Effects of crystallite stacking on adsorption of ethylene on graphitized thermal carbon black

Adsorption of gases on graphitized thermal carbon black (GTCB) is frequently modelled as a homogeneous planar graphite with infinite extent in the directions parallel to the surface. TEM micrographs of GTCB, however, show polyhedral crystallites with a finite basal plane on their faces. The stacking of these crystallites results in wedge shaped spaces in which the interaction energy with an adsorbate is enhanced by overlapping potentials from the graphitic basal planes of adjacent crystallites in the vicinity of their junction. This enhancement facilitates clustering of molecules and the subsequent formation of a condensate, in addition to molecular layering on the basal planes. As a result, contributions to the adsorbed density observed experimentally originate from both molecules on the basal graphitic planes, and those at the junction of the planes. The former is favoured entropically because of the large area of the basal planes, while the latter is favoured energetically. We have tested this new wedge model by comparison with the experimental data of Menaucourt, Thomy and Duval [Le Journal de Physique, 1977, 38, C4-195] for ethylene adsorbed on exfoliated graphite at three temperatures and show that the simulation results describe the data better than those based on a homogeneous planar graphite model.