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

The pH-independent hydrolysis of 4-nitropbenyl chloroformate in the presenc e of aqueous micelles of sodium dodecyl sulfate, sodium dodecylbenzene sulf onate, alkyltrimethylammonium chlorides, alkyldimethylbenzylammonium chlori des (alkyl group = cetyl and dodecyl) and polyoxyethylene (9) nonylphenyl e ther was studied spectrophotometrically. The observed rate constants, k(obs ), decrease in the following order: cationic micelles > bulk water > non-io nic micelles > anionic micelles. Surfactant-substrate association constants , K-s, were determined from the dependence of kobs on surfactant concentrat ion, and were found to be only slightly dependent on the charge of the surf actant and, for similarly charged micelles, on the length of their hydropho bic tail. A H-1 NMR study of the solubilization of a model compound, 4-nitr ophenyl 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 do es 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 micr oscopic polarity at the reaction site retards the reaction, whereas electro static interactions of the polar transition state with the charged interfac e result in a rate decrease by anionic micelles and a rate enhancement by c ationic micelles. Copyright (C) 1999 John Wiley & Sons, Ltd.