Stiff water down the nanotubes

Water is a strange substance, it often bucks the physicochemical trends that other simpler substances follow. At sea level atmospheric pressure it boils at 100 degrees Celsius, which is a temperature a lot higher than you might anticipate based on its structure and ignoring hydrogen bonds. It is even stranger given that it freezes just one hundred degrees below that temperature. Confine water molecules in a small space and you can nudge down its boiling and freezing temperatures by as much as 10 degrees.

Now, a team at Massachusetts Institute of Technology (MIT) has demonstrated once again that water can behave in unexpected ways. Squirt a tiny drop down a carbon nanotube and you can make ice even at temperatures close to its normal boiling point. The finding opens up the possibility of ice-filled wires that exploit the unique electrical and thermal properties of ice but remain stable at room temperature. [Strano et al., Nature Nanotechnol (2016); DOI: 10.1038/nnano.2016.254] For instance, such wires could be potent proton transporters for a range of energy conversion applications with water being a proton transporter at least ten times better than other conductors.

"If you confine a fluid to a nanocavity, you can actually distort its phase behavior," team leader Michael Strano explains. In the case of water down the nanotubes, the big change in the opposite direction to that expected was a complete surprise. Indeed, in one demonstration the team observed solidified water at well above 105 degrees Celsius and possibly at a temperature as high as 151 degrees. Thermometers don't work well in carbon nanotubes so it is difficult to be certain by just much above the normal boiling point, water remained frozen.

Strano points out that earlier simulations had been contradictory partly because measuring the size of the carbon nanotubes with precision was problematic and a fractional differences in internal diameter made all the difference to just what temperature the scientists could sustain the little freeze. Of course, perhaps more surprising is that water enters the largely hydrophobic nanotubes at all. However, the team's vibrational spectroscopy tracked the movement of water within the nanotubes. "We can tell if it's vapor or liquid, and we can tell if it's in a stiff phase," Strano explains. The researchers avoid using the term "ice" as that implies a crystalline structure and they are yet to determine conclusively determine the detailed structure of the water inside the nanotubes.

"So far, we have not studied the phase transitions of other liquids inside the nanotubes, but we are interested in doing these experiments with other substances such as alcohols to see whether we observe similar changes in the phase transition temperatures," team member Jesse Benck told Materials Today. "These studies will provide more fundamental insights into the mechanisms through which geometrical confinement affects phase transition temperatures."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".