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

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)

A new interpretation of chemical potential in adsorption systems and the vapour-liquid interface

Chemical potential is a fundamental thermodynamic quantity that is constant everywhere in uniform or non-uniform systems at equilibrium. Because it is not a mechanical variable, its clear interpretation is elusive and its relationship to the energetics of the molecules that make up the system has not been established. In this work, we present a link between the chemical potential and molecular energetics, using a kinetic Monte Carlo scheme. We illustrate this new interpretation using argon as a model species giving examples for adsorption on a graphite surface and for a bulk vapour-liquid equilibrium (VLE). It was found that in either an adsorbed phase or a bulk liquid phase, the chemical potential is associated with repelling molecules, despite the number of these molecules being very small. In a rarefied phase it is associated with attracting molecules. In the interfacial regions in an adsorption system or in a VLE, the energetics of the repelling and attracting molecules contribute equally to the chemical potential

On the canonical isotherms for bulk fluid, surface adsorption and adsorption in pores: a common thread

Kinetic Monte Carlo simulated isotherms calculated in the canonical ensemble, at temperatures below the critical temperature, for bulk fluid, surface adsorption and adsorption in a confined space, show a van der Waals (vdW) loop with a vertical phase transition between the rarefied and dense spinodal points at the co-existence chemical potential, µ . Microscopic examination of the state points on this loop reveals features that are common to these systems. At state points with chemical potentials greater than μ the microscopic configurations show clusters, which coalesce to form two co-existing phases along the vertical section of the loop (the coexistence line). As more molecules are added, the dense region expands at the expense of the rarefied region, to the point where the rarefied region becomes spherical (cylindrical for 2D-systems) with a curvature greater than that of the coexisting phases. This results in a decrease of chemical potential from µ to the liquid spinodal point where the rarefied region disappears. With a further increase in loading, the chemical potential and the density increase. The existence of a vdW loop is the microscopic reason for the hysteresis observed in the grand canonical isotherm, where the adsorption and desorption boundaries of the hysteresis loop are first-order transitions, enclosing the vertical section of the vdW loop of the canonical isotherm. However, a first-order transition is rarely observed in experiments where transitions are usually steep, but not vertical. From our extensive simulations, we provide two possible reasons: (1) the finite extent of the system and (2) the existence of high energy sites that localize the clusters. In the first case, the desorption branch, and in the second case the adsorption branch, either comes close to, or collapses onto the coexistence line. When both occur, the hysteresis loop disappears and the isotherm is reversible, as often observed experimentally

Phase properties and wetting transitions of simple gases on graphite─characteristic temperatures of monolayer adsorbate

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.This work is supported by the Australian Research Council (DP160103540)

Nonwetting/prewetting/wetting transition of ammonia on graphite

Simulations of ammonia adsorption on graphite were carried out over a range of temperatures to investigate the transition from nonwetting to wetting. The process is governed by a subtle interplay between the various interactions in the system and the temperature. At temperatures below the bulk triple point, the system is nonwetting; above the triple point, we observed continuous wetting, preceded by a prewetting region in which the so-called thin-to-thick film transition occurs. This system serves as an excellent example of wetting/nonwetting behavior in an associating fluid as a function of temperature because the heat of sublimation (or condensation) is greater than the isosteric heat of adsorption at zero loading. The nonwetting-to-wetting transition (NW/W) is also strongly affected by the adsorbate-adsorbate interaction, which becomes important when this contribution to the isosteric heat is of a similar magnitude to the heat of condensation. An appropriate indicator of a NW/W transition at a given loading is therefore the difference between the isosteric heat and the heat of sublimation (or condensation). Our simulation results show the "thin-to-thick" film transition in the temperature range between 195 and 240 K, which has not been previously explained. Above 240 K, continuous wetting occurs. This study provides a basis for a better understanding of adsorption in a range of systems because ammonia is an intermediate between simple molecules, such as argon, and strongly associating fluids, such as water

Wedge pore modelling of gas adsorption in activated carbon:Consistent pore size distributions

The pore size distribution (PSD) of a porous solid is usually obtained by matching the experimental adsorption isotherm for argon or for nitrogen at their boiling points against a theoretical isotherm constructed with models of non-connected pores. The seminal work of Rosalind Franklin in 1951, showed that the pore spaces have wedge geometry rather than the parallel side slits commonly used in carbon pore modelling. Based on this, we proposed a wedge geometry for pores as a basis for modelling adsorption in activated carbon; thereby introducing a paradigm shift from the customary slit geometry. Not only does the wedge geometry better reflect the physical pore space, it accounts for the linear connectivity which is absent in the current PSD modelling. We have used extensive simulations to obtain a theoretical isotherm and use it to match experimental isotherms for argon and nitrogen to check the invariance of the PSD with respect to the adsorbate molecule. In this new model, the artefact of zero pore volume at 1 nm, which can occur in the slit model, is absent. Apart from the self-consistency of the derived PSD, the new wedge model also gives a good account for the isosteric heat versus loading

Order-disorder transition of an argon adsorbate in graphitic wedge pores

From her exhaustive X-ray studies of graphitized and non-graphitized carbons, Rosalind Franklin (1951), Franklin (1950, 1951,1953) and Harris (2001) deduced that pores in these materials exist as wedge shaped spaces in the interstices between graphitic micro-crystallites. In this paper we investigate the order-disorder transition of adsorbed argon in carbonaceous materials, modelled as a closed wedge formed by two sets of graphene layers. Despite its simplicity, the model captures the physics of adsorption in a non-uniform pore and gives rise to a rich adsorption behaviour. Our particular focus in this work is on the wetting/filling of argon and its variation with the wedge angle and temperature. We find that incomplete wetting occurs at temperatures below the roughening temperature, T = 70 K, and that continuous wetting occurs above this temperature. We summarise our results as a phase diagram for wetting/filling of argon adsorption in graphitic wedges as a function of wedge angle and temperature

Effects of temperature on the transition from clustering to layering for argon adsorption on substrates of different strength - Parametric map of wetting, pre-wetting and non-wetting

We have carried out extensive simulations of argon adsorption on substrates of different strengths to investigate the effects of temperature on the transition from non-wetting to wetting. For moderately weak substrates, pre-wetting in the so-called “thin-to-thick film” transition occurs. We show that at a microscopic level this is a transition from an adsorbate composed of clusters to one of adsorbed layers. At the wetting temperature (T), this transition occurs at the saturation vapour pressure, and T depends on the relative strength of the intermolecular interaction between adsorbate molecules and between the adsorbate and the adsorbent, and that T increases as the adsorbent strength decreases. The appropriate parameter to account for the role of temperature in the wetting transition is the ratio of the isosteric heat to the heat of condensation, which is a measure of the relative strength between adsorption and cluster formation. At temperatures greater than T, the adsorption mechanism is layer-by-layer wetting if the wetting temperature is lower than the roughening temperature (T). On the other hand, if the wetting temperature is greater than T the mechanism is either continuous wetting or pre-wetting, which then transitions to continuous wetting when the temperature is greater than the critical pre-wetting temperature. Our study shows that the transitions from non-wetting to pre-wetting to wetting, occur not only in the first adsorbate layer, but also in higher layers