CO ADSORPTION-ISOTHERMS ON ICE BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY AND NEW INSIGHTS OF THE ICE SURFACE FROM QUANTUM AB-INITIO INVESTIGATIONS

Various ice samples are prepared from highly concentrated H2O/Ar matri xes submitted to various annealing treatments. Each of these samples c orresponds to a different stage in the solid organization from amorpho us to nanocrystalline clusters. The type II isotherms are drawn from a 0.1 to a few monolayers surface coverage in the 43-48 K temperature r ange as a function of the integrated absorbance of the carbon monoxide (GO) vibrational mode, measured by FTIR spectroscopy, vs the CO equil ibrium pressure. In the frame of the Brunauer, Emmett, and Teller (BET ) model the mean enthalpy of adsorption of the first CO monolayer is e valuated to be about 10 kJ mol(-1) and is independent of the history o f the ice surfaces. The quantum modeling on a perfect ice surface lead s to adsorption energies in good agreement with experimental results f or an isolated admolecule as well as for the monolayer. The CO adsorbs perpendicularly to the surface plane, and the two orientations of CO or OC are energetically equivalent. Adsorption on a surface defect mod eled as a hole indicates hydrogen bonding between the admolecule and t he substrate. Comparing experimental and quantum results, we may concl ude that the CO molecule sees the ice surface as covered by protons th at make the adsorption dynamic process insensitive to the presence of surface defects.