(426f) Transition from Fickian to Single-File Diffusion for Binary Lennard-Jones Mixtures In Single-Walled Carbon Nanotubes

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).

1 J. Goldberger, R. Fan, and P. D. Yang, Accounts Chem.
Res. 39, 239 (2006).

2 D. M. Guldi, G. M. A. Rahman, V. Sgobba, and C. Ehli, Chem.
Soc. Rev. 35, 471 (2006).

3 Dubbeldam, D.
and Snurr, R.Q., Molecular Simulation, 33, 305 (2007).

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