On the description of argon adsorption on graphite for temperatures below the 2D-critical temperature

Abstract

Computer simulations have been carried out in order to study the adsorption of argon on planar graphite substrates to shed light on the evolution of the physical structure of the adsorbate as a function of pressure. Planar substrates are generally modelled as having an infinite extent using periodic boundary conditions (PBCs) in the directions parallel to the surface, in order to take advantage of the simple expression for the adsorbate-adsorbent interaction energy. This model has proved to be successful as a description for experimental isotherms at temperatures above the 2D-critical temperature for the first layer, T2D,c. However, it fails at temperatures below T2D,c because the application of PBCs results in a number of unphysical sub-steps in the isotherm in the course of formation of the first layer. It is shown here that these anomalous steps arise from the interactions of molecules in the main box with molecules in the image boxes, creating artefact domains and gaps in the adsorbate. To overcome this we propose a 2D-model for the planar substrate with finite dimensions in the directions parallel to the surface, thereby removing the artificial interactions from the image boxes. Applied to argon adsorption on graphite we find that this finite 2D model describes the experimental adsorption isotherms better than the infinite model, especially at temperatures below the 2D-critical temperature. These simulations suggest that a free boundary on the adsorbate plays an important role in 2D-condensation.