The role of adsorbate size on adsorption of Ne and Xe on graphite

Abstract

We have carried out an extensive grand canonical Monte Carlo simulation to investigate the adsorption of neon and xenon on graphite. The adsorbate collision diameters of neon and xenon are smaller and greater respectively, than the commensurate graphite lattice spacing λ=3×3R30 of 0.426 nm. Simulated isotherms and isosteric heats were obtained using a graphite model that has been shown to describe successfully the adsorbate transitions for krypton, methane and nitrogen by Prasetyo et al. (2017), which have collision diameters close to λ. Neon does not exhibit commensurate (C) packing because the gain in the intermolecular potential interactions in the incommensurate (IC) packing when molecules move away from carbon hexagon centres, does not compensate for the increase in the solid-fluid potential energy. Xenon, on the other hand, exhibits IC packing because its molecular size is greater than λ. Nevertheless, at a sufficiently high chemical potential, the first layer of xenon changes from the IC to C packing (in contrast to what is observed for krypton, nitrogen and methane). This transition occurs because the decrease in the xenon intermolecular interactions is sufficiently compensated by the increase in the solid-fluid interaction, and the increase in the fluid-fluid interactions between molecules in the first layer and those in the second layer. This finding is supported by the X-ray diffraction study by Mowforth et al. (1986) and Morishige et al. (1990).

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

“…24 Panels b and c show the adsorption/desorption isotherms determined from GCMC of argon on an infinite surface and a finite surface, respectively. the pairwise intermolecular separations at the LJ potential minimum, 2 1/6 σ ff , for krypton (0.41 nm) and methane (0.42 nm) are slightly smaller than the lattice spacing of graphite of 0.426 nm, 27 while that for xenon (0.44 nm) is slightly larger. At low temperatures, krypton and methane pack in registry with the hexagonal structure of the surface graphene layer, resulting in a commensurate packing, and transition to an incommensurate packing (i.e., out of registry) as the monolayer is densified.…”

“…At low temperatures, krypton and methane pack in registry with the hexagonal structure of the surface graphene layer, resulting in a commensurate packing, and transition to an incommensurate packing (i.e., out of registry) as the monolayer is densified . On the other hand, adsorbed xenon forms an incommensurate packing, and when the monolayer is densified, the adsorbate becomes commensurate because the xenon molecules are squeezed closer together with an intermolecular spacing smaller than the equilibrium pairwise spacing in registry with the graphene lattice . Since our focus here is to investigate the different characteristic transition temperatures for methane, krypton, and xenon, compared to those of argon, we do not model graphite as a discrete lattice.…”

“…25 On the other hand, adsorbed xenon forms an incommensurate packing, and when the monolayer is densified, the adsorbate becomes commensurate because the xenon molecules are squeezed closer together with an intermolecular spacing smaller than the equilibrium pairwise spacing in registry with the graphene lattice. 27 Since our focus here is to investigate the different characteristic transition temperatures for methane, krypton, and xenon, compared to those of argon, we do not model graphite as a discrete lattice.…”

Fundamental study of adsorption of non-polar and polar fluids on hydrophobic ordered solids

“…For example, the adsorption of Ar/Mg (D*=3.5) 92 shows non-wetting and pre-wetting transitions for temperature above the bulk triple point of Ar whereas Ar/Graphite (D*=9) always shows continuous wetting for the same temperature range 43,49,109 . This contrast of behaviour is also observed for the adsorption of other simple gases on alkali metals 86,92,93,97,98,102 and graphite 26,28,44,110 . Macroscopically, the wetting/non-wetting is determined from the contact angle between the liquid film and solid substrate at the saturation vapour pressure P0 88 .…”

Porosity of Rigid Dendrimers in Bulk: Interdendrimer Interactions and Functionality as Key Factors

“…Therefore, the S and V values found may not reflect the real porosity of rigid-chain dendrimers and may be bigger if gases whose molecules are smaller than CO 2 are used, such as H 2 (molecule size of 2.8 A), Ne (2.4 A), or He (2.0 or 1.95 A). Certainly, this assumption requires an experiential verification since the adsorption of each gas mentioned above and its diffusion into the micropores have some specific features [48,52].…”