Paper title:
A molecular dynamics simulation of water transport through C and SiC nanotubes: application for desalination
Journal:
Int. J. Nano Dim. 2(3): 151-157, 2011
Author/s:
F. Moradi Garakani* and R. Kalantarinejad
Aerospace research Institute, Tehran, Iran.
Summary:
In this paper, the conduction of salt solution through two separated bundles of carbon and silicon carbide nanotubes was simulated using molecular dynamics (MD). Both nanotubes were armchair-type with chiral vectors (7,7) and (5,5) for C and Si-C, respectively. Applied pressure was varied to obtain its effect on water and ion flowrates, which would determine the more suitable nanotube for desalination.
Simulations and analyses were performed using NAMD and VMD. Each bundle was made up of four (4) C or Si-C nanotubes having a length and diameter of 11.45 and 9.45 ?, respectively. The water box containing 2 pairs of NaCl salt ions had dimensions of 27 x 21 x 30 nm3. TIP3 model was used to represent water during the simulations. The system was equilibrated for 20 ns to achieve an NVT ensemble and to relax the system and its components. Afterwards, succeeding simulations were performed with a timestep of 2 fs.
To apply hydrostatic pressure differences, forces directed along the z-direction were applied on the O atoms of the water molecules. These forces added to the opposing damping forces originating from the Langevin thermostat were the two forces that acted on the particles in the system. Overall pressure difference was calculated by multiplying the number of water molecules to pressure (force over cross-sectional area of membrane).
In order to determine the relationship between water flow and applied pressure, the total number of water molecules inside each nanotube was first counted. It was observed that at 10 MPa, water molecules fill the four tubes in an alternating manner while for 100 MPa, they fill the tubes simultaneously. In addition, water molecules inside the CNTs were arranged in a double chain structure for all three applied pressures. For Si-C nanotubes, the water structure at pressures below 150 MPa was single chain while for higher pressures, it became double chain.
For the ions, it was observed that at higher pressures, Na+ ions entered both C and Si-C nanotubes more easily than Cl- ions because of the larger solvated layer around the heavier chloride ions. However, for Si-C tubes, the differences between the positive and negative ions entering the tubes were less pronounced.
Finally, it was also observed that both C and Si-C nanotubes were slightly elongated after the simulation.
The paper concluded by stating that it is more efficient to use Si-C nanotubes for lower pressures (below 150 MPa) while for higher pressures, CNTs were better. It was also found that CNTs are more sensitive to pressure changes than Si-CNTs.
Contribution and application:
This paper is another proof that MD simulations may be used to investigate the feasibility of using existing and new materials for desalination. It also presented a relatively detailed procedure on how to practically apply hydrostatic pressure differences in MD simulations.
By: Hannah Ebro
hannah@gist.ac.kr