Effects of a Free Adsorbate Boundary on the Description of an Argon Adsorbed Film on Graphite below the Bulk Triple Point

Abstract

Monte Carlo simulations have been carried out to study argon adsorption on graphite at temperatures below the bulk triple point temperature, T tr (bulk) = 83.8 K. Two models for graphite have been used to investigate the effects of an adsorbate patch with a free boundary on the layering temperatures, the two-dimensional (2D)-triple point and the 2D-critical point for the three adsorbate layers on the surface. The first model (S-model) has a planar surface of infinite extent in the two directions parallel to the surface, and the second is a finite (2D-patch model). Although simulations using both models describe the characteristic temperatures, only the 2D-patch model can represent the experimental isotherms accurately, and the condensation pressures at which firstorder transitions occur, while simulations with the S-model yield many unphysical substeps that are not observed experimentally in the first layer adsorbate, which leads to a poor description of higher adsorbate layers. These results support the interpretation that boundary growth of an adsorbate patch is the mechanism for argon adsorption at temperatures below the bulk triple point temperature. Combining the results derived from this simulation study for temperatures below the bulk triple point temperature, with results reported in the literature for temperatures above T tr (bulk) and experimental data, we have constructed a generic pattern for the adsorption isotherms of simple gases on graphite at temperatures ranging from well below the bulk triple point temperature up to the bulk critical temperature, a comprehensive description not widely recognized in the literature.

Adsorption Mechanism and Characteristic Temperatures of the Monolayer Adsorption of CO2 on Graphite: The Role of Graphene Dimensions

“…At this temperature, the grand canonical isotherm exhibits a first-order transition, from points A to E, which occurs at a constant reduced pressure of 0.79, but the isosteric heat across this first-order shows a modest increase. This distinct feature for CO 2 is different from what was observed earlier for argon where its isosteric heat across the first-order transition is constant, supporting that the boundary growth is the sole mechanism of adsorption for argon where an incoming molecule interacts with three adsorbed molecules at the periphery of the adsorbate patch. On the other hand, the modest increase in the isosteric heat for CO 2 across the first-order transition corroborates the 2D-LDD and the cluster analysis about the additional mechanism for the adsorption of carbon dioxide, the vacancy-filling mechanism.…”

“…This indicates that adsorbed molecules lay flat and incoming molecules attaching and interacting with three molecules at the periphery of the adsorbate patch because the pairwise interaction energy between two CO 2 molecules is approximately 2 kJ/mol. This is governed by the boundary growth mechanism and it has been also observed with adsorption of simple gases on a graphene layer …”

“…Figure has shown the effects of the finite dimensions of the graphene layer, especially for 122.5 K below the bulk triple point temperature, While the finite graphene model correctly describes the free boundary of the adsorbate that are always present in physical systems, which allows molecules to densely pack on the surface, , Figure has shown the inadequacy of the smallest graphene layer (model A) in not only the description of the isotherms (that were discussed in Figure ) but also the unusual distribution of the adsorbed molecules. This means that larger graphene layers should be used in simulations to correctly describe the experimental data, and the obvious question that one needs to address is, “What is the smallest suitable dimension of a graphene layer that should be used in simulation?” If the layer is too large, the computation time will be excessive, beyond the capability of a supercomputer, because of not only the greater number of molecules in the system but also the greater number of calculations involved in the determination of the interaction energy between a molecule and the graphene layer of finite dimension.…”

“…The top and bottom boundaries of the simulation box in the z-direction were modeled as a hard wall. 15,16 Grand-Canonical Monte Carlo Simulation. The grand canonical Monte Carlo method (GCMC) was used to simulate adsorption.…”

“…13,14 To resolve this issue, Loi et al 15 were able to describe experimental data of argon for a wide range of temperatures, including temperatures below the bulk triple point of argon. 15,16 To apply the finite model for the description of experimental data, the dimensions of the surface need not be too small to avoid the finite-size effect. 14,15,17 Although surfaces of large dimensions minimize the finite-size effect and better mimic the physical system, simulations require excessive computation times, making them impractical.…”