A team at the Massachusetts Institute of Technology (MIT) has found that inside carbon nanotubes whose inner dimensions are not much bigger than a few water molecules water can freeze solid even at temperatures that would normally make it boil.

The discovery illustrates how materials can change their behavior when trapped inside structures measured in nanometers, or billionths of a meter. The finding might lead to applications such as ice-filled wires that take advantage of the electrical and thermal properties of ice while remaining stable at room temperature.

Water's behavior inside the nanotubes depends on the diameter of the tubes. The results are reported in the journal Nature Nanotechnology, in a paper by Michael Strano, the Carbon P. Dubbs Professor in Chemical Engineering at MIT; postdoc Kumar Agrawal; and three others.

According to their findings, confining fluid to a nanocavity can distort its phase behavior, referring to how and when the substance changes between solid, liquid, and gas phases. Such effects were expected, but the magnitude of the change and its direction (raising rather than lowering the freezing point), were surprising: In one of the team's tests, the water solidified at a temperature of 105 C or more.

Water's behavior changes inside the carbon nanotubes depending on the diameter of the tubes. In the experiments, the nanotubes were left open at both ends, with reservoirs of water at each opening.

Even the difference between nanotubes 1.05 nanometers and 1.06 nanometers across made a difference of tens of degrees in the freezing point, the researchers found. Such extreme differences were unexpected.

Researchers used a technique called vibrational spectroscopy that could track the movement of water inside the nanotubes, thus making its behavior subject to detailed measurement.

The team can detect not only the presence of water in the tube, but also its phase. While the water definitely goes into a solid phase, the team avoids calling it "ice" because that term implies a certain kind of crystalline structure, which they haven't yet been able to show exists in these confined spaces.

Because this solid water doesn't melt until well above the normal boiling point of water, it should remain stable indefinitely under room-temperature conditions. That makes it potentially a useful material for a variety of applications.

For example, it should be possible to make "ice wires" that would be among the best carriers known for protons because water conducts protons at least 10 times more readily than typical conductive materials.