### Abstract

In this paper we consider the adsorption of argon on the surface of graphitized thermal carbon black and in slit pores at temperatures ranging from subcritical to supercritical conditions by the method of grand canonical Monte Carlo simulation. Attention is paid to the variation of the adsorbed density when the temperature crosses the critical point. The behavior of the adsorbed density versus pressure (bulk density) shows interesting behavior at temperatures in the vicinity of and those above the critical point and also at extremely high pressures. Isotherms at temperatures greater than the critical temperature exhibit a clear maximum, and near the critical temperature this maximum is a very sharp spike. Under the supercritical conditions and very high pressure the excess of adsorbed density decreases towards zero value for a graphitesurface, while for slit pores negative excess density is possible at extremely high pressures. For imperfect pores (defined as pores that cannot accommodate an integral number of parallel layers under moderate conditions) the pressure at which the excess pore density becomes negative is less than that for perfect pores, and this is due to the packing effect in those imperfect pores. However, at extremely high pressure molecules can be packed in parallel layers once chemical potential is great enough to overcome the repulsions among adsorbed molecules.

This project is supported by the Australian Research Council.

I. INTRODUCTION

II. THEORY

A. Fluid-fluid potential

B. Molecular parameters

C. Solid-fluid interaction energy

D. Grand canonical Monte Carlo simulation

III. RESULTS AND DISCUSSION

A. GCMC simulation of GTCB

1. Effects of temperature

B. Graphitic perfect slit pores

1. 6.5-A pore (one-layer pore)

2. 9.5-A pore (two-layer pore)

3. 12.5-, 15.5-, and 19-A pores (three-, four- and five-layer pores)

C. Graphitic imperfect slit pores

IV. CONCLUSIONS

### Key Topics

- Adsorption
- 64.0
- High pressure
- 39.0
- Supercritical fluids
- 13.0
- Carbon
- 11.0
- Intermolecular potential energy surfaces
- 11.0

## Figures

Surface excess (top) and bulk gas density (bottom) vs pressure for argon adsorption on graphite at 253 K.

Surface excess (top) and bulk gas density (bottom) vs pressure for argon adsorption on graphite at 253 K.

Surface excess vs bulk gas density for argon adsorption on graphite surface at 253 K.

Surface excess vs bulk gas density for argon adsorption on graphite surface at 253 K.

2D-density distribution of argon vs distance from the surface at three values of pressure (10, 20, and 100 MPa).

2D-density distribution of argon vs distance from the surface at three values of pressure (10, 20, and 100 MPa).

Snapshots of argon particles in the first layer at 253 K for three values of pressure, (a) 10 MPa, (b) 20 MPa, and (c) 100 MPa.

Snapshots of argon particles in the first layer at 253 K for three values of pressure, (a) 10 MPa, (b) 20 MPa, and (c) 100 MPa.

Surface excess (top graph) and bulk gas density (bottom graph) vs pressure for argon adsorption on graphite at 150, 158, 166, and 253 K (the vertical dashed line is the vapor pressure at 150 K).

Surface excess (top graph) and bulk gas density (bottom graph) vs pressure for argon adsorption on graphite at 150, 158, 166, and 253 K (the vertical dashed line is the vapor pressure at 150 K).

(Top graph) Plot of the first derivative of the bulk gas density vs pressure. (Bottom graph) Plots of the surface excess vs pressure at 158, 166, and 253 K.

(Top graph) Plot of the first derivative of the bulk gas density vs pressure. (Bottom graph) Plots of the surface excess vs pressure at 158, 166, and 253 K.

Surface excess vs bulk density for three values of supercritical temperatures, 158, 166, and 253 K.

Surface excess vs bulk density for three values of supercritical temperatures, 158, 166, and 253 K.

Plot of the excess density (circle symbols), absolute pore density (triangle symbols), and bulk gas density (solid line). (a) Log scale of pressure axis and (b) linear scale of pressure axis.

Plot of the excess density (circle symbols), absolute pore density (triangle symbols), and bulk gas density (solid line). (a) Log scale of pressure axis and (b) linear scale of pressure axis.

Plot of the excess and absolute pore density vs bulk gas density for slit pore of 6.5 A and at 253 K.

Plot of the excess and absolute pore density vs bulk gas density for slit pore of 6.5 A and at 253 K.

Comparison between subcritical adsorption at 87.3 K and supercritical adsorption at 253 K (solid line with symbols: absolute pore density and solid line: excess pore density).

Comparison between subcritical adsorption at 87.3 K and supercritical adsorption at 253 K (solid line with symbols: absolute pore density and solid line: excess pore density).

Snapshots of argon in 6.5-A slit pore at 253 K (, 10, and 1000 MPa).

Snapshots of argon in 6.5-A slit pore at 253 K (, 10, and 1000 MPa).

Local-density distribution vs distance for argon adsorption in 9.5-A pore at 253 K.

Local-density distribution vs distance for argon adsorption in 9.5-A pore at 253 K.

Left figure: Plot of absolute pore density (triangle symbols), excess pore density (circle symbols), and bulk gas density (solid) vs pressure. Right figure: Plot of absolute pore density (triangle symbols) and excess pore density (circle symbols) vs bulk gas density.

Left figure: Plot of absolute pore density (triangle symbols), excess pore density (circle symbols), and bulk gas density (solid) vs pressure. Right figure: Plot of absolute pore density (triangle symbols) and excess pore density (circle symbols) vs bulk gas density.

(a) Snapshot at 100 MPa and 253 K of the first layer in 9.5-A pore. (b) Snapshot at 1000 MPa and 253 K. (c) Snapshot at 100 kPa and 87.3 K.

(a) Snapshot at 100 MPa and 253 K of the first layer in 9.5-A pore. (b) Snapshot at 1000 MPa and 253 K. (c) Snapshot at 100 kPa and 87.3 K.

Plot of absolute pore density (solid line), excess pore density (symbols), and bulk gas density (dashed line) vs pressure (left figure: low-pressure range and right figure: high-pressure range).

Plot of absolute pore density (solid line), excess pore density (symbols), and bulk gas density (dashed line) vs pressure (left figure: low-pressure range and right figure: high-pressure range).

Plot of excess pore density (top graph) and absolute pore density (bottom graph) vs bulk gas density for pores 9.5, 12.5, 15.5, and 19 A.

Plot of excess pore density (top graph) and absolute pore density (bottom graph) vs bulk gas density for pores 9.5, 12.5, 15.5, and 19 A.

Local-density distribution vs distance from pore wall for 12.5-, 15.5-, and 19-A pores.

Local-density distribution vs distance from pore wall for 12.5-, 15.5-, and 19-A pores.

Plot of excess pore density (circle symbols), absolute pore density (triangle symbols), and bulk gas density (dashed line) vs pressure for 8- and 11.5-A pores (left figure: linear pressure scale and right figure: logarithm pressure scale).

Plot of excess pore density (circle symbols), absolute pore density (triangle symbols), and bulk gas density (dashed line) vs pressure for 8- and 11.5-A pores (left figure: linear pressure scale and right figure: logarithm pressure scale).

Plot of absolute and excess pore densities vs bulk gas density for 8- and 11.5-A pores at 253 K.

Plot of absolute and excess pore densities vs bulk gas density for 8- and 11.5-A pores at 253 K.

Local-density distribution vs distance from pore wall with various values of pressure (left figure: 8-A pore and right figure: 11.5 A pore).

Local-density distribution vs distance from pore wall with various values of pressure (left figure: 8-A pore and right figure: 11.5 A pore).

Plot of absolute and excess pore densities vs bulk gas density for the 9.5-A pore at 158 and 253 K.

Plot of absolute and excess pore densities vs bulk gas density for the 9.5-A pore at 158 and 253 K.

## Tables

Experimental studies of supercritical fluids.

Experimental studies of supercritical fluids.

Summary of the literature review on empirical and semiempirical approaches.

Summary of the literature review on empirical and semiempirical approaches.

Summary of the literature review on potential, DFT, and molecular simulation approaches.

Summary of the literature review on potential, DFT, and molecular simulation approaches.

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