Credit: © 2007 Nature Materials

Ice nucleation — the process by which ice crystals form — is important in diverse fields such as meteorology, biology and astrophysics. Now, Angelos Michaelides from the Fritz-Haber-Institut in Berlin and Karina Morgenstern from the Leibniz University of Hannover have improved our molecular understanding of ice nucleation by providing images and simulations of the smallest ice crystals possible1.

Water was deposited on copper and silver substrates in a vacuum at 17 K (-256 °C) and scanning tunnelling microscopy was used to image ice crystals formed on their surfaces. Cyclic hexamer structures were observed that consist of six water molecules arranged in a hexagon. Density functional theory calculations predict that the most energetically favourable structure of such a hexamer should have water molecules at alternate high and low positions. This arrangement reduces the symmetry of the crystal, and is reminiscent of Kekulé's alternating bond model of the benzene molecule.

Michaelides and Morgenstern believe the up–down arrangement stems from the competition between the ability of the water molecules to bond to the substrate and to accept hydrogen bonds from other water molecules. However, this alternating arrangement was not apparent in their images — possibly because the electric field from the microscope tip rearranges the molecules as they are imaged.