Reaction of bromide with bromate in thin-film water.

Thin-film water is ubiquitous in nature, occurring on virtually all surfaces exposed to the ambient environment. In particular, alkali halide salts below their deliquescence point are expected to be coated with water films from one molecular layer to a few nanometers thick. While salt ion mobility in thin-film water has been characterized in the literature, little is known about the chemistry occurring within these films. Here we investigate the surface chemistry change of a mixed bromine salt (KBr/KBrO(3)) using X-ray photoelectron spectroscopy, secondary electron microscopy, and energy-dispersive X-ray spectroscopy. At 68% relative humidity, the Br(-) surface concentration was observed to deplete with increasing water vapor exposure time. Known bulk solution kinetics for the reaction of Br(-) + BrO(3)(-) has a second-order dependence on H(+) concentrations. However, in the present experiments there was no addition of an external acid. These results suggest that the pH and chemical reactions within thin-film water are uniquely differently from bulk solution. Because bromine chemistry in the atmosphere is strongly influenced by pH, these results have implications for the cycling of bromine where thin-film water is present.