Adsorption of HCN at the Surface of Ice: A Grand Canonical Monte Carlo Simulation Study

Adsorption of HCN molecules at the surface of hexagonal (I-h) ice is studied under tropospheric conditions by a set of grand canonical Monte Carlo simulations. Although the adsorption isotherm is of Langmuir shape and the saturated adsorption layer is practically monomolecular, lateral interactions are found to have a non-negligible effect on the adsorption. The Langmuir shape of the isotherm can be rationalized by the fact that the interaction energy for HCN-water and HCN-HCN pairs is rather close to each other, and hence, monomolecular adsorption even in the presence of lateral interactions turns into a special case. At low surface coverages the HCN molecules prefer a tilted orientation, pointing by the N atom flatly toward the ice surface, in which they can form a strong O-H center dot center dot center dot N-type hydrogen bond with the surface water molecules. At high surface coverages, an opposite tilted orientation is preferred, in which the H atom points toward the ice phase and the HCN molecule can form only a weak C-H center dot center dot center dot O-type hydrogen bond with a surface water molecule. This orientational change is dictated by the smaller surface area occupied by a H than a N atom, and the corresponding energy loss is (over)compensated by formation of C-H center dot center dot center dot N-type hydrogen bonds between neighboring HCN molecules. The obtained results have several consequences both on the atmospheric effect of HCN and also on the possible prebiotic formation of precursor molecules of adenine. These consequences are also discussed in the paper.