Computer simulations provide a molecule's eye view of the melting of ice nanoparticles, predicting melting at very low temperatures.
The melting of ice is a very familiar process but its ubiquity belies its importance. It plays a central role in a wide variety of chemical processes, and is particularly relevant to environmental and atmospheric chemistry. However, despite being an everyday process, melting is not as well understood as one might have thought, particularly on the nanoscale.
Ice cubes, like those you might put in a gin and tonic melt at zero degrees Celsius, but at what temperature do ice nanoparticles (that is particles about 0.000000001 metre large) melt? An international team of researchers from the Thomas Young Centre, the London Centre for Nanotechnology, UCL Chemistry Department and Peking University set about to answer this question with computer simulation techniques and some of the most powerful computers in the UK (the HECToR and Legion Supercomputers). The answer they got is around a cool minus 100 degrees Celsius. That is, on the nanoscale the melting point of ice particles is about 100 degrees less than it is for macroscopic (everyday) ice cubes.
The strong dependence of melting temperature on particle size sounds remarkable but is not that surprising: it is due to the large surface to volume ratio of the nanoparticles and can be explained pretty well with textbook thermodynamic theories that relate melting temperatures with particle size.
These results will be of relevance in understanding the size dependence of ice crystal morphology and the surface reactivity of ice particles under atmospheric conditions, particularly polar mesospheric clouds, which occur at altitudes of about 80 to 90 km.
Figure: A partially melted ice nanoparticle at about -100 degrees Celsius