Anastasios I. Skoulidas

Adsorption and diffusion of carbon dioxide and nitrogen through single-walled carbon nanotube membranes

We have used atomically detailed simulations to examine the adsorption and transport diffusion of CO2 and N2 in single-walled carbon nanotubes at room temperature as a function of nanotube diameter. Linear and spherical models for CO2 are compared, showing that representing this species as spherical has only a slight impact in the computed diffusion coefficients. Our results support previous predictions that transport diffusivities of molecules inside carbon nanotubes are extremely rapid when compared with other porous materials. By examining carbon nanotubes as large as the (40,40) nanotube, we are able to compare the transport rates predicted by our calculations with recent experimental measurements. The predicted transport rates are in reasonable agreement with experimental observations.

Diffusivities of Ar and Ne in Carbon Nanotubes

Atomically detailed simulations are used to compute the self-diffusivity and transport diffusivity of Ar and Ne through single walled carbon nanotube (SWNT) pores at room temperature. The diffusivities are computed over a range of loadings, corresponding to external equilibrium bulk pressures ranging from 0 to 100 bar. The diffusivities in carbon nanotubes are compared with diffusivities of the same gases in silicalite, a common zeolite, under the same conditions. We find that self-diffusivities are one to three orders of magnitude faster in carbon nanotubes than in silicalite, depending on loading. The transport diffusivities are about three orders of magnitude faster in nanotubes than in silicalite over all loadings studied. The equilibrium adsorption isotherms and computed diffusivities are used to predict fluxes through hypothetical membranes of nanotubes and silicalite. The fluxes for both Ar and Ne are predicted to be four orders of magnitude greater through nanotube membranes than through silicalite membranes of the same thickness.

Kinetics of hard sphere and chain adsorption into circular and elliptical pores

Transport of molecules through microporous crystals can be influenced by transport limitations at the mouths of pores. In the limit of high temperatures, the kinetics of adsorption into micropores is dominated by steric effects. To assess the role of molecular shape and pore size on pore mouth adsorption kinetics, discontinuous molecular dynamics has been used to simulate a variety of hard-sphere molecules adsorbing into slit, circular, and elliptical pores. Particular attention has been given to pores with length scales similar to zeolites. By simulating a range of straight chain, branched, and cyclic molecules, the selectivities that can arise due to differences in adsorption kinetics as functions of molecular size and shape have been probed.