Computational modeling of adsorption of microporous adsorbents. Communication 1. Water molecules on a carbon surface

Abstract

1. The interaction energy of a molecule of water with a model of a fragment of a carbon surface, calculated with atom-atom potential functions in the optimum position of the molecule, is ∼13 kJ/mole. 2. Numerical modeling by the Monte Carlo method showed that in clusters containing ∼20 molecules of water near a carbon surface, the molecules are not optimally oriented with respect to the surface due to formation of hydrogen bonds between them. However, the interactions with the surface are sufficient to stabilize the flattened, spread shape of the cluster over the surface. The water molecules in this cluster form a slightly lower average number of hydrogen bonds than in a “free” cluster. 3. The interaction of a water molecule with a model primary adsorption site (carbonyl group) with optimum placement is ∼-32 kJ/mole. Addition of a molecule of water to a molecule located near a carbonyl group results in approximately the same decrease in the energy. Water molecules adsorbed on primary sites (or simply on a carbon surface) can be effective secondary adsorption sites. 4. In a cluster of water molecules located on a surface containing carbonyl groups, there are fewer hydrogen bonds per molecule of water than in the case of a cluster on a purely carbon surface on the average. However, due to the interaction with the “primary sites,” the energy of such a system is lower and the adsorption layer is stabilized.