Fouling Mechanisms in Y-shaped Carbon Nanotubes

1. Fouling Mechanisms in Y-shaped Carbon Nanotubes Jason Myers, SeongJun Heo, and Susan B. Sinnott Department of Materials Science and Engineering University of Florida Funded by the Network for Computational Nanotechnology at Purdue University, NSF Grant No. EEC-02288390

2. Outline • Background • Computational Methods • System Design • Results • Conclusions

3. The Need for Filtration The chemical and biomedical fields have a constant demand for solutions of greater purity. Current filtration methods (zeolites) do not offer uniform pore size, and are susceptible to fouling. Carbon nanotubes (CNTs) have the potential to be custom designed for optimal molecular filtration.

4. Nanofluidics: Confinementof fluids to nanopores Carbon nanotubes (CNTs): Honeycomb graphene lattice rolled into a cylinder Ajayan and Zhou (2001) Sinnott et al. (2002) Carbon Nanotubes • Discovered by Ijima, et al in 1991 • Outstanding mechanical properties • Nanometer size enables precise molecular transport

5. Zigzag (10,0) Chiral (7,4) Armchair (6,6) Analagous to a rolled graphene sheet. - One-dimensional axial symmetry. - Spiral conformation: Chirality Carbon Nanotubes, cont. Chiral vector Ch = (n,m) = na1 + ma2

6. Small arm – large molecule is energetically discouraged from entering Y-shaped CNTs Large arm – no similar barrier for large molecules Result? Only the small molecule will pass through the small arm.

7. Outline • Background • Computational Methods • System Design • Results • Conclusions

8. Classical molecular dynamics (MD) simulations (numerically integrating F = ma) Reactive Empirical Bond Order (REBO) Potential Lennard-Jones (LJ) Potential van der Waals Interaction Covalent Interaction Molecular Dynamics • For more details on REBO-MD, see Wen-Dung Hsu’s Breeze presentation.

9. Outline • Background • Computational Methods • System Design • Results

10. Y-shaped CNTs “Ytube1” “Ytube2” CNT Diameter, Å CNT Diameter, Å Branch: Big arm: Small arm:

11. Reservoirs Three different molecules: Methane N-butane Rigid Argon Box Isobutane Push-plate 10, 5, 3, and 0 m/s

12. Rigid Active Thermostat Reservoir Branch Arms Direction of Flow Each system consists of a Y-shaped CNT and reservoir. System Design

13. Outline • Background • Computational Methods • System Design • Results • Methane • Isobutane + Methane • N-butane + Methane • Conclusions

14. Methane Ytube1 Ytube2 6.35 Å 6.92 Å

15. Isobutane + Methane Ytube1 Ytube2 Filtered methane Blocking Isobutane

16. Ytube2, 10 m/s at… N-butane + Methane 0.48ns 0.27ns Stationary n-butane Aligned n-butane

17. Summary Ytube1 shows no tendency for filtration. There is evidence of size-based diffusion in the methane systems. Ytube2 shows no similar behavior. The isobutane + methane systems exhibit fouling. This is attributed to the steric interactions of the isobutane molecule with the junction area, and is not due to a potential energy well. Prior to the formation of the block, filtration occurred in ytube2. There is neither fouling nor filtration in the n-butane + methane systems. Once the driving force is sufficient, the n-butane aligns itself to pass easily down both arms.

18. Outline • Background • Computational Methods • System Design • Results • Conclusions

19. Conclusions • Y-shaped carbon nanotubes exhibit promising signs of filtration. • However, they tend to clog due to molecular steric interactions. • Linear molecules (n-butane) avoid fouling, but prevent filtration. • System redesign with these factors in mind is needed.