Ice Pops Thermal Phasing

Carbon nanotubes have been found to distort phase behaviour between solid/liquid/gas to a far greater extent than predicted.

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It has been established for a while that by confining water in tiny spaces its boiling and freezing points can fluctuate, typically dropping by around 10C.

But a remarkable discovery has shown that when water is placed in nanocavities it defies expectations and actually freezes solid.

MIT researchers found that water inside carbon nanotubes with minute inner dimensions could freeze solid in temperatures high enough to boil it under standard conditions. The icy water doesn’t melt until well above the normal boiling point of water, so is likely to remain stable at room-temperature indefinitely.

“The effect is much greater than anyone had anticipated,” said Michael Strano, Carbon P. Dubbs Professor of Chemical Engineering and co-author of a paper on the subject in *Nature Nanotechnology*.

“If you confine a fluid to a nanocavity, you can actually distort its phase behaviour,” he explained referring to how and when the substance changes between solid, liquid and gas phases.

Such effects were expected but the enormous magnitude of the change, and its direction (raising rather than lowering the freezing point), were a complete surprise. In one of the team’s tests, the water solidified at a temperature of at least 105C.

The way water’s behaviour changes inside the tiny carbon nanotubes is dependent on the exact diameter of the tubes.
Unexpectedly, researchers found that even the difference between nanotubes 1.05 nanometers and 1.06 nanometers across made a difference of tens of degrees in the apparent freezing point.

Research teamIn carbon nanotubes water can freeze solid even at high temperatures that would normally set it boiling. The finding might lead to new applications such as ice-filled wires.

The team used highly sensitive vibrational spectroscopy to track the movement of water inside the nanotubes.

Not only the presence of water in the tube, but also its phase, could be identified.

Prof Strano said: “We can tell if it’s vapor or liquid, and we can tell if it’s in a stiff phase.”

The word “ice” is avoided by the team because, although solid, the structure cannot be shown conclusively to be crystalline in the confined spaces studied.

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

One application envisaged by scientists is so-called ‘ice-filled wires’ having the electrical and thermal properties of ice while remaining stable at room temperature.

The work was supported by the U.S. Army Research Laboratory and the U.S. Army Research Office through the MIT Institute for Soldier Nanotechnologies, and Shell-MIT Energy Initiative Energy Research Fund.

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