Paper title:

Intrinsic Ion Selectivity of Narrow Hydrophobic Pores

 

Journal:

J. Phys. Chem. B 2009, 113, 7642?7649

 

Author/s:

Chen Song and Ben Corry

 

School of Biomedical, Biomolecular and Chemical Sciences, The University of Western Australia, Crawley, WA 6009, Australia

 

Summary:

The article demonstrated the ion selectivity of carbon nanotubes (CNTs). As narrow hydrophobic pores, CNTs of radii between 3.4 to 6.1 ? were found to be intrinsically selective of certain ions. In this study, three ions were used: Na⁺, K⁺, and Cl^?.

 

The potential of mean force (PMF) profile and the free energy of perturbation (FEP) were both used to investigate the ion selectivity of the hydrophobic porous membranes. Both are used for quantifying the energy barrier experienced by ions in different locations, most importantly at the location of the CNT entrance. They are also used to calculate the dehydration energies of the ions.

 

Water and ions face different amounts of free energy barriers. Water was able to spontaneously enter all CNT pores because of the small energy barrier it encountered unlike the ions. For ions, the PMF calculations revealed that CNT pores have intrinsic ion selectivities. As an example, in a (5,5) CNT, the lowest free energy barrier was encountered by K, followed by Na then Cl. For a (6,6) CNT, K also has the lowest barrier but the energy barrier of Na and Cl are almost equal. More apparent difference is found for (7,7) and (8,8) CNTs wherein Na faces the lowest energy barrier, followed by K, then Cl. For the biggest CNT tested in the study, which is the (9,9) CNT, there was no distinguishable differences in energy barriers observed.

 

In addition, the study also observed the structure of the solvation shells of each ion and found that they change upon entering the narrower tubes. The coordination numbers of the ions decrease from the normal/bulk hydration conditions (6?8) to about 2 water molecules only upon entering the (5,5) CNT.

 

This study has shown the importance of precise diameter control in fabricating CNTs. A few angstrom difference change in the diameter would alter the selectivity and consequently, the performance of the CNT membrane as an ion-separation device.

 

Contribution and application:

This paper gave a detailed description of how PMF and FEP calculations are done and interpreted. These two are very important for analyzing the energetics of a system. These can support the investigation of using CNTs for seawater desalination.

 

By: Hannah Ebro

hannah@gist.ac.kr