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Fouling Mechanisms in Y-shaped Carbon Nanotubes

Jason Myers, SeongJun Heo, and Susan B. Sinnott

Department of Materials Science and EngineeringUniversity of Florida

Funded by the Network for Computational Nanotechnology at Purdue University, NSF Grant No. EEC-02288390

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• Background• Computational Methods• System Design• Results• Conclusions

Outline

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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.

The Need for Filtration

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Carbon nanotubes (CNTs):Honeycomb graphene lattice rolled into a cylinder

Ajayan and Zhou (2001)

Sinnott et al. (2002)

Nanofluidics: Confinement of fluids to nanopores• Discovered by Ijima, et al in 1991

• Outstanding mechanical properties• Nanometer size enables precise molecular transport

Carbon Nanotubes

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Analagous to a rolled graphene sheet.- One-dimensional axial symmetry.- Spiral conformation: Chirality

Carbon Nanotubes, cont.

Chiral vector Ch = (n,m) = na1 + ma2

Zigzag (10,0)

Armchair (6,6)

Chiral (7,4)

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Small arm – large molecule is energetically discouraged from entering

Large arm – no similar barrier for large molecules

Result? Only the small molecule will pass through the small arm.

Y-shaped CNTs

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• Background• Computational Methods• System Design• Results• Conclusions

Outline

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Molecular Dynamics

•Classical molecular dynamics (MD) simulations (numerically integrating F = ma)

i ij

ijvdwijAijijR rVrVbrVE

Reactive Empirical Bond Order (REBO)

Potential

Lennard-Jones (LJ) Potential

Covalent Interaction van der Waals

Interaction•For more details on REBO-MD, see Wen-Dung Hsu’s Breeze

presentation.

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• Background• Computational Methods• System Design• Results

Outline

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Y-shaped CNTs

(14,0) 11.12

(10,0) 8.18

(8,0) 6.35

“Ytube1” “Ytube2”

(8,8) 11.00

(6,6) 8.23

(5,5) 6.92

Branch:

Big arm:

Small arm:

CNT Diameter, Å CNT Diameter, Å

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Three different molecules:

MethaneIsobutane

N-butaneRigid Argon Box

Push-plate10, 5, 3, and 0 m/s

Reservoirs

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Each system consists of a Y-shaped CNT and reservoir.

Reservoir Branch Arms

Direction of Flow

Thermostat

Rigid

Active

System Design

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• Background• Computational Methods• System Design• Results

• Methane• Isobutane + Methane• N-butane + Methane

• Conclusions

Outline

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Methane

Ytube1 Ytube2

6.35 Å6.92 Å

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Isobutane + Methane

Ytube1 Ytube2

Filtered methane

Blocking Isobutane

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N-butane + Methane

Ytube2, 10 m/s at…

0.48ns0.27ns

Stationary n-butane

Aligned n-butane

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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.

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• Background• Computational Methods• System Design• Results• Conclusions

Outline

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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.