Ninety molecules—plus or minus 10—is the threshold at which nanosized water droplets can form ice crystals such as those we see in snowflakes or ice cubes, according to a new study (Proc. Natl. Acad. Sci. U.S.A. 2019, DOI: 10.1073/pnas.1914254116).
Previous theoretical studies put that lower limit at between 100 and 300 water molecules. Now, Valeria Molinero of the University of Utah, Francesco Paesani of the University of California San Diego, Thomas Zeuch of the University of Göttingen, and colleagues have combined experimental and theoretical work to find a more precise answer.
The researchers generated water clusters of varying sizes at temperatures between 40 and 150 K using a high-pressure nozzle. Infrared spectroscopy revealed that crystalline-ice hydrogen bonds form between molecules in clusters of about 90 molecules and bigger at around 150 K. Up to about 150 molecules, the molecules in these clusters oscillate between the solid and liquid phases. The researchers used their experimental results to fine-tune molecular dynamics simulations of similarly sized clusters and showed that they could reproduce their experimental spectroscopic results.
Clusters of these sizes exist inside large proteins and some materials, so the researchers think that this work will help predict water’s behavior in those situations. They also say that nanosized systems of other molecules are likely to have similar regimes in which bulk characteristics take over and phase oscillations may exist, even systems with liquid-liquid rather than liquid-solid phase separations.