Nanotubes, which are closely related to buckyballs, are cylinders of carbon, structured as if a sheet of hexagonal carbon were wrapped up into a cylinder. One end of the cylinder is closed with a half-buckeyball structure, as shown in the diagram. The diameter of the nanotube is about a nanometer.

Water molecules can fit inside a nanotube, but there is not a lot of room to spare. The hydrogen-oxygen bond length in water is about 10^-10 meters, compared with the nanotube diameter of about 10^-9 meters, so the fit is tight. Moreover, ordinary ice will not fit because of its open structure (see diagram in the previous section).

This image shows nanotube water inside a nanotube at low temperatures. In the drawing, the carbon atoms of the nanotube are brown; the oxygen and hydrogen atoms of the ice sleeve just inside the nanotube are red and white; the oxygen and hydrogen atoms of the liquid water inside the ice cylinder are gold. Notice that there is insufficient space for the open structure of ordinary ice. (image credit: Christian J. Burnham )

This image shows only the nanotube water, again at low temperatures. The red and white water molecules are the ice sleeve, and the green and white molecules are the liquid water inside. (image credit: Christian J. Burnham )

To study nanotube water, physicists marshalled several different experimental approaches and computer simulation as well. Neutron diffraction can be used to determine the structure of the water confined inside the nanotubes. Then neutron inelastic scattering revealed how energy passed from the neutrons to the nanotubes and water, particularly at low temperatures. This showed how the water molecules were moving and vibrating and also how the bonds among them formed and reformed. Finally, computer simulations of how the water molecules moved showed that the structure and the observed motion were consistent.

Here are the results. At low temperatures, the tight fit inside the nanotube forces the water molecules into an unusual structure. At eight kelvins (i.e., eight degrees above absolute zero), the water inside forms a thin sleeve, a cylinder of ice one molecule thick, as shown in the diagram, with liquid water inside—still liquid at -265° C! The two hydrogens of each interior molecule bind to oxygens of others, but the average number of bonds for these molecules is less than two, completely different from liquid water at room temperature. The molecules of low-temperature nanotube water are far more mobile than the molecules of bulk water at room temperature, and the bonds constantly break and reform. Contrast the diagrams of the nanotube ice sleeve and the liquid water inside with those of normal ice and water in the previous section.

The researchers plan to work with nanotubes with even smaller diameters, which may help them understand how membrane proteins transport water. Also, they plan to investigate what happens to the water inside as it is heated and cooled.

And for the field of neutron scattering as a whole, an important new facility—the Spallation Neutron Source at the Oak Ridge National Laboratory—is coming on line. It will be the most intense source of pulsed neutrons available and should become a center for the growth of neutron science.