Relation between the IR spectrum of water and decarboxylation kinetics in microemulsions

The properties of water solubiliséd by cetyltripropylammonium bromide (CTPABr) and sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in CCl₄ were studied as a function of the number of water molecules per surfactant molecule(W) by the kinetics of decarboxylation of the 6-nitrobenzisoxazole-3-carboxylate ion (6NBIC) and IR spectroscopy in the O—H stretching region in the region of low water (0 < W < 10) where the properties depend strongly upon W. Decarboxylation of 6NBIC in CTPABr reverse micelles is strongly inhibited by an increase in W and the first-order constants level off at W > 1. The rate effects are ascribed to increases in polarity and hydrogen bonding in the reaction region as water is added and are related to changes in the IR spectrum. For the AOT system, the kinetics is independent of W and this difference from the behaviour of the CTPABr system is ascribed to the difference in the interactions of 6NBIC with surfactant headgroups.