A Computational Study of Molecular Diffusion and Dynamic Flow through Carbon Nanotubes

Abstract

Molecular dynamics simulations are used to study the flow of methane, ethane, and ethylene through carbon nanotubes at room temperature. The interatomic forces in the simulations are calculated using a classical, reactive, empirical bond-order hydrocarbon potential coupled to Lennard-Jones potentials. The simulations show that the intermolecular and molecule-nanotube interactions strongly affect both dynamic molecular flow and molecular diffusion. For example, molecules with initial hyperthermal velocities slowed to thermal velocities in nanotubes with diameters less than 36 Å. In addition, molecules moving at thermal velocities are predicted to diffuse from areas of high density to areas of low density through the nanotubes. Normalmode molecular thermal diffusion is predicted for methane for nearly all the nanotube diameters considered. In contrast, ethane and ethylene are predicted to diffuse by normal mode, single-file mode, or at a rate that is transitional between normal-mode and single-file diffusion over the time scales considered in the simulations, depending on the diameter of the nanotube. When the nanotube diameters are between 16 and 22 Å, ethane and ethylene are predicted to follow a helical diffusion path that depends on the helical symmetry of the nanotube. The effects of atomic termination at the nanotube opening and pore-pore interactions within a nanotube bundle on the diffusion results are also considered.

Strong correlations and Fickian water diffusion in narrow carbon nanotubes

“…Beckstein et al [3,4] have extended such simulations to 2 study the density fluctuations of water inside other hydrophobic pores. Transport of oxygen and organic molecules such as methane, ethane and ethylene through carbon nanotubes has also been studied [13,14,15,16] through MD simulations.…”

“…The simulations of Mao et al [14] suggest that spherical methane molecules exhibit normal mode diffusion inside closed carbon nanotubes, whereas aspherical molecules such as ethane and ethylene diffuse by a mode which is between normal and single-file. In all these simulations, the time over which the MSD is measured is rather short (100 − 500 ps).…”

A novel algorithm to model the influence of host lattice flexibility in molecular dynamics simulations: Loading dependence of self-diffusion in carbon nanotubes

“…It is therefore not comparable to our work. Sinnott and co-workers [29][30][31][32] simulated the carbon nanotube as a flexible framework. They studied entrance effects, transport diffusion, and molecular diffusion for various carbon nanotubes and various guest molecules.…”

Separation of Co2 From Flue Gases by Carbon-Multiwall Carbon Nanotube Membranes

“…While this simplifies the simulation considerably, our previous work has demonstrated that molecular shape can have a significant influence on molecular diffusion through nanopores [6]. Therefore the approach that is being taken is one where the atoms are treated explicitly and the molecules are characterized with Coulomb and Lennard-Jones (LJ) potentials [7,8].…”