Calculation of the adsorption isotherm of formaldehyde on ice by grand canonical Monte Carlo simulation

The adsorption isotherm of formaldehyde at the surface of I-h ice has been calculated at 200 K by performing a set of grand canonical Monte Carlo simulations, systematically varying the chemical potential of formaldehyde. In a clear contrast to other partially oxidized small hydrocarbons (POHs), such as methanol, the obtained isotherm does not show any particular stability of the saturated adsorption monolayer of the formaldehyde molecules. Further, in contrast to that of methanol, the obtained adsorption isotherm can be well described with the functional form of the Langmuir isotherm, indicating the relative weakness of the lateral interactions between the adsorbed molecules. The obtained results show that the main thermodynamic driving force of the adsorption is the possibility of the formation of water-formaldehyde hydrogen bonds. However, an adsorbed formaldehyde molecule has never been found to form more than one hydrogen bond with the surface waters, and due to the requirement of maximizing the number of water-formaldehyde hydrogen bonds per surface unit, these hydrogen bonds are often rather distorted, hence they are relatively weak in the case of the saturated adsorption monolayer. Besides these water-formaldehyde hydrogen bonds, the adsorbed molecules are further stabilized by lateral dipole-dipole type interactions, which are, however, considerably weaker than the water-formaldehyde hydrogen bonds, and also weaker than the lateral hydrogen bonding that occurs in the adsorption layer of other POH molecules, such as methanol.