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Transition from Fickian to Single-File Diffusion for Binary Lennard-Jones Mixtures In Single-Walled Carbon Nanotubes

Joshua D. Moore1, Qu Chen2, Ying-Chun Liu2, Thomas R. Roussel1, Qi Wang2, and Keith E. Gubbins1. (1) Department of Chemical and Biomolecular Engineering and Center for High Performance Simulation, North Carolina State University, Raleigh, NC 27695-7905, (2) Department of Chemistry, Zhejiang University, Hangzhou, 310027, China

Carbon nanotubes (CNTs) are important building blocks for nanocomposite materials and nanomachinery due to their unique physical properties (electronic, optical, thermal, mechanical, etc.)1,2 and have received extensive interest in research for materials science. One possible application is the use of CNTs as host structures for gas separation as they yield faster diffusion rates of fluid molecules than other widely used materials (e.g., zeolites).  Several diffusion mechanisms are possible in narrow cylindrical pores, but these are still poorly understood3,4.  Among the most important are ballistic motion, Fickian diffusion, and single-file diffusion. Ballistic motion occurs for very short times, before the molecule has had a chance to collide with anything.  For longer times the motion becomes Fickian or single-file.  If the pore is large enough, the molecules will diffuse in 3-dimensions as they would in a bulk fluid, and diffusion obeys Fick's Law.  If the diameter of the pore becomes small enough, the diffusion will crossover from 3-dimensions to a single-file diffusion mechanism, where the molecules can no longer pass each other.  Clearly single-file motion is much slower than Fickian, which is much slower than ballistic. 

We report molecular simulation results on the adsorption and the self-diffusion of binary Lennard-Jones mixtures in narrow armchair (n,n), zigzag (n,0), and chiral (n,m) carbon nanotubes using a combination of GCMC (Grand Canonical Monte Carlo) and MD (molecular dynamics) simulations.  We investigate the effect of properties such as pore loading, pressure, temperature, intermolecular interactions (fluid-wall and fluid-fluid), and CNT flexibility on the mechanism of diffusion (Fickian or single-file).


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4  Kärger, J., Valiullin, R. and Vasenkov, S., New Journal of Physics, 7, 15 (2005).