Effects of charge and structure of surfactants on kinetics of water reactions: The pH-independent hydrolysis of bis (2,4-dinitrophenyl) carbonate

The pH-independent hydrolysis of bis (2,4-dinitrophenyl) carbonate 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 has been studied spectrophotometrically. Observed rate constants, k(obs), decrease in the following order: alkyldimethylbenzylammonium chlorides > bulk water = alkyltrimethyl- ammonium chlorides > nonionic micelles > anionic micelles. Surfactant-substrate association constants have been determined from the dependence of k(obs) on [surfactant]. H-1 NMR study of the solubilization of a model compound, phenyl-2,4-dinitrophenyl carbonate, showed that all surfactant segments are affected by the solubilizate, and that the ester penetrates deeper in micelles of alkyldimethylbenzylammonium chlorides relative to the corresponding alkyltrimethylammonium chlorides. Micellar effects on k(obs) are analyzed in terms of "medium" and "electrostatic" effects. The lower microscopic polarity at the reaction site cause rate decrease, whereas electrostatic interactions of the dipolar (C) 1999 Elsevier Science B.V. All rights reserved.