Adsorption of benzaldehyde at the surface of ice, studied by experimental method and computer simulation.

Adsorption study of benzaldehyde on ice surfaces is performed by combining experimental and theoretical approaches. The experiments are conducted over the temperature range 233-253 K using a coated wall flow tube coupled to a mass spectrometric detector. Besides the experimental way, the adsorption isotherm is also determined by performing a set of grand canonical Monte Carlo simulations at 233 K. The experimental and calculated adsorption isotherms show a very good agreement within the corresponding errors. Besides, both experimental and theoretical studies permit us to derive the enthalpy of adsorption of benzaldehyde on ice surfaces DeltaH(ads), which are in excellent agreement: DeltaH(ads) = -61.4 +/- 9.7 kJ/mol (experimental) and DeltaH(ads) = -59.4 +/- 5.1 kJ/mol (simulation). The obtained results indicate a much stronger ability of benzaldehyde of being adsorbed at the surface of ice than that of small aliphatic aldehydes, such as formaldehyde or acetaldehyde. At low surface coverages the adsorbed molecules exclusively lie parallel with the ice surface. With increasing surface coverage, however, the increasing competition of the adsorbed molecules for the surface area to be occupied leads to the appearance of two different perpendicular orientations relative to the surface. In the first orientation, the benzaldehyde molecule turns its aldehyde group toward the ice phase, and, similarly to the molecules in the lying orientation, forms a hydrogen bond with a surface water molecule. In the other perpendicular orientation the aldehyde group turns to the vapor phase, and its O atom interacts with the delocalized pi system of the benzene ring of a nearby lying benzaldehyde molecule of the second molecular layer. In accordance with this observed scenario, the saturated adsorption layer, being stable in a roughly 1 kJ/mol broad range of chemical potentials, contains, besides the first molecular layer, also traces of the second molecular layer of adsorbed benzaldehyde.