Bin-Monte Carlo simulation of ethylene coexistence and of ethylene adsorption on graphite

Bin-Canonical and Grand Canonical Monte Carlo (Bin-CMC and Bin-GCMC) simulations have been carried out to study vapour–liquid equilibrium (VLE) and vapour–solid equilibria (VSE) of ethylene and its adsorption on a graphite surface. Earlier experimental studies of this system have shown a number of interesting features including: (1) constant isosteric heat in the sub-monolayer coverage region for temperatures below the triple point, (2) a finite number of layers at the saturation vapour pressure at low temperatures, (3) a very low heat of adsorption (5 kJ/mol) at saturation. Our Bin-CMC simulation results agree with the experimental observations (1) and (2), but do not reproduce the third observation. We attribute this failure to the well-known fact that simulation is not able to reproduce the crystallisation process which is believed to take place in this system, followed by boundary growth on the crystallites. Our Bin-CMC simulations show clear spikes in the isosteric heat curves in the vicinity of monolayer formation over a range of temperature from 98 K to 120 K. Although not reported in earlier experiments, we conjecture that these spikes should be observable using a continuous calorimetric technique.

Figure optionsDownload as PowerPoint slideHighlights
► Detailed description of vapour–liquid and vapour–solid equilibria of ethylene with the effective Bin-Monte Carlo simulation.
► Correct description of constant heat of adsorption in the sub-monolayer coverage region for temperatures less than the triple point.
► Correct description of finite number of layers at the saturation vapour pressure, in agreement with experimental data.
► Description of the evolution of heat spike with temperature in the plot of isosteric heat versus loading.