Inside carbon nanotubes, MIT researchers found water solidifies at temperatures that would normally see it boiling. Credit: MIT

Phase change — the transition substances make from solid, liquid, gas — is a lot more dynamic than most people think. At sea-level, water starts boiling at around 100 degrees Celsius and freezes at zero C. If you confine the same volume of water in a small space, thus exerting pressure, you’ll need more energy to boil water. This is common knowledge but few thought you could drastically alter normal boiling point conditions by trapping water in small cavities. MIT did this recently with carbon nanotubes and found water started freezing at 100 C.

The weirdest ice

Carbon nanotubes are among the thinnest materials we know. Inside them, you can’t cram something thicker than a couple of water molecules. “If you confine a fluid to a nanocavity, you can actually distort its phase behavior,” Michael Strano, the Carbon P. Dubbs Professor of Chemical Engineering at MIT. That’s to say everyone expected phase change conditions to be altered but the magnitude blew everyone away.

Experiments suggest the water inside the carbon nanotubes solidified at 105 degrees Celsius. At this scale, well below the billionth of a meter, reading temperature can prove challenging. It’s possible that the actual freezing temperature is in excess of 150 degrees Celsius, the MIT researchers note in their paper.

Nevertheless, the challenge of, firstly, squeezing water inside carbon nanotubes, a typically hydrophobic material, and, secondly, measuring the interactions inside this tiny space was huge. But the MIT researchers rose to it.

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Highly sensitive imaging systems were used to measure temperature and behaviour of water inside the carbon nanotubes. The technique called vibrational spectroscopy can not only detect the presence of water in the tube, but also its phase changes. Concerning the water’s phase trapped inside the tubes, the MIT researchers are still reserved. They don’t want to call it ice yet because the crystalline structure is unknown but it’s definitely a solid. “It’s an ice-like phase,” Strano said in a statement. 

The behaviour change directly depends on the diameter of the tubes. Even a tiny difference between nanotubes 1.05 nanometers and 1.06 nanometers resulted in a difference of tens of degrees in the apparent freezing point. “All bets are off when you get really small,” Strano says. “It’s really an unexplored space.”

Besides being an excellent science experiment, the MIT research could actually lead to a new class of materials. To liquefy in this state, the water needs to reach temperatures exceeding its boiling point at sea level. This suggests that at room temperature, it will stay solid. It’s not just water — anything can be squeezed inside, theoretically. One potential application is ‘ice wires’ — cables with carbon nanotubes which trap ice inside. Previous research suggests ice can conduct protons 10 times better than conventional conductors.