The preferential permeation of ions across carbon and boron nitride nanotubes
Chemical Physics 403 (2012) 105?112
Jaber Jahanbin Sardroodi, Jafar Azamat, Alireza Rastkar, Negar Rad Yousefnia
Molecular Simulations Lab., Azarbaijan University of Shahid Madani, BP 5375171379 Tabriz, Iran
This paper used molecular dynamics simulations to investigate the preferential permeation of calcium and chloride ions across carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs).
The system, composed of 1,190 water molecules, 11 Ca2+ or Cl? ions, armchair-type (7,7) and (8,8) nanotubes, and silicon-nitride membrane matrix, was simulated in the presence of an electric field. The diameter of each nanotube is large enough to accept both Ca2+ and Cl?.
Ionic current was calculated by obtaining the normalized transport rate of water with respect to number of transported ions, retention time of ions, as well as the ion-water radial distribution function (RDF). The structure of water inside the nanotubes was also observed. Potential of mean force (PMF) was also obtained for the system to determine the existence of energy barriers for the permeation of ions.
Results show that Ca2+ ions can pass through the (7,7) nanotubes while Cl? ions permeate through the (8,8) nanotubes. This may be attributed to the different orientations of the dipole moment vector of the water molecules and to the electric field inside the nanotubes. Upon examination of the PMFs, it was also shown that there is an energy barrier for Cl? ions inside the (7,7) nanotubes while in the (8,8) nanotubes, there is an energy barrier for Ca2+ ions. In addition, results reveal that there is a linear relation between current and electric field, implying that the number of ions and water molecules that can permeate through nanotubes increases linearly with applied electric field.
Contribution and application:
In this study, it was shown that CNTs and BNNTs can be used for desalination because of their ability to allow water and to separate ions. Calcium ion, which is one of the components of inorganic scaling in SWRO systems, could be handled using the nanotubes studied here. On a different note, this paper also presents some useful insights and approach on how to analyze results from MD simulations that can be applied in our own analyses.
By: Hannah Ebro