Water Stays Frozen Above Boiling Point When Trapped In Carbon Nanotubes

Artistic impression of the structure of carbon nanotubes. nobeastsofierce/Shutterstock

You know those reassuring physics facts, like water freezes at 0°C (32°F) and boils at 100°C (212°F)? Well, it turns out that when scientists start playing around with things, those become a lot more arbitrary.

Researchers at the Massachusetts Institute of Technology (MIT) were able to keep water frozen inside carbon nanotubes at temperatures of at least 105°C (221°F) and as high as 151°C (304°F). Nanotubes are hollow cylinders made entirely of carbon, and they tend to be only a few billionths of a meter across, so when filled with water only a few molecules can comfortably sit side by side.

The research was published in Nature Nanotechnology and focused on the water’s phases – solid, liquid, or gas – and its changes. People had previously seen small changes in the boiling and freezing point when fluid is trapped in a narrow cavity, but this was completely unexpected.

“If you confine a fluid to a nanocavity, you can actually distort its phase behavior,” lead author Michael Strano said in a statement. “The effect is much greater than anyone had anticipated. All bets are off when you get really small. It’s really an unexplored space.”

Before the experiment, the team ran a variety of simulations to work out what the expected behavior of fluids would be in such a unique environment. Those simulations showed contradictory results, due to the incredibly strong relationship between nanotube size and fluid arrangement.

The team discovered that even between nanotubes of 1.05 and 1.06 nanometers across, there was a difference of tens of degrees in the freezing point of water. To work out the freezing point, the researchers used a technique called vibrational spectroscopy, which allowed them to track how (or if) the water was flowing in the nanotube.

“We can tell if it’s vapor or liquid, and we can tell if it’s in a stiff phase," added Strano. "It’s not necessarily ice, but it’s an ice-like phase." For frozen water to be ice, it requires certain crystalline properties, and the researchers are not sure of them just yet.

The team is looking at how to exploit this discovery for technological breakthroughs. Water inside a carbon nanotube wire, for example, can conduct photons 10 times more readily than a typical conductive material. Their stability over a wide range of temperatures is certainly exciting enough to imagine several potential applications in materials in the future.

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