‘Ice-wires’ at room temperature have potential for proton electronics

Confining water inside carbon nanotubes changes its physical properties so much that it can be solid at 100°C. This discovery, while not fully understood, could lead to new advances in nanoelectronics.

One of the few scientific facts that everybody knows is that water freezes at 0°C (32°F) and boils at 100°C (212°F). It slightly less well-known that these figures change depending on the pressure exerted on the water. Scientists have known for many years that confining water into small spaces can also change the phase-transition temperatures. But the team at Massachusetts Institute of Technology has now discovered that if the water is confined inside a cylindrical nanotube, these changes can be very large indeed.

According to research leader Michael Strano, the team expected to see distortions of phase behaviour when confining water inside a nano-cavity. However, they did not expect to see such a large effect; and also did not expect to see the melting point go up. In one experiment, the water was in a solid state at 105°C.

The team used nanotubes that were extremely narrow (a little above a nanometre in diameter) and open at both ends. One still-unanswered question is how the water got into the nanotubes in the first place: carbon nanotubes are hydrophobic.

The state of the water inside the tube is also still a mystery. Although Rahman spectroscopy confirmed that it is solid, the researchers cannot confirm that it has the characteristic crystalline structure of ice. “It’s not necessarily ice, but it’s an ice-like phase,” Strano said.

Even very small differences in diameter caused a large difference in melting point. A difference of just 0.01 nanometres raised the melting point by tens of degrees; again, the team cannot explain this.

One of the unusual properties of water is it is a very good conductor of protons: 10 times better than conventional conductive materials. If the water inside a nanotube is solid above 100°C, it will definitely be solid at room temperature. “This gives us very stable ice wires,” Strano said. Discussing the research in Nature Nanotechnology, he explained this could have applications in electronics.