Molecular Dynamics Study on the Storage of Carbon Dioxide in Single-Walled Carbon Nanotubes at Low Pressures

The storage of carbon dioxide (CO₂) in single-walled carbon nanotubes was studied with molecular dynamics simulation. The influences of the temperature, system average density, and nanotube size on the CO₂ pressure, density distribution, and intermolecular forces were investigated. Multilayer adsorption inside nanotubes was observed as average density increases at lower pressures, which is desirable in industry. Meanwhile, a nanobubble was gradually formed in the center of the nanotube, and the system with the nanobubble was stabilized by the balance between the positive Laplace pressure and the negative liquid pressure when the size of the nanobubble was higher than the critical size. The adsorption effect of the nanotube wall leads to high local condensed density near the wall and stronger intermolecular repulsion, while Laplace pressure results in a low local condensed density in the adsorbed CO₂ near the bubble interface and stronger intermolecular attraction. The stretching effect that originates from the intermolecular force dominated by attraction in the condensed phase leads to low pressure. At the critical nanobubble size, a higher CO₂ average density can be achieved by lowering the temperature and increasing the nanotube radius or length. When the adsorption impact of the nanotube wall on bubble destabilization becomes negligible as the adsorption layer thickens, further increasing the nanotube radius leads to limited increase of the average density at the critical nanobubble size. The simulation of a graphene-sealed nanotube confirmed the formation of a vapor nanobubble under more realistic conditions. This work provides insights into utilizing carbon nanotubes as a material for CO₂ capture with multilayer adsorption at lower pressures.