Adsorption of acetamide on crystalline and amorphous ice under atmospheric conditions. A grand canonical Monte Carlo simulation study

Abstract

The adsorption of acetamide at the surface of crystalline (Ih) and low density amorphous (LDA) ices under tropospheric conditions is investigated by performing a set of grand canonical Monte Carlo simulations at the temperature of 200 K. Our results show that the adsorption is governed by the possibility of new H-bond formation. Further, acetamide-water and acetamide-acetamide hydrogen bonds are found to be of rather similar strengths, and hence these H-bonds are exchangeable with each other around an adsorbed molecule for practically no energy cost. In the case of monolayers being well below the saturation surface concentration, acetamide molecules prefer to stay parallel with the ice surface and form 3H-bonds with the surface waters. The energy of the adsorption at infinitely low surface concentration is estimated to be –64.5 kJ/mol on LDA and –63.7 kJ/mol on Ih ice. Increasing saturation of the surface triggers a marked change in the orientational preference of the adsorbed molecules, thus, once the interaction between the adsorbed molecules is no longer negligible (which occurs already at mildly unsaturated coverages) they prefer to stay perpendicular to the surface, forming 2 H-bonds with the ice phase and another 2 with each other. As a consequence, the ice and adsorbed phases give similar contributions to the adsorption energy at this coverage, which results in a remarkable stability of the adsorption monolayer. Indeed, after their initial rise the adsorption isotherms exhibit a plateau, corresponding to the more or less saturated adsorption monolayer, in a rather broad range of chemical potential values. The surface concentration of the saturated adsorption monolayer is estimated to be 10.2 μmol/m2 on LDA and 8.1 μmol/m2 on Ih ice. Besides adsorption, non-negligible dissolution of the acetamide molecules is found in both ice phases; however, the concentration of the dissolved molecules is still low enough that the solution exhibits ideal behaviour.