Kinetics of the pH-independent hydrolysis of 4-nitrophenyl chloroformate in aqueous micellar solutions: effects of the charge and structure of the surfactant

Abstract

The pH-independent hydrolysis of 4-nitrophenyl chloroformate in the presence of aqueous micelles of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, alkyltrimethylammonium chlorides, alkyldimethylbenzylammonium chlorides (alkyl group = cetyl and dodecyl) and polyoxyethylene (9) nonylphenyl ether was studied spectrophotometrically. The observed rate constants, kobs, decrease in the following order: cationic micelles > bulk water > non-ionic micelles > anionic micelles. Surfactant–substrate association constants, Ks, were determined from the dependence of kobs on surfactant concentration, and were found to be only slightly dependent on the charge of the surfactant and, for similarly charged micelles, on the length of their hydrophobic tail. A 1H NMR study of the solubilization of a model compound, 4-nitrophenyl chloroacetate, showed that all surfactant segments are affected by the solubilizate and the effect is more pronounced toward the middle of the hydrocarbon chain. The average solubilization site of the acetate ester does not depend on the charge of the micelle or the length of the surfactant hydrophobic tail. Micellar effects on observed rate constants are analyzed in terms of a ‘medium’ effect and an ‘electrostatic’ effect. The lower microscopic polarity at the reaction site retards the reaction, whereas electrostatic interactions of the polar transition state with the charged interface result in a rate decrease by anionic micelles and a rate enhancement by cationic micelles. Copyright © 1999 John Wiley & Sons, Ltd.