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

The adsorption process of molecular pollutants on water layers supported by well-defined ionic surfaces can be used, in a first approximation, to modelize precursor mechanisms implied in the pollution of ice, as studied in glaciology or in polar stratospheric clouds. We use theoretical methods to calculate the electric field and field gradients at the surface of ordered mono and bilayer phases of ice observed at low temperature (≤200 K) on MgO(100). We analyze the adsorption properties of several pollutants (N2, CO2, HCl, HOCl) as a function of the site geometry and field intensity on the basis of semi-empirical potentials and optimization procedures. The layer structure strongly influences the intensity and the direction of the electric field experienced by the pollutant, and the stable adsorption site appears to be a compromise which tends to maximize the field effects and the coordination of the pollutant with the water molecules of the outermost layer. Moreover, the ionic support polarizes strongly the closest water layer and enhances consistently the water field, but its influence becomes rapidly negligible when the substrate–water layer distance increases.