Atomistic Simulations of Rare Gas Transport through Breathable Single-wall Nanotubes with Constrictions and Knees

Abstract

We present the results of molecular dynamics (MD) computer simulations of rare gas diffusion through breathable nanotubes with pentagon–heptagon pair defects resulting in constrictions and knees. Diffusion involves interrupted high speed “choppy” motion with intermittent reversal in velocity direction. Single atoms exhibit a spiral-like path, in contrast to atoms traveling in groups. Considerable resistance to flow appears to reside in the upstream section of the nanotube where density gradients are small, prior to the constriction. Subsequently, considerable density gradients are present and speeds increase, becoming greatest at the tube exit. For the nanotubes examined, Kr and Xe diffusion was too hindered to provide reliable results. Diffusion of He through the nanotubes with knees occurs in a single-file fashion nearly along the center of the tube and the knee has no detectable effect on the diffusion kinetics. Transport diffusion coefficients are in the order of 10^-4–10^-2 cm²/s.

Keywords:

• Nanotube diffusion
• Molecular dynamics
• Rare gases
• Permeation
• Transport diffusivity

Acknowledgements

The first author gratefully acknowledges illuminating discussions with J. Che. Both authors are grateful to the University of Northern Iowa for support of this work through a summer 2002 undergraduate research fellowship.