Effect of crystallization and surface potential on the nitrogen adsorption isotherm on graphite: A refined Monte Carlo simulation

The low temperature nitrogen-graphite system, being relatively simple, presents, however, a significant challenge in accurate description of the experimental adsorption isotherm at 77 K, which is widely used as a reference system in characterization of porous carbon materials. So far all attempts to rigorously describe the isotherm failed to reproduce an experimentally observed sub-step at about 1 kPa corresponding to the two-dimensional fluid-solid transition and significantly underestimated the onset of the second layer formation. To eliminate these discrepancies a recently developed methodology based on a kinetic Monte Carlo method has been applied to this system, which explicitly models the gas-fluid/solid equilibrium. It has been shown that the crystallization of nitrogen in the contact layer at 77 K occurs only due to a high modulation of the potential exerted by graphite arising from anisotropy of the C-N potential and a lower value of carbon atom collision diameter (0.26 nm instead of 0.34 nm suggested by Steele). The latter results in a sharper decay of the adsorption potential with the distance from the graphite surface, which increases the equilibrium pressure in the multilayer region and, therefore, provides excellent agreement between the simulated and experimental isotherms.