Kinetics of the pH-independent hydrolysis of 4-nitrophenyl chloroformate in aqueous micellar solutions: effects of the charge and structure of the surfactant
S. Possidonio et al., Kinetics of the pH-independent hydrolysis of 4-nitrophenyl chloroformate in aqueous micellar solutions: effects of the charge and structure of the surfactant, J PHYS ORG, 12(4), 1999, pp. 325-332
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.