Nucleophilic Reactivity of the CTACl-Micelle-Bound Fluoride Ion: The Influence of Water Concentration and Ionic Strength at the Micellar Interface

Abstract

The nucleophilic reactivity of the fluoride ion, F-, toward PNDPP, p-nitrophenyldiphenyl phosphate, was investigated in buffered aqueous solutions of CTACl, cetyltrimethylammonium chloride, at pH 9.8 and 7.9. The pH- and buffer-independent micellar rate constant, k2,m = 0.096 L mol-1 s-1, is surprisingly similar to that measured for the reaction in bulk water (k2,m/k2,w very close to unity) despite its marked difference with the micellar interface. To reproduce k2,m outside the micellar domain, the kinetic salt (Me4NCl) and solvent (dioxane−water) effects in bulk media were studied: both effects are found to be small and positive (increasing [salt] or [water] increases the rate constant). The micellar rate constant was satisfactorily reproduced only in a medium containing water and Me4NCl concentrations similar to those measured at the interfacial region of CTACl. The unusual success in reproducing the micellar rate constant, due to the negligible role of electrostatic interactions, is discussed by comparing the reactions of the F- and OH- ions with PNDPP, as well as some H2O reactions. For the reaction of the former anion, its small kinetic dependence on the medium is shown to result from a compensation between the changes in the activity coefficients of PNDPP and of the corresponding transition state on going from bulk to interfacial water whereas the effect of F- and OH- desolvation is small in this water-rich medium. The negligible dependence of the activation parameters on solvent composition or salt concentration is interpreted in the same terms.