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Simulations help explain fast water transport in nanotubes
9/16/08
James E. Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@illinois.edu
Click photo to enlarge Photo by L. Brian Stauffer Narayana R. Aluru, professor of mechanical science and engineering, left, and doctoral student Sony Joseph have discovered the physical mechanism behind the rapid transport of water in carbon nanotubes. Image in background shows the trajectory of water molecules in a carbon nanotube moving in the direction of their orientations due to rotation-translation coupling.
CHAMPAIGN, Ill. — By discovering the physical mechanism behind the rapid transport of water in carbon nanotubes, scientists at the University of Illinois have moved a step closer to ultra-efficient, next-generation nanofluidic devices for drug delivery, water purification and nano-manufacturing.
“Extraordinarily fast transport of water in carbon nanotubes has generally been attributed to the smoothness of the nanotube walls and their hydrophobic, or water-hating surfaces,” said Narayana R. Aluru, a Willett Faculty Scholar and a professor of mechanical science and engineering at the U. of I.
“We can now show that the fast transport can be enhanced by orienting water molecules in a nanotube,” Aluru said. “Orientation can give rise to a coupling between the water molecules’ rotational and translational motions, resulting in a helical, screw-type motion through the nanotube,” Aluru said.
Using molecular dynamics simulations, Aluru and graduate student Sony Joseph examined the physical mechanism behind orientation-driven rapid transport. For the simulations, the system consisted of water molecules in a 9.83 nanometer long nanotube, connected to a bath at each end. Nanotubes of two diameters (0.78 nanometers and 1.25 nanometers) were used. Aluru and Joseph reported their findings in the journal Physical Review Letters.
For very small nanotubes, water molecules fill the nanotube in single-file fashion, and orient in one direction as a result of confinement effects. This orientation produces water transport in one direction. However, the water molecules can flip their orientations collectively at intervals, reversing the flow and resulting in no net transport.
This illustration demonstrates one form a carbon nanotube can take.
Experiments during the past several years examining membranes formed from carbon nanotubes have demonstrated potential for a variety of applications, perhaps most notably economical filtration of water for desalination.  But something that has consistently confused researchers is that the rate of flow for water passing through these special membranes is considerably higher than would be predicted by classical physics. In some experiments the flow rate was up to 10,000 times the speed expected.  In addition, recent measurements by a team at Lawrence Livermore National Laboratory conducted via nuclear magnetic resonance showed that the water passing through nanotubes exhibits very different properties in comparison to bulk water.

In a press release Tuesday the News Bureau of the University of Illinois at Urbana-Champaign highlighted a recent finding that offers one possible explanation for the phenomenon.  The research, published in ''Physical Review Letters'' last month (), used molecular modeling to analyze the predicted behavior and interactions of individual water molecules passing through a nanotube.

UIUC researchers Narayana R. Aluru, a professor of mechanical science and engineering, and doctoral student Sony Joseph found that the chemical polarity of H2O and what they describe as "confinement effects" within the bounds of smaller nanotubes cause the molecular movements to couple.  Due to the water molecules interacting and orienting themselves in response to one another they flow in a more orderly helical or corkscrew pattern through the nanotube.  This flow pattern may explain some of the experimental results obtained by other scientists.

Hopefully this discovery is another step towards an affordable technology to produce plentiful potable water for drinking, irrigation of crops, and other purposes, a vital resource that is in short supply in many parts of the world.