Explanation of the unusual peak of calorimetric heat in the adsorption of nitrogen, argon and methane on graphitized thermal carbon black

Heats of adsorption and adsorption isotherms of argon, nitrogen and methane on a perfect graphitic surface and a defective graphitic surface are studied with a Grand Canonical Monte Carlo Simulation (GCMC). For the perfect surface, the isosteric heat versus loading shows a typical pattern of adsorption of simple fluids on graphite. Depending on adsorbate, degree of graphitization and temperature, a spike in the heat curve versus loading is observed when the first layer is mostly covered with adsorbate molecules. The heat spike is observed for argon and nitrogen at 77 K while for argon at 87.3 K it is no longer present. These simulation results are consistent with the experimental data of J. Rouquerol, S. Partyka and F. Rouquerol, J. Chem. Soc., Faraday Trans. 1, 1977, 73, 306. In the case of methane we observe heat spikes at low temperatures, 84.5, 92.5 and 104 K. The heat spike shifts to higher loading with temperature and it then disappears at high temperatures. These observations are in qualitative agreement with the experimental data of A. Inaba, Y. Koga and J. A. Morrison, J. Chem. Soc., Faraday Trans. 2, 1986, 82, 1635. In all cases where heat spikes are observed, the GCMC simulation results indicate that the heat spike is associated with the squeezing of molecules into the already dense first layer, and the rearrangement of molecules to form a highly structured fluid of this layer. While this squeezing into the first layer is happening, molecules continue to adsorb onto the relatively sparse second layer.