Adsorption of CO and N 2 molecules at the surface of solid water. A grand canonical Monte Carlo study.

The adsorption of carbon monoxide and nitrogen molecules at the surface of four forms of solid water is investigated by means of grand canonical Monte Carlo simulations. The trapping ability of crystalline Ih and low-density amorphous ices, along with clathrate hydrates of structures I and II, is compared at temperatures relevant for astrophysics. It is shown that when considering a gas phase that contains mixtures of carbon monoxide and nitrogen, the trapping of carbon monoxide is favored with respect to nitrogen at the surface of all solids, irrespective of the temperature. The results of the calculations also indicate that some amounts of molecules can be incorporated in the bulk of the water structures, and the molecular selectivity of the incorporation process is investigated. Again, it is shown that incorporation of carbon monoxide is favored with respect to nitrogen in most of the situations considered here. In addition, the conclusions of the present simulations emphasize the importance of the strength of the interactions between the guest molecules and the water network. They indicate that the accuracy of the corresponding interaction potentials is a key point, especially for simulating clathrate selectivity. This highlights the necessity of having interaction potential models that are transferable to different water environments.