Molecular dynamics simulation of the adsorption of oxalic acid on an ice surface.

The adsorption properties of oxalic acid molecules on the surface of hexagonal ice are investigated by means of molecular dynamics simulations performed at tropospheric temperatures. Although the oxalic acid-water interaction is strong at low coverage, due to the possible formation of a large number of hydrogen bonds between the adsorbed oxalic acid and the surface water molecules, the results of the simulations at finite coverage show the predominant role played by the oxalic acid-oxalic acid lateral interactions in the adsorption/desorption process. These interactions are even stronger than the water-water or water-oxalic acid interactions. With increasing temperature these strong lateral interactions favor the formation of oxalic acid aggregates on the ice surface, with the concomitant departure of water molecules through the ducts in the adsorbed layer created by the aggregation process. These results support conclusions of experimental data on the oxalic acid-ice interactions. Moreover, in comparison to previously obtained results for formic and acetic acid adsorbed on ice, the present study suggests that not only the organic functionality is of importance for atmospheric implications of partially oxidized hydrocarbons (POH) interactions with ice, but also the balance between water-ice, water-POH, and POH-POH interactions.