RELATION BETWEEN THE INFRARED-SPECTRUM OF WATER AND DECARBOXYLATION KINETICS IN CETYLTRIMETHYLAMMONIUM BROMIDE IN DICHLOROMETHANE

The structure of water in cetyltrimethylammonium bromide (CTABr) rever se micelles in dichloromethane has bees studied as a function of W = [ H2O]/[surfactant] by using the IR absorption due to O-H stretching mod es in the 3500-3000 cm(-1) range. We restricted our analysis to the re gion of small amounts of water (0 < W < 20) where the properties of th e system depend strongly upon W. The IR spectra can be represented as a sum of contributions from interfacial and bulklike water, with a min or contribution from water in dichloromethane. The fractions of water in the two ''regions'' within the reverse micelle were evaluated as a function of W. Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate i on in the reverse micelles is strongly inhibited by an increase in W, and first-order rate constants level off at W > 5. The rate effects ca n be ascribed to an increase ix polarity and hydration of the reaction region as water is added and are related quantitatively to changes in the IR spectrum.