Adsorption of small polar molecules as a probe of the surface electric field created by water layers supported by MgO(100): a theoretical study

The adsorption process of molecular pollutants on water layers supported by well-defined ionic surfaces can be used, in a first approximation, to mode lize precursor mechanisms implied in the pollution of ice, as studied in gl aciology or in polar stratospheric clouds. We use theoretical methods to ca lculate the electric field and field gradients at the surface of ordered mo no and bilayer phases of ice observed at low temperature (less than or equa l to 200 K) on MgO(100). We analyze the adsorption properties of several po llutants (N-2, CO2, HCl, HOCl) as a function of the site geometry and field intensity on the basis of semi-empirical potentials and optimization proce dures. The layer structure strongly influences the intensity and the direct ion of the electric field experienced by the pollutant, and the stable adso rption site appears to be a compromise which tends to maximize the field ef fects and the coordination of the pollutant with the water molecules of the outermost layer. Moreover, the ionic support polarizes strongly the closes t water layer and enhances consistently the water field, but its influence becomes rapidly negligible when the substrate-water layer distance increase s. (C) 1999 Elsevier Science B.V. All rights reserved.