We present a grand canonical Monte Carlo simulation study of deformation in graphitic slit pores induced by argon adsorption at sub- and supercritical temperatures. We find that solvation pressure is the driving force for the deformation. This is analyzed by studying its spatial distribution across the pore in order to understand the effects of adsorbate location on the deformation. We find that (1) pore width affects the packing of the adsorbate molecules and note that the zero solvation pressure at saturation pressure could be used to distinguish between commensurate and incommensurate pores, and (2) thermal fluctuation increases with temperature meaning that molecular excursions are closer to the pore walls at high temperatures, resulting in greater repulsion compared to that at lower temperatures. Consequently, the pore deformation depends on an intricate interplay between packing and thermal fluctuation.

The displacement process of CH 4 by the CO 2 injection in the shale micropores plays a dominant role in the CO 2 enhanced shale gas recovery (CO 2 -ESGR). In this paper, we have addressed the displacement of CH 4 by CO 2 in the micropores, and particularly, we have investigated the contribution of each specific pore size from 0.50 to 2.0 nm to the competitive adsorption of CH 4 and CO 2 in terms of the CH 4 recovery and residual CH 4 and CO 2 adsorption after the displacement. The results showed that the micropores have different contributions to the CH 4 recovery depending on the pore size, CO 2 ratio, temperature, and pressure. The pores below 0.61 nm make no contribution to CH 4 recovery, but the 0.55−0.60 nm pores are beneficial for CO 2 storage. The 0.65−0.70 nm pores show the highest CH 4 storage capacity and a high selectivity for CO 2 . As a result, the CH 4 recovery reaches the maximum and is not affected by CO 2 ratio. Besides, the pores above 1.3 nm provide little to the CH 4 recovery at lower pressures, and the injected CO 2 ratio changes the optimum pore size in terms of the maximum CH 4 recovery. The pore size for the maximum CH 4 recovery decreases slightly with the increase of pressure. In addition, the CH 4 recovery density is higher at lower temperatures due to higher preadsorption of CH 4 and lower residual CH 4 capacity. Furthermore, the distribution of the adsorbed CH 4 and CO 2 after the displacement showed that the residual CH 4 distribution is not affected by the injected CO 2 and is randomly located among the adsorbed CO 2 molecules.

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The adsorption and desorption of Kr on graphite at temperatures in the range 60 K to 88 K, was systematically investigated using a combination of several simulation techniques including: grand canonical Monte Carlo (GCMC), canonical kinetic-Monte Carlo (C-kMC) and the Mid-Density scheme (MDS). Particular emphasis was placed on the gas-solid, gas-liquid and liquid-solid 2D phase transitions. For temperatures below the bulk triple point, the transition from a 2D-liquid-like monolayer to a 2D-solid-like state is manifested as a sub-step in the isotherm. A further increase in the chemical potential leads to another rearrangement of the 2D-solidlike state from a disordered structure to an ordered structure that is signalled by (1) another sub-step in the monolayer region and (2) a spike in the plot of the isosteric heat versus density at loadings close to the dense monolayer coverage concentration. Whenever a 2D transition occurs in a grand canonical isotherm it is always associated with a hysteresis, a feature that is not widely recognized in the literature. We studied in details this hysteresis with the analysis of the canonical isotherm, obtained with C-kMC, which exhibits a van der Waals (vdW) type loop with a vertical segment in the middle. We complemented the hysteresis loop and the vdW curve with the analysis of the equilibrium transition obtained with the MDS, and found that the equilibrium transition coincides exactly with the vertical segment of the C-kMC isotherm, indicating the coexistence of two phases at equilibrium. We also analysed adsorption at higher layers and found that the 2D-coexistence is also observed, provided that the temperature is well below the triple point. Finally the 2D-critical temperatures were obtained for the first three layers and they are in good agreement with the experimental data in the literature.

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A grand canonical Monte Carlo simulation study of argon adsorption in deformable graphitic slit mesopores has been carried out, to determine the mechanisms of deformation in the different stages of adsorption and desorption. At pressures less than the condensation pressure, especially in the submonolayer coverage region, the pore walls are slightly compressed. This is due to the decrease in the potential energy of interaction between the adsorbate and the second graphene layer (the interaction with the outermost graphene layer remains the same) and this enhancement of the solid-fluid interaction compensates for the repulsive penalty incurred by compressing of the graphene layers. This mechanism holds, irrespective of pore size and temperature, because at low loadings the two pore walls behave like two independent surfaces. At higher loadings, after condensation has taken place, adsorbate molecules in the interior of the pore attract the pore walls, while those close to the 2 surface repel them. As a result the pore can either contract or expand at high loadings, depending on the balance between these two mechanisms. Across the capillary condensation, the attraction of the condensate in the pore interior is greater than the repulsion by the adsorbate close to the surface, resulting in pore contraction and a corresponding sharp decrease in the solvation pressure. After the capillary condensation, the pore either expands or contracts, depending on the balance between these two processes, which is a function of pore width and temperature, which in turn determine the commensurate or incommensurate packing and thermal motion of the molecules.

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