Water within carbon nanotubes does not behave as it should.
At sea level, water has a set boiling and freezing temperature. Researchers have previously observed that water in very small, confined spaces will behave differently, changing boiling and freezing points, but never to the extent that that it turns solid when it should be boiling.
However, this is exactly what Michael Strano and colleagues at MIT observed in water within carbon nanotubes, publishing their findings in the journal Nature Nanotechnology.
Normally, confinement can change within a temperature range of 10 degrees Celsius (50 degrees Fahrenheit). The researchers for the current study were completely surprised then when one of their tests resulted in water solidifying at a minimum of 105 degrees Celsius (221 degrees Fahrenheit).
After more observations, the researchers came to the conclusion that water behavior varies when inside carbon nanotubes depending on the exact diameters of the tubes. Even a difference of 0.01 nanometers can cause a change!
Earlier studies have tried to shed light on how water and other types of fluids will behave in confined, small spaces but have yielded contradictory results. One reason for this may be that researchers from the earlier studies were not able to measure their nanotubes exactly, having not realized that even the slightest difference in diameter will produce different outcomes.
Water's Many Surprises
Aside from behaving differently in terms of changing phases, water should also not even be able to get into nanotubes in the first place. Carbon nanotubes are considered to be hydrophobic so they repel water. This means it's difficult for water molecules to get inside the nanotubes, and how they do it is another mystery to the researchers.
Observing Water In Nanotubes
Strano and colleagues used vibrational spectroscopy to track water movement precisely inside the carbon nanotubes. This was what made it possible for them to acquire such detailed measurements of the nanotubes.
With the imaging technique, the researchers were able to check not only for water presence but water's phase as well. When water turns solid inside the nanotubes, it is not referred to as "ice" because that implies the existence of a crystalline structure, which the researchers have not been able to conclusively show.
"It's not necessarily ice, but it's an ice-like phase," said Strano.
This solid water the researchers observed also doesn't revert back to liquid at room temperature so it's a potentially useful material for a number of applications. "Ice wires" is one example, as water conducts protons better than the usual conductive materials.