Simulation of absorption sites of acetone at ice: (0001) surface, bulk ice and small-angle grain boundaries

Abstract

Local structures and energies were calculated for the interaction of acetone molecules with ice I_h at the (0001) surface, in the bulk and at small-angle grain boundaries. Force-field methods were used; for the surface additionally ab initio calculations were done. An ordered crystal-structure model of ice I_h in space groupP112₁ (Z = 8) was used. The small-angle grain boundary was set up as a series of line defects with Burgers vectors of [2/3 1/3 1/2] (in the hexagonal lattice of ice I_h). All calculations were carried out with one or two acetone molecules in a sufficiently large simulation box containing up to 4608 water molecules, representing the low concentration of acetone in the atmosphere. The adsorption on the surface is energetically preferred. The acetone molecule is bound to the surface by two hydrogen bonds. This result is in contrast to earlier works with high acetone concentrations where only one hydrogen bond is formed. With two hydrogen bonds the adsorption enthalpy is calculated as −41.5 kJ mol⁻¹, which is in agreement with experimental results. The interaction at small-angle grain boundaries is energetically less favourable than at the surface but much more favourable than in the bulk ice. In bulk ice and at small-angle grain boundaries the acetone molecule is bound by two hydrogen bonds like at the surface. The incorporation of acetone in bulk ice distorts the crystal structure significantly, whereas an incorporation at a small-angle grain boundary leads only to a minor distortion.