Water conduction through the hydrophobic channel of a carbon nanotube
Nature 414, 188-190 (8 November 2001)
1. Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-0520, USA
2. Department of Chemistry, University of Maine, Orono, Maine 04469, USA
This paper introduced the ability of carbon nanotubes (CNTs) to be spontaneously and continuously filled with water. By using molecular dynamics simulations, the authors presented the reasons why a hydrophobic channel can still able to conduct water.
It was also observed that there is a reduction in the attraction between the atoms in interior of the CNT and the water molecules, influencing the hydration of the CNT pore.
The study modeled a single-walled CNT having a diameter of 8.1 ? and used TIP3P as the water model. A total of 66 ns was used for the simulation. It was observed that water molecules spontaneously and rapidly filled the CNT. The authors mentioned that water occupancy depends on the free energy of removing a water molecule from the channel. This is influenced not by the strength of bonding of a water molecule but by the number of weakly bound states inside the CNT. These binding energies inside the pore were found to be sharply distributed and high energy states were seen to be less frequently occupied. The net result was that there was a lower excess chemical potential for water molecules inside the CNT compared to bulk water. It was also shown that hydrogen bonds of water molecules inside the CNT were highly oriented compared to bulk water, but they were still able to retain the entropy while inside. This allowed the water molecules to rotate freely about the aligned series of hydrogen bonds, resulting in a degenerate energetic “ground state” and the narrow distribution of binding energies.
The study also observed that water molecules were able to exit the other end of the CNT opening at a rate of 17 molecules per nanosecond. According to the authors, the pulse-like transmissions of water molecules were caused by the tight network of hydrogen bonds inside the pore. During these “pulses” or “bursts”, there was greater available energy which rendered the interactions of water with the hydrophobic wall negligible. Hence, water molecules were found to move with little resistance through the CNT.
Contribution and application:
This paper is one of the first to establish the possibility of using CNTs for desalination by showing the ability of CNTs to conduct water.
By: Hannah Ebro