Nat. Nanotech. http://doi.org/bvpt (2016)

The phase diagram of water never ceases to amaze — and drive research forward. One of the many questions regarding water's behaviour is what happens when it's confined. With the advent of carbon nanotubes, which can be filled with water molecules, cylindrical nanoscale confinement can now be experimentally investigated.

Kumar Agrawal and colleagues looked at how the size of a nanotube affects the phase behaviour of encapsulated water molecules. They filled centimetre-long tubes of different diameters, ranging from 1.05 to 1.52 nm, with water and recorded Raman spectra. Specifically, they looked at the frequency of the so-called radial breathing mode (RBM): the vibrational mode of a tube where its diameter oscillates around the equilibrium value. When varying the temperature of their water-in-tube systems, the authors observed shifts of the RBM frequencies, which can be explained as liquid–solid transitions. The smallest tubes displayed melting around 105–151 °C, whereas the transition temperature for the largest tube was around 0 °C.