When water molecules are placed inside tiny carbon nanotubes, they can stay frozen even when heated to boiling temperatures. The MIT researchers that discovered this phenomenon hope to use the “ice-filled tubes as wires to move protons,” a key step in creating hydrogen fuel cells. The results are reported on in a research paper published by the journal Nature Nanotechnology.
The Discovery
To see what phase water has taken with the naked eye at a nanoscale is impossible. The researchers had to employ a technique called “vibrational spectroscopy,” which enabled them to track the movement of water inside the nanotube and determine whether it was in a liquid, solid, or gas phase. It is normal for water at sea level to freeze at 0°C (32°F) and boil at 100°C (212°F).
What they saw was completely unexpected. They had supposed it would change its phase, but the particular phase it changed to came to them as a complete surprise, says Michael Strano—the Carbon P. Dubbs Professor in Chemical Engineering at MIT—and his fellow researchers. What they realized was that water in nanotubes can freeze solid when heated up to beyond its natural boiling point. Therefore, temperature isn’t the only variable that can drastically change the state of water. Other forces can too: several emerging kitchen technologies steer water with electric fields and superheated steam to cook and freeze food more gently.
This experiment proved that if the size of the nanotube is small enough and the temperature is at least 105°C (221°F) or more, then instead of boiling as it naturally would at this temperature, the water solidifies. Now, this depends crucially on the exact diameter of the tubes.
Strano explains, “These are really the smallest pipes you could think of. In the experiments, the nanotubes were left open at both ends, with reservoirs of water at each opening.”
Even the difference between 1.05 nanometers and 1.06 nanometers across made a difference of tens of degrees in the apparent freezing point. Such extreme differences were completely shocking. Strano said, “If you confine a fluid to a nanocavity, you can actually distort its phase behavior… [but] the effect is much greater than anyone had anticipated. All bets are off when you get really small. It’s really an unexplored space.”
If this alone isn’t strange enough… there’s more. Up until this experiment, carbon nanotubes were thought to be hydrophobic, which means that it would be very difficult for water molecules to enter. However, this test proves that it is possible for water to get into the tiny space. Although, they have not yet been able to explain how this is so.
They also have yet to test if the solidified state of the water indeed contains the typical crystalline properties of ice. What they do have is evidence of the water solidifying, but that doesn’t necessarily mean that it turned to ice. “It’s not necessarily ice, but it’s an ice-like phase,” Strano says. Nevertheless, all questions aside, there’s a lot they can do with this discovery.
Possible Applications
They can make “ice wires” that remain stable even at room temperature. It should remain perfectly stable indefinitely under room-temperature conditions because this solid water doesn’t melt until well above the normal boiling point of water. Such a wire would maintain the unique electrical and thermal properties of ice, thus allowing for better conductivity of protons. Water conducts protons at least 10 times more readily than typical conductive materials.
Strano says, “This gives us very stable water wires at room temperature.”
