Water activity at the surface of ionic micelles as measured by the extent of hydration of 3-formyl- N-tetradecylpyridinium bromide

Abstract

An effort was made to estimate the water activity at the surface of ionic micelles in aqueous media by determining the extent of hydration of the carbonyl group of 3-formyl- N-tetradecylpyridinium bromide as a function of surfactant concentration. Over the range of concentration zero to 0.5 M of both N-tetradecyltrimethylammonium bromide and sodium dodecyl sulfate, the UV spectrum of this aldehyde was not distinguishable from that in water. In contrast, the UV spectrum of the same aldehyde in aqueous solutions of sulfuric acid changed progressively from that typical of pure water to that typical of dry acetonitrile with increasing concentrations of acid and, hence, decreasing activity of water. A change of 30% in water activity is sufficient to yield a detectable change in the UV spectrum of the aldehyde, reflecting a change in the extent of carbonyl function hydration. We conclude, in agreement with earlier suggestions, that the activity of water at the surface of ionic micelles is not very different from that in the bulk aqueous environment.