Effects of Sulfobetaine-Sodium Dodecanoate Micelles on Deacylation and Indicator Equilibrium

Article abstract of DOI:10.1021/j100029a030

Fractional micellar ionization, α, and micelle concentrations have been measured for mixed micelles of sodium dodecanoate (SDOD) and sulfobetaines (CⁿH²ⁿ⁺¹N⁺Me2(CH2)3SO3^-, SB3-n, n = 10, 12, 14, 16), and values of pH_app have been calculated from deprotonation of 1-dodecylpyridinium-4-aldoxime bromide.For 0.1 M total surfactant first-order rate constants of hydrolyses of 2,4-dinitrophenyl acetate and octanoate (DNPA and DNPO, respectively) and benzoic anhydride (Bz2O) go through maxima at a mole fraction of SDOD of ca. 0.5 for DNPA and DNPO and 0.8 for Bz2O.These rate maxima are fitted quantitatively in terms of reactions of OH^- and dodecanoate ion on the assumption that sulfobetaine micelles increase the nucleophilicity of the carboxylate ion.This rate effect is ascribed to decreased hydration and ion-pairing of the dodecanoate ion in the mixed micelles, on the basis of 1H NMR spectrometry in mixtures of SDOD and SB3-14.Therefore, incorporation of dodecanoate ion in SB3-n slows reaction by "dilution" in the micelle, but this inhibition is offset by activation of the nucleophilic dodecanoate ion.