Keywords: nanotechnology, carbon nanotubes, molecular dynamics, carbon dioxide, greenhouse gases, density layers, diffusion, adsorption.

Understanding the flow throughout carbonaceous materials is crucial for a great number of applications varying from bio-engineering to environmental sciences. The limitations, in terms of cost and accuracy, of the experimental methods when applied to nano-scales have motivated both the industrial and scientific community to embrace computational studies for studying phenomena at these scales [1]. Among the computational tools, molecular dynamics (MD) has a vital role in understanding the fluid flow through nanostructures such as carbon nanotubes (CNT) [1]. CNTs are considered one of the most prominent nanomaterials for gaseous and liquid mixtures separation.

The present paper investigates the capture of greenhouse emissions by CNTs through MD simulations. The greenhouse gasses are in reality mixtures and therefore in the paper we study the behaviour of a CO₂-Ar mixture passing through single-walled (SWNT) CNTs. The layering phenomena of CO₂ and N₂ molecules along with their relative adsorption are investigated as the mixture is transported through the nanotubes. The effects of pore size, temperature and pressure to the layering formation of the binary mixtures are also studied. Different carbon nanotubes are examined, spanning from (6,6) to (30,30) either in a self cylindrical (SWNT). Simulations are performed for pressures ranging from 5-20 bar at 300K, respectively keeping both gasses in non-supercritical conditions in bulk [2,3]. The simulations are carried out using the NVE ensemble with the number of particles ranging from 1,000 to 15,000, depending on the values of pressure and temperature along with a Langevin thermostat. The particle positions are updated using the leapfrog Verlet scheme with a timestep of 1fs. The shake algorithm is used to apply bond and angle constraints in the CO₂ and Ar molecules. An initial number of 2x10⁵ time-steps are performed for equilibration and an additional 2x10⁸ time-steps, are carried out for analysing the adsorption process, studying the density's profiles, and calculating the gas self diffusion coefficients.

Numerical simulations show that well defined layers are developed around the internal and external surface of the nanotubes for all the cases examined. The number of layers along with their relative strength varies as a function of the nanotube chirality, size, partial densities and gas-structure interactions.

1

A. Fujiwara, K. Ishii, H. Suematsu, H. Kataura, Y. Maniwa, S. Suzuki, Y. Achiba, "Gas adsorption in the inside and outside of single-walled carbon nanotubes", Chemical Physics Letters, 336(3-4), 205-211, 2001. doi:10.1016/S0009-2614(01)00111-7

2

A.I. Skoulidas, D.S. Sholl, J.K. Johnson, "Adsorption and diffusion of carbon dioxide and nitrogen through single-walled carbon nanotube membranes", Journal of Chemical Physics, 124(5), 1-7, 2006. doi:10.1063/1.2151173

3

P. Atkins, J. de Paula, "Atkins' Physical Chemistry", 7th edition, Oxford University Press, New York, 2002.

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