Predictions of a Spiral Diffusion Path for Nonspherical Organic Molecules in Carbon Nanotubes

The diffusive behavior of ethane and ethylene in single-walled carbon nanotubes is investigated using classical molecular dynamics simulations and density functional theory calculations. At low molecular densities, these nonspherical molecules follow a spiral path inside nanotubes with diameters of $13–22\AA$, which maximizes the interaction of molecular C-C bonds with the C-C bonds in the nanotubes. Spherical molecules, such as methane, are not predicted to follow a spiral diffusion path. This result quantifies the manner in which molecular shape and chemical bonding affects molecule-nanotube interactions and indicates the generality of spherical transport through nanotubes.

Figure 1

The line represents the averaged path followed by 18 ethane molecules diffusing through a (10,10) CNT at 300 K in the MD simulation trajectories.

Figure 2

Molecule-CNT interaction energies and shapshots from the DFT calculations of ethane molecules oriented along the axis of (a) a (10,10) CNT at kinetic energy cutoffs of 100 and 250 eV and (b) a (19,0) CNT at a kinetic cutoff of 100 eV. The energies are for the ethane in various orientations while the snapshots show the optimal configurations predicted by the DFT calculations. The distance between the molecule and the nanotube wall is 0.35–0.45 nm.