Adsorption isotherm of formic acid on the surface of ice, as seen from experiments and grand canonical Monte Carlo simulation

The adsorption isotherm of formic acid on proton disordered hexagonal (I-h) ice is determined by computer simulation (grand canonical Monte Carlo, GCMC) at 200 K and by experiments (flow tube measurements) at 187, 197, 209 and 221 K. The simulation results are in a good agreement with the experimental data; all of the deviations can be explained by a small (10-20 K) temperature shift of the properties of the model system simulated on the phase diagram. The adsorption behavior of formic acid is found to be very complex. Up to the relative pressure of about 0.04, the isotherms exhibit Langmuir behavior. In this pressure range, only the adsorption sites corresponding to the deepest adsorption energy are occupied, and the orientation of the adsorbed molecules is such that there is no considerable lateral interaction between them. Above this pressure, the adsorption isotherm exhibits another steeply rising part, corresponding to the filling of the first molecular layer by molecules occupying different adsorption sites in different orientations. This part of the isotherm can still be described by the Brunauer-Emmett-Teller (BET) equation. In this pressure range, the adsorbed molecules strongly interact with each other, often forming cyclic hydrogen bonding dimers. Finally, the first molecular layer becomes saturated at the relative pressure of about 0.5; above this pressure, multilayer adsorption occurs, as reflected in the sudden increase of the adsorption isotherm. Here, the neighboring formic acid molecules often form cyclic hydrogen-bonding dimers with each other both within the same layer and also between two consecutive layers, involving both oxygen-hydrogen...oxygen (O-H center dot center dot center dot O) and carbon-hydrogen...oxygen (C-H center dot center dot center dot O) type hydrogen bonds.