Carbon nanotubes make water freeze solid at boiling temperatures
Water freezes at 0° C (32° F) and boils at 100° C (212° F) at sea level, right? Normally, yes. But researchers at MIT have found that, when contained inside the tiny cavities of carbon nanotubes, water can actually freeze solid at temperatures well above its usual boiling point. This finding may have applications in creating proton-conducting "ice wires".
Temperature alone isn't the sole factor in determining when water shifts between a solid, liquid and gas. As demonstrated in a Cody's Lab video, pressure plays a big part as well, allowing water to effectively be boiled until it freezes by lowering the pressure.
Water has also been known to behave strangely when it's confined to spaces on the scale of nanometers: earlier this year scientists at Oak Ridge National Lab discovered water has a freaky fourth state of matter when it's put under extreme pressure in these tiny spaces. And research as far back as the 1990s has observed water spontaneously vaporizing when it's tightly surrounded by hydrophobic materials – a phenomenon seen when scientists accidentally created nanorods that appear to harvest water from the air.
"If you confine a fluid to a nanocavity, you can actually distort its phase behavior," says Michael Strano, lead author of the study. "The effect is much greater than anyone had anticipated."
The researchers expected to see changes in the temperature at which water boils and freezes, but were still surprised by the results. The changes were far more pronounced than they'd anticipated, and in the other direction – the freezing point increased instead of lowering. In one test, water froze in a nanotube at a temperature of between 105° C and 151° C (221° F and 304° F).
This caught the team off-guard partly because earlier simulations suggested very different outcomes. These false findings were likely the result of imprecise measurements, which can be really difficult to get right at such tiny scales and yet cause very drastic differences. The researchers found, for instance, that changing the diameter of a nanotube by as little as 0.01 nanometers could alter the freezing point of water by dozens of degrees.
"All bets are off when you get really small," says Strano. "It's really an unexplored space."
To study this, the team placed reservoirs of water at both ends of carbon nanotubes, which were left open. How the water actually got inside them is still a bit of a mystery, given that nanotubes are generally thought to repel water, but it found a way.
Then, using vibrational spectroscopy, the team was able to track not only the movement of water within a nanotube, but whether it's in a solid, liquid or vapor phase. And while the water does solidify, that doesn't necessarily mean it's ice, because the team hasn't yet been able to confirm that it contains the usual crystalline structure of ice.
So what can we do with this strange material? Since this solid water stays stable at room temperature, the researchers suggest a type of "ice wire" may be developed, which could conduct protons 10 times better than existing materials.
The research was published in the journal Nature Nanotechnology.