Inside very tiny tubes, water is doing the complete opposite of what you’d expect

Inside very tiny tubes, water is doing the complete opposite of what you’d expect

MIT News and World Economic Forum published a very interesting article titled – ‘Inside very tiny tubes, water is doing the complete opposite of what you’d expect’ – that we thought would be good to share with you.

It is well-known that water at sea level starts to boil at 212 degrees Fahrenheit, or 100 degrees Celsius. Scientists have long observed that water’s boiling and freezing points also change when confined in very small spaces, dropping by around 10 C or so.

However, the team at MIT has discovered a completely unexpected behaviour of water. They have observed that inside the tiniest of spaces like in carbon nanotubes that is not much bigger than a few water molecules, water can freeze solid even at high temperatures that would normally set it boiling.

The results of the study are reported in the journal Nature Nanotechnology, in a paper by Michael Strano, the Carbon P. Dubbs Professor in Chemical Engineering at MIT; postdoc Kumar Agrawal; and three others.

“If you confine a fluid to a nanocavity, you can actually distort its phase behavior,” Strano says, 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 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.)

“The effect is much greater than anyone had anticipated,” Strano says.

The water’s behaviour changes and is dependant upon the exact diameter of the tiny carbon nanotubes made from carbon atoms. “These are really the smallest pipes you could think of,” Strano says. In the experiments, the nanotubes were left open at both ends, with reservoirs of water at each opening.

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, the researchers found. Such extreme differences were completely unexpected. “All bets are off when you get really small,” Strano says. “It’s really an unexplored space.”

It is surprising that water even enters into these tiny tubes in the first place. Strano explains that carbon nanotubes are thought to be hydrophobic, or water-repelling, so water molecules should find it difficult to make their way inside. The fact that they do remains a mystery.

Strano and his team used highly sensitive imaging systems, a technique called vibrational spectroscopy, that could track the movement of water inside the nanotubes, hence making its behaviour subject to detailed measurement for the first time.

The team can detect not only the presence of water in the tube, but also its phase, he says: “We can tell if it’s vapour or liquid, and we can tell if it’s in a stiff phase.” While the water definitely goes into a solid phase, the team avoids calling it “ice” because that term implies a certain kind of crystalline structure, which they haven’t yet been able to show conclusively exists in these confined spaces. “It’s not necessarily ice, but it’s an ice-like phase,” Strano says.

This study illustrates the potential of leading to new applications such as “ice wires”, that can take advantage of the unique electrical and thermal properties of ice where it is among the best carriers of protons; as water conducts protons 10 times more readily than typical conductive materials. “This gives us very stable water wires, at room temperature,” Strano says.

The research team also included MIT graduate students Steven Shimizu and Lee Drahushuk, and undergraduate Daniel Kilcoyne. 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.

To read the source article, click here.

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