Modelling Water Flow through Carbon Nanotubes

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• Modelling Water Flow through Carbon Nanotubes
• S. P. Shaw and D. A. Faux
• Advanced Technology Institute, Faculty of
  Engineering and Physical Sciences, University of
  Surrey, Guildford, GU2 7XH

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Outline

• Motivation
• Carbon Nanotubes (CNTs)
• Modelling CNTs
• Modelling water
• Simulation cell
• The current picture
• Next steps

Faculty of Engineering and Physical Sciences ATI
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Motivation

• Considerable interest in the general field of
  water in confined spaces
• Potential applications of water flow through
  nanotubes include
• Desalination
• Biosensors
• Nanovalves

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Confinement of water molecules within the
one-dimensional nanotube structure has a profound
effect on the diffusive behaviour of water

• Fast molecular transport through single walled
  nanotubes
• Formation of an inner tube of water molecules
  with a liquid-like chain of water molecules
  inside
• Temperature dependence effect of CNT diameter
  not well characterised for water
• More work is needed on molecular adsorption and
  transport properties

Faculty of Engineering and Physical Sciences ATI
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Carbon Nanotubes

• Carbon Nanotubes are a allotrope of carbon
• Sheets of graphite, just one atom thick rolled
  into a cylinder

• They can be semiconducting or metallic depending
  on the way they are rolled, described by their
  chiral vector, (n,m)

• If 2n m 3q (qinteger), CNT is metallic

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Modelling Carbon Nanotubes

• Carbon-Carbon interactions are modelled using a
  Tersoff Potential
• Strength of C-C bond depends on
• Distance between carbon atoms AND
• The coordination number (number of neighbours
  each atom has)
• High coordination number weak C-C bonds
• Potential includes
• Sum of pair-like interactions
• Attractive term is dependent on the local
  environment

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Modelling Water

• Simple models
• Rigid bonds
• Point charges on nuclei
• Lennard-Jones potential between molecules

• More complex models
• Flexible, harmonic potentials for bond length
  and bond angle
• Additional charges at zero-mass sites

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Modelling water II

• Water is modelled using
• a flexible single point charge interaction
  (SPC/Fw) potential
• that has been optimised to accurately reproduce
  the self-diffusion coefficient

Ds Exp. SPC SPC/Fw
Diffusion coefficient 2.3 x10-5 cm2 s-1 4 x10-5 cm2 s-1 2.4 x10-5 cm2 s-1
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Putting the pieces together

• Simulation cell consists of
• 2300 water molecules, modelled with SPC/Fw
• 4 (15,0) 1.19nm diameter carbon nanotubes, each
  with 450 carbon atoms
• 1 atom at either end of each tube have fixed
  x,y,z
• Fixed atoms keep the CNTs aligned parallel to z
  axis
• All simulations are performed using the DL_POLY
  molecular dynamics simulation package, developed
  by Daresbury Laboratory

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Interaction between the pieces

• Interaction between the water molecules and the
  CNTs is modelled with a Lennard-Jones pair
  potential acting between the oxygen and the
  carbon atoms.

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The simulation cell
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Next steps

• Calculate diffusion rate mechanism(s) as a
  function of
• CNT diameter
• Temperature
• Model the diffusive properties using analytic
  diffusion models
• Investigate the diffusive behaviour of salinated
  water

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Thank You
Faculty of Engineering and Physical Sciences ATI