Paper title: Molecular Dynamics Simulation of Salt Rejection in Model Surface-Modified Nanopores
Journal: J. Phys. Chem. Lett., 2010, 1 (2), pp 528?535

Author/s: Jacob Goldsmith and Craig C. Martens*
Department of Chemistry, University of California, Irvine, Irvine, California 92697-2025
*cmartens@uci.edu

Summary: 
The article is about using nonequilibrium molecular dynamics (MD) simulations to investigate the pressure-induced flow of salt and water through modeled nanopores. Models of armchair-type (n,n) CNTs were constructed. The paper found out the effects of nanopore diameter, charge patterns and pressure difference to salt and water flux. The transport of ions and that of water were driven by the pressure difference caused by externally applied pressure.

Five different models corresponding to (8,8), (10,10), (12,12), (14,14) and (16,16) CNTs were constructed by using graphene sheets. The paper described the specifications (size, number of atoms, etc.) of each system as well as the simulation parameters (boundary conditions, temperature, pressure etc.). To facilitate the comparison of the surface-modified model CNTs, a control, which is a native hydrophobic uncharged CNT, was also constructed.

The models had two types of surface-modified systems. The first has alternating positive and negative charges of same magnitude on rings. The separation between these rings increases with increasing distance from the entrance of the pore (z). The second one, on the other hand, has equally spaced rings but the magnitude of the charges are increasing with z. Despite these two cases, the total charge for each still remains zero while the individual charges are kept low, at a value similar to the polarization of carbon atoms in molecular environments.

The nanotubes differing in sizes were compared by calculating both water and ion fluxes through unmodified and surface-modified nanotubes. The flux rates were also compared to the flux prediction using Hagen?Poiseuille (HP) equation of laminar incompressible flow. It was also observed and explained how applied pressure affects the salt and water fluxes.

The paper generalizes that surface patterning of nanotubes with partial charges leads to decreased overall flux of water but increased selectivity of the system. It was also shown that this method of surface modification increases the ion rejection capabilities of larger pores.

Contribution and application:
Similar to other published works about nanotubes, this paper confirms not only the viability of nanotubes but also their efficiency when used in desalination. It shows that surface modification of nanopores might be able to achieve desalination with lower energy requirements since they allow higher flux while maintaining significant ion rejection.



By: Hannah Ebro
hannah@gist.ac.kr