Researchers discover a totally unexpected behaviour of water inside carbon nanotubes
We all know that, at sea level, water starts to boil at 100 degrees Celsius (212 degrees Fahrenheit). And in very confined spaces, researchers have long observed that this boiling point can change, usually dropping by several degrees.
But now a team has shown for the first time that water confined in carbon nanotubes – some of the tiniest spaces on Earth – behaves in a completely unexpected way. Instead of boiling at lower temperatures, researchers have shown that water in these tubes can actually freeze solid at temperatures well above boiling point.
“The effect is much greater than anyone had anticipated,” said lead researcher Michael Strano from the Massachusetts Institute of Technology (MIT). “If you confine a fluid to a nanocavity, you can actually distort its phase behaviour.”
Phase behaviour describes the way that water changes between the solid, liquid, and gas phases we know so well.
Although it was expected that being confined inside a carbon nanotube would change these phase transitions, no one thought the effect would be so extreme, or that it would occur in the direction it did.
It was thought that the small space would lower, rather than raise, the freezing point, and only by about 10 degrees Celsius (18 degrees Fahrenheit) or so, which is what usually happens when water is trapped in small spaces.
But the opposite turned out to be true inside carbon nanotubes.
In one of the team’s tests, the water froze at a temperature above 105 degrees Celsius (221 degrees Fahrenheit) – well above boiling point.
It was hard for the researchers to measure the exact temperature within the tube without interfering with results, but 105 degrees Celsius was the minimum value being tested, and the actual freezing point could have been as high as 151 degrees Celsius, or 305.6 degrees Fahrenheit.
The discovery shows that even though water is one of the most abundant substances on our planet, we still have so much to learn about how it works. Earlier this month, researchers found evidence that there are actually two liquid states of water, not just the one we learnt about in high school.
So what’s going on inside the carbon nanotubes to mess with water’s behaviour so much?
First of all, keep in mind that these tubes are really tiny. Structurally, they look like a straw, or a cylinder with openings on either side. But the diameter of the tubes themselves are so small, they need to be measured in nanometres, or billionths of a metre.
Once the water was inside these nanotubes, the researchers were able to use a technique called vibrational spectroscopy, which allowed them to track how the water was moving and determine whether it was in gas, liquid, or solid phase.
This is the first time anyone has used this technique, and it allowed the team to show that it’s the diameter of the nanotubes that makes all the difference when it comes to freezing point.
The team found that even the difference between a nanotube with a 1.05 nanometre diameter and a 1.06 nanometre diameter had freezing points that varied by tens of degrees Celsius (more than 18 degrees Fahrenheit).
Earlier research trying to test how water behaved in these small spaces had provided contradictory results, but that’s because past experiments haven’t been able to measure the exact size of their nanotubes as precisely as the MIT team, and no one had ever thought that such small differences in diameter could produce such different outcomes.
“All bets are off when you get really small,” said Strano. “It’s really an unexplored space.”
In fact, there’s still a lot the team doesn’t understand – such as why water even enters the carbon nanotubes in the first place, seeing as they’re thought to be hydrophobic, or water repelling.
The researchers also aren’t willing to call the solid water inside the nanotubes ‘ice’, seeing as they haven’t been able to conclusively show that the water has the specific crystalline structure required of ice.
“It’s not necessarily ice, but it’s an ice-like phase,” said Strano.
His team is now going to continue researching the water inside these tubes to figure out what’s going on.
And in addition to reminding us just how weird and unpredictable water really is, the research could lead to practical new applications.
Most promisingly, these carbon nanotubes could be used to develop ‘ice wires’ that remain stable at room temperature, but maintain the unique electrical and thermal properties of ice – such as being able to conduct protons at least 10 times better than other conductive materials.
“This gives us very stable water wires, at room temperature,” said Strano.
The results have been published in Nature Nanotechnology.