Phase Properties and Wetting Transitions of Simple Gases on Graphite─Characteristic Temperatures of Monolayer Adsorbate

Abstract

Computer simulations were performed to study the characteristic transition temperatures of the adsorbate monolayer transitions on graphite and to determine the layering temperatures for higher layers at temperatures less than the bulk triple point temperature. Two models for graphite were studied to examine the effects of finite size of the graphene layer on the evolution of the characteristics of the monolayer, its boundary with the gas phase, and the resulting isotherm and isosteric heat versus loading. Both models give good agreement with experiment for the 2D-critical point and the 2D-triple point, but the finite model is more successful in representing the experimental isotherm and isosteric heat. Radial density distribution for the monolayer supports this, and it illustrates the manner in which the monolayer is compressed with loading, by mass transfer of molecules from the gas phase through the 1D-boundary of the 2D monolayer adsorbate. As the adsorbed phase grows beyond the monolayer, the structure of the thick adsorbed film was shown to lie between the crystalline structure and the dense supercooled liquid, as reflected in partial wetting, defined as finite loading of the adsorbed film at the bulk sublimation pressure.

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

“…(1) = 60 K, and it was found that the 2D-triple point temperature is around 47 K, in agreement with the simulation results of Loi et al 30 and the experimental data of Migone et al 41 To further substantiate the transition from a gas-like state to a solid-like state for adsorption at 30 K, we analyzed in detail the isosteric heat as a function of loading, shown in Figure 6. Canonical ensemble simulations give the isosteric heats through the first-order transition from the gas-like state to a solid-like state, and these have a constant value of about 12 kJ/ mol, supporting the boundary growth mechanism proposed earlier from analysis of isotherms and molecular configurations in Figures 3 and 4.…”

“…Although these characteristic temperatures follow the above inequality for each layer, it is interesting to investigate the behavior of each characteristic temperature across different layers. We have established the following inequalities from simulations over a wide range of temperatures with both grand canonical and canonical simulations ,,,, We note that the layering temperature increases for higher layers (eq ) and that the difference between the layering temperature of the first layer and that of the second layer is 34 K, and that the difference between the second and third layers is only 13 K. It is expected that the onset of higher layers will be no longer first order, i.e., layering is no longer distinct; not only because the difference between successive higher layers is smaller, and because of the temperature increase toward the bulk triple point (where thermal fluctuations are greater), but also because there will be a weaker contribution from the graphite potential energy. Furthermore, when multiple layers have been formed on the surface, it is expected that condensation of adsorbate at the junctions between microparticles would interfere with the molecular layering.…”

“…We next consider the microscopic mechanism of condensation from a gas-like state to a condensed state across the first-order transition, which shows whether the packing of the adsorbate is liquid-like or solid-like after the condensation. At P6, the density is 11.6 μmol/m 2 , which is slightly less than the dense monolayer density of 12.3 μmol/m 2 . The packing at P6 is solid-like, which raises the question: why is the density at P6 less than the density of a dense monolayer?…”

“…At P6, the density is 11.6 μmol/m 2 , which is slightly less than the dense monolayer density of 12.3 μmol/m 2 . 30 The packing at P6 is solid-like, which raises the question: why is the density at P6 less than the density of a dense monolayer? There are two reasons: (1) the adsorbate patch does not fully cover the 2Dpatch surface, and (2) the adsorbate does not have a perfect close-packed structure because of the effects of the boundary of the adsorbate, where the interactions between molecules and the 2D-graphene patch are weaker than those in the interior of the adsorbate patch.…”

“…29 Although the dimensions of the box can be adjusted to match the lattice spacing of the adsorbate, simulation results still exhibit many substeps in the isotherm, which are artifacts, and not experimentally observed. To circumvent this, Loi et al 30 proposed a 2D-patch model to imitate the effects on the first adsorbate layer, of the finite size of a graphene layer in the plane parallel to the surface. Their simulation results showed good agreement with the experimental isotherms for the monolayer at temperatures below the bulk triple point temperature.…”