Carbon Nanotubes Full O' Water

Fascinated by the ship-in-a-bottle trick and other 3-D puzzles? Then here’s one to consider: How do you put a chain inside of a tube and insert the tube into another tube, when the opening of the largest piece measures barely more than one billionth of a meter across?

That’s exactly what researchers at Argonne National Laboratory have unintentionally accomplished using nothing more than a little water and carbon nanotubes—and letting self-assembly do all the heavy lifting.

By exposing open-ended single-walled carbon nanotubes with diameters of roughly 14 Å to warm water vapor and then cooling to low temperature, the researchers were able to induce the spontaneous assembly of a chain of water molecules inside a cylindrical sheath of water molecules that is, in turn, inside a carbon nanotube. This unusual nanostructure has never been reported before nor even predicted to exist, and the properties it exhibits are unexpected.

“We thought it would be quite interesting to study water confined in carbon nanotubes,” says Argonne staff researcher Alexander I. Kolesnikov, who led the study. Kolesnikov explains that forcing water into one-dimensional channels with nanometer-scale diameters should endow the water sample with properties that differ from ordinary bulk water. Such studies may help explain proton and water transport in plants and transmembrane proteins and other systems that feature nanometer-scale confinement.

To test for those properties, Kolesnikov and coworkers at Argonne and MER Corp., Tucson, Ariz., and University of Utah postdoctoral associate Christian J. Burnham examined the water-filled nanotubes using a combination of neutron-scattering techniques and molecular dynamics simulations. Neutron diffraction methods provided information about the structure of the confined water, and inelastic neutron-scattering measurements probed the sample’s vibrations and dynamic behavior.

In addition to detecting the surprising structure, the group found that the confined water is quite mobile and fluidlike even at cryogenic temperatures far below the freezing point of ordinary water. At 50 K, for example, hydrogen bonds that hold water molecules in the chain are constantly breaking and re-forming, Kolesnikov notes.

The researchers attribute the behavior to the unique bonding arrangement of molecules in the water chain. Unlike in bulk water, where each molecule is hydrogen-bonded to four nearest neighbors, in “nanotube water,” molecules bond to just two neighbors. The relatively loose bonding configuration leads to “fluctuations” and a lot of motion along the length of the chain, Kolesnikov comments. “It’s very unusual behavior.”

The study will be published in a forthcoming issue of Physical Review Letters.