Paper title:  Ion-separation and water-purification using single-walled carbon nanotube electrodes
Journal: Desalination 277 (2011) 236-243

Author(s): Amir Taghavi Nasrabadi, Masumeh Foroutan* 
Department of Physical Chemistry, School of Chemistry, College of Science, University of Tehran, Tehran, Iran

Summary:
The authors of the paper used molecular dynamics (MD) simulation to show that carbon nanotubes (CNTs), which are charged either positively or negatively, may be used as nanoelectrodes to separate sodium and chloride ions in aqueous solution. As a consequence, the ion-pair of the salt breaks up and desalination takes place.

Simulations were done using two (10,10) single-walled CNTs (SWCNTs) having a diameter of 13.56 A and length of 11.22 A. The volume of the system, which was 27.5 × 27.5 × 33.66 A3, contained the nanotubes surrounded by aqueous sodium chloride solution. There were 400 atoms of for the SWCNTs, 40 sodium ions, 40 chloride ions, and 820 water molecules. The system also employed periodic boundary conditions in all directions. Water, ions and nanotubes were setup using established models. Long-range interactions were taken care of by Particle Mesh Ewald method having a cut-off distance of 10 A. Same cut-off distance was applied to calculate short-range van der Waal’s forces.

The velocity Verlet algorithm was employed for the simulations together with the Nose-Hoover thermostat, which acted as the temperature regulator. The simulations were performed under NVT ensemble (300 K). When all parameters were set, equilibriation was done for 1 ns MD run. Actual simulations lasted for 2 ns, integrating every 1 fs and saving coordinates every 1 ps.

Three different electrostatic charge densities and one neutral charge density were used for the study. It was proved that neutral CNTs are hydrophobic because of their graphitic structure. On the other hand, charged nanotubes caused water molecules to be attracted on their walls, as proven by the radial distribution function (RDF) profiles of both positively- and negatively-charged CNTs. It is also important to note that as the charge density increases, the height of the first maximum peak in the RDF profile for both C-Na+ and C-Cl? also increases. This further implies an increase in the attractive interactions between CNTs and the ions.

The paper also investigated about the encapsulation and contact adsorption of ions. In addition, it observed that there was a significant increase in the first max peak of the RDF of oxygen-oxygen when the highest charge density was used. These and other results of the study confirmed the viability of using CNTs as nanoelectrodes in achieving ion transport and separation and water purification. In summary, it demonstrated that by simply varying the charge density of carbon nanotubes, the extent of ion separation may be adjusted accordingly.


Contribution and application:

The paper validated the flexibility of using MD in analyzing molecular systems, specially in the area of desalination. Aside from this, it showed how carbon nanotubes are easily modified to study the separation of ions during seawater reverse osmosis.



By: Hannah Ebro
hannah@gist.ac.kr