Ice crystals have small beginnings—even smaller than previously believed. Challenging the existing belief that around 1000 molecules are required to generate a particle of ice with a true crystalline structure, a team of researchers working with Thomas Zeuch from the University of Göttingen, Petr Slavíček from the Technical University in Prague, and Udo Buck from the Göttingen-based Max Planck Institute for Dynamics and Self-Organization has now shown experimentally that crystalline order starts to form with just 275 water molecules, and that only 475 can generate a real crystalline structure.
The water molecules in ice crystals arrange themselves in a hexagonal lattice in which each water molecule forms hydrogen bonds to four adjacent molecules, and which occupies more space than liquid water, which is an unusual behavior. In the experiments performed, clusters below the minimum size for a crystal are generated with temperatures of around −180°C to −160°C. As they are too small to crystallize, they instead form a disordered, amorphous spatial lattice.
As such clusters increase in size, the water molecules at their core can change at some stage from a disordered into an ordered crystalline structure. This was first observed with 275 water molecules, where the first crystalline structure is observed in the interior of the cluster and comprises a ring of six hydrogen-bonded water molecules in a tetrahedral configuration.
To begin with, this structure is still slightly deformed. However, as the cluster increases in size, this interior grows to become a nicely ordered ice crystal, while the outer layers remain amorphous. “When there are 475 molecules, the very core is already perfect,” says Buck.
As reported in the September 21 issue of Science (DOI: 10.1126/science.1225468; p. 1529), the researchers doped the cluster with a sodium atom and used IR excitation–modulated photoionization spectroscopy to pinpoint the onset of crystallization. Zeuch says that the sodium atom enables the cluster to be gently ionized, sorted with an electric field, and measured specifically.
With increasing size, very cold clusters of water molecules arrange themselves as a real ice crystal. (a) When there are 123 water molecules, clusters are still completely unstructured—like a rigid liquid; (b) when there are just under 300 molecules, the hexagonal structure of the ice crystal is already discernible in the cluster’s core; (c) when there are 600 molecules, the interior of the ice crystal is already perfectly formed, only the outer layer is still amorphous.Credit: Udo Buck
The sodium atom on the surface of the water cluster also has a second function. “It acts as a type of photographic paper,” says Zeuch. “We initially irradiate the clusters containing the sodium atom with the infrared light. Then we ‘develop’ it with a laser pulse of ultraviolet light.” The sodium atom provides an infrared spectrum of the tiny water cluster. This decisive trick was the breakthrough.
The researchers now want to experimentally investigate the crystallization of other substances and their surface properties as well—accurate to one molecule, where possible.
