Water Inside Carbon Nanotubes Stays Frozen At Boiling Point

The action of water in carbon tubes at the nanometer scale was a surprise to researchers as well.

Water, at sea level, starts to boil at a temperature of 212 degrees Fahrenheit, or 100 degrees Celsius. That much is well known.

Scientists have also long known that when water is confined in very small spaces, its boiling and freezing points can change a bit, usually dropping by around 10 C or so.

But a team at MIT has discovered that inside carbon nanotubes, whose inner diameters are not much larger than a few water molecules, water can freeze solid even at high temperatures that would usually have it boiling.

Completely Unexpected Results

A paper detailing the results, by Michael Strano, Professor in Chemical Engineering at MIT; postdoc Kumar Agrawal; and three others is published in the journal Nature Nanotechnology.

If you confine a fluid to a nano-cavity, you can actually distort its phase behavior, Strano explains. Phase behavior refers to how and when a substance changes between solid, liquid, and gas phases.

Such effects were expected, but the huge magnitude of the change, and its direction, raising instead of lowering the freezing point, were completely unexpected.

nanotube

Image: Courtesy of the researchers

In one of the experiments, the water solidified at a temperature of 105 C or more. (The exact temperature is hard to determine, but 105 C was considered the minimum value in this test; the actual temperature could have been as high as 151 C.)

Since carbon nanotubes are hydrophobic, one question that remains unanswered is just how the water entered the nanotubes in the first place. The nature of the water in the tubes is also still unclear.

“It’s not necessarily ice, but it’s an ice-like phase,” Strano said.

Although the water does go into a solid phase, the research team hesitates to call it “ice”. That term implies a certain kind of crystalline structure, which they haven’t yet been able to conclusively show exists in these confined spaces.

Strano says 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. “This gives us very stable water wires, at room temperature,” he says.

Study: Observation of extreme phase transition temperatures of water confined inside isolated carbon nanotubes