On the hysteresis of low temperature adsorption of xenon in graphitic wedge pore

Abstract

We used Monte Carlo simulation with canonical and grand canonical ensembles to investigate the structure of the adsorbate and the hysteresis in the isotherm for xenon adsorption in a closed-end graphitic wedge pore at temperatures below the simulated bulk triple point of 159 K. The simulation results are compared with those of the corresponding open-end wedge to reveal the interesting behaviours in the presence of the closed end. We found that when the apex angle is less than a critical angle hysteresis occurs because of the ordering of the adsorbate. This is due to the enhancement in the solid-fluid interactions that induces the adsorbate into a sequence of domains of ordered layers, separated by smaller disordered junctions, and each domain has its own characteristic temperature above which it becomes disordered. The difference in the pressure at which the ordered domain forms during adsorption and that at which the evaporation occurs upon desorption is the microscopic reason behind the hysteresis, and we can view the alternate domain/junction as the solid-like structure of the adsorbate in the mesoscopic scale. However, as the adsorbate progresses further away from the apex, it becomes disordered with a clear interface separating the adsorbate and the bulk gas, and it is found that the functional form of the Cohan equation can be used to relate the radius of curvature of the interface to the bulk pressure. Through this equation we have found that the adsorbate is denser than the supercooled liquid, and yet is not as structured as the bulk solid for temperatures below the bulk triple point temperature.