ABSTRACT
The coadsorption of two atmospheric trace gases on ice is characterized by using, for the first time, grand canonical Monte Carlo (GCMC) simulations performed in conditions similar to those of the corresponding experiments. Adsorption isotherms are simulated at tropospheric temperatures by considering two different gas mixtures of 1-butanol and acetic acid molecules, and selectivity of the ice surface with respect to these species is interpreted at the molecular scale as resulting from a competition process between these molecules for being adsorbed at the ice surface. It is thus shown that the trapping of acetic acid molecules on ice is always favored with respect to that of 1-butanol at low pressures, corresponding to low coverage of the surface, whereas the adsorption of the acid species is significantly modified by the presence of the alcohol molecules in the saturated portion of the adsorption isotherm, in accordance with the experimental observations. The present GCMC simulations thus confirm that competitive adsorption effects have to be taken into consideration in real situations when gas mixtures present in the troposphere interact with the surface of ice particles.
ABSTRACT
In this paper, we report grand canonical Monte Carlo simulations performed to characterize the adsorption of four linear alcohol molecules, comprising between two and five carbon atoms (namely, ethanol, n-propanol, n-butanol, and n-pentanol) on crystalline ice in a temperature range typical of the Earth's troposphere. The adsorption details analyzed at 228 K show that, at low coverage of the ice surface, the polar head of the adsorbed molecules tends to optimize its hydrogen bonding with the surrounding water, whereas the aliphatic chain lies more or less parallel to the ice surface. With increasing coverage, the lateral interactions between the adsorbed alcohol molecules lead to the reorientation of the aliphatic chains that tend to become perpendicular to the surface; the adsorbed molecules pointing thus their terminal methyl group up to the gas phase. When compared to the experimental data, the simulated and measured isotherms show a very good agreement, although a small temperature shift between simulations and experiments could be inferred from simulations at various temperatures. In addition, this agreement appears to be better for ethanol and n-propanol than for n-butanol and n-pentanol, especially at the highest pressures investigated, pointing to a possible slight underestimation of the lateral interactions between the largest alcohol molecules by the interaction potential model used. Nevertheless, the global accuracy of the approach used, as tested under tropospheric conditions, opens the way for its use in modeling studies also relevant to another (e.g., astrophysical) context.
