Adsorption of Fluorinated Methane Derivatives at the Surface of Ice under Tropospheric Conditions, As Seen from Grand Canonical Monte Carlo Simulations

The adsorption of the fluorinated methane derivatives, CHnF4-n, at the (0001) surface of Ih ice is studied by grand canonical Monte Carlo computer simulation at the tropospheric temperature of 200 K. It is found that CH4 and CF4 adsorbs rather weakly, while CH3F, CH2F2, and CHF3 exhibit multilayer adsorption. The vapor phase of CH4 and CF4 turns out to be rather dense, in accordance with the fact that CF4 is already rather close to, while CH4 is already above, its critical point. Adsorbed CH3F molecules, being in contact with the ice phase, turn with their H atoms toward the ice surface, forming several weak, C-H donated hydrogen bonds with the surface water molecules. By contrast, CH2F2 and CHF3 molecules are found to turn at least one of their F atoms toward the ice phase, forming strong, O-H donated hydrogen bonds with surface waters, in accordance with former infrared (IR) spectroscopy data. Once all hydrogen-bonding positions are occupied, the first molecular layer of these molecules is not yet saturated. Thus, further molecules can be adsorbed in contact with the ice phase, but without forming hydrogen bonds with it