle-Water Interfaces
GERMANI, Raimondo;SAVELLI, Gianfranco;
1993
Abstract
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 modestlyto maxima, but in 0.6 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 23-56, 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, a.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
