Adsorption and Solvation of HCl into Ice Surfaces

Abstract

The kinetics of HCl adsorption and incorporation into crystalline and amorphous ice were studied by using a thermal molecular beam, with FTIR spectroscopy to characterize the products. Thin films of hexagonal ice were grown on Pt(111) to provide a highly ordered surface on which to test models for HCl adsorption. Absolute HCl uptakes, product H2O:HCl stoichiometries, and sticking probabilities were measured as a function of HCl exposure and temperature between 85 and 145 K. Adsorption proceeds via a trapping mechanism, with the barrier to HCl adsorption into the final state being 7 kJ mol-1 lower than for desorption at low coverages. Adsorption becomes less favorable with increasing HCl coverage, saturating with one HCl adsorbed for each surface H2O molecule, independent of the ice thickness for T ≤ 120 K. Above 125 K, HCl is incorporated into the ice film, absorption showing a complex exposure and flux dependence. HCl absorption disrupts the ice lattice, causing the rate of HCl uptake to increase as adsorption proceeds. At high HCl fluxes, the creation of favorable adsorption sites on the ice surface is limited by the rate of HCl transport into the film and the sticking probability drops. The saturation product for 130 ≤ T ≤ 140 K is the amorphous trihydrate HCl·(3.1 ± 0.3)H2O and RAIR spectra for this, and for the surface-adsorbed monohydrate species, showed no bands due to molecular HCl. Amorphous ice films show a similar behavior, but with a greater density of water in the surface and more facile HCl transport into the film.