But now, a group at MIT has identified a completely unexpected set of changes. Inside the smallest spaces, in carbon nanotubes whose inner dimensions are not that much stretched than a few water molecules, water can freeze and become solid even a high temperatures that would normally lead it to boil.
The study demonstrates how even very common substances can drastically alter their behavior when captured inside structures estimated in nanometers, or billionths of a meter, and the research might result in novel applications, like essentially, ice – filled wires that take benefit of the unique thermal and electrical properties of ice while remaining constant at the room temperature.
“If you confine a liquid to a nanocavity, you can actually distort its phase behavior,” says Strano, referring to how and when the material alters between liquid, solid and gas transitions. Such effects were expected, but the huge dimensions of the alteration and its direction rising rather than diminishing freezing point were an absolute surprise. In one of the experiments from the team, the water became solid at a temperature of 105 C or more. The precise temperature is difficult to analyze, but 105 C was considered the minimum value in the test, the actual temperature could have been as high as 151C.
“Such an effect is much bigger than anyone had expected,” says Strano. It turns out to be the way water behavior alters inside the small carbon nanotubes, structures the shape of a soda straw, formed completely from carbon atoms, but just a few nanometers in diameter – depends critically on the exact radius of the tubes. “These are actually the tiniest pipes you could consider,” says Strano. In the studies, the nanotubes were left open at both the ends with water reservoirs at each opening.
Even the difference between nanotubes 1.05 nanometers and 1.06 nanometers across made a huge difference of tens of degrees in the apparent freezing point, the scientists identified. Such extreme differences were absolutely unexpected. “As the bets are off when you appear really small,” says Strano. “It is truly an unexpected space.”
The group can identify not just the presence of water inside the tube, but also its phase, he confirms. “We can inform if the liquid or vapor and we can inform if it is in a stiff phase.” While the water surely rests into a solid phase, the group avoids considering it ‘ice’ as the term means a specific sort of crystalline structure, which they have not been able to disclose.
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