Public Affairs : Newsroom : News Releases : 2008 News Release Archive :: NR-08-08-09 News Release Printer-Friendly Contact: Anne M. Stark
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June 25, 2008
NR-08-08-09 LLNL researchers peer into water
in carbon nanotubes LIVERMORE, Calif. – Researchers have identified a signature for water inside single-walled carbon nanotubes, helping them understand how water is structured and how it moves within these tiny channels. This is the first time researchers were able to get a snapshot of the water inside the carbon nanotubes.
An NMR spectrum showing features associated with water external and internal to the carbon nanotube (see inset).
Figure by Charles Chen and Yue Wu
University of North Carolina, Chapel Hill.
Click for high resolution image Single-walled carbon nanotubes (SWCNTs) offer the potential to act as a unique nanofiltration system. While experiments have demonstrated extremely fast flow in these channels, it is still unclear why, and few studies have experimentally probed the detailed structure and movement of the water within nanotubes. That’s where Lawrence Livermore scientists Jason Holt, Julie Herberg, and University of North Carolina’s, Yue Wu and colleagues come in. As described in an article appearing in the July edition of Nanoletters, they used a technique called Nuclear Magnetic Resonance (NMR) to get a glimpse of the water confined inside one-nanometer diameter SWCNTs. The nanotubes, special molecules made of carbon atoms in a unique arrangement, are hollow and more than 50,000 times thinner than a human hair. The confined water exhibited very different properties from that of bulk water, and this allowed it to be distinguished in the NMR spectrum. Carbon nanotubes have long been touted for their superior thermal, mechanical and electrical properties, but recent work suggests they can be used as nanoscale filters.
Earlier Livermore studies have suggested that carbon nanotubes may be used for desalination and demineralization because of their small pore size and enhanced flow properties. Conventional desalination membranes are typically much less permeable and require large pressures, entailing high energy costs. However, these more permeable nanotube membranes could reduce the energy costs of desalination significantly. While the technology offers great promise, there still are important unanswered scientific questions. “There have been many predictions about how water behaves within carbon nanotubes,” said Holt, the principal investigator of the project, which is funded through LLNL’s Laboratory Directed Research and Development (LDRD). “With experiments like these, we can directly probe that water and determine how close those predictions were.” Founded in 1952, Lawrence Livermore National Laboratory is a national security laboratory, with a mission to ensure national security and apply science and technology to the important issues of our time. Lawrence Livermore National Laboratory is managed by Lawrence Livermore National Security, LLC for the U.S. Department of Energy's National Nuclear Security Administration.
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.