Molecular dynamics simulation of carbon dioxide in single-walled carbon nanotubes in the presence of water: structure and diffusion studies

Abstract

We present a molecular dynamics study on the structure and diffusion of carbon dioxide in single-walled carbon nanotubes (SWCNTs) in the presence of water. We consider (10,10) and (15,15) SWCNTs of nanotube diameter 13.6 and 20.3 Å, respectively, with amounts of pre-adsorbed water equal to 0, 0.025, 0.1, and 0.2 g/cm3. The density of the carbon dioxide in the SWCNTs corresponds to the maximum amount adsorbed at 300 K and 37.6 bar, as previously determined by grand canonical Monte Carlo simulation. We determine the structure of the confined fluids by calculating density distributions in the nanotube axial and radial directions, along with a second-order Legendre polynomial to elucidate their molecular orientations. The diffusion of the confined molecules is then analysed via both the overall time-dependent and space-dependent mean-square displacements in the nanotube axial direction. We find that the systems display distinct axial regions comprised of either pure carbon dioxide or water clusters that have been penetrated by CO2 molecules. The confined molecules form layered structures that strongly depend on the nanotube diameter. However for particular SWCNTs, the shape of the layered structures is not affected by the presence of pre-adsorbed water. The amount of pre-adsorbed water strongly influences the overall diffusion, while the space variation of fluid densities across the nanotubes has a relatively small effect on the space-dependent diffusion.