How Ice Forms On Salt

Ultrahigh resolution scanning tunnelling microscopy has overturned the decades-old understanding of ice structure based on Bernal-Fowler-Pauling ice rules.

AsianScientist (Jun 19, 2014) – Scientists from the University of Peking have discovered that ice adopts an unusual structure when formed on salt, a finding that could influence phenomena such as ice nucleation and salt dissolution.

Water-solid interactions are of broad importance in both nature and technology. For decades, scientists have used the Bernal-Fowler-Pauling ice rules to describe the structure of water at interfaces, predicting that ice should have a hexagonal bilayer arrangement. However, the validity of the hexagonal bilayer model has been challenged by recent studies which report structural distortions in ice.

A research team led by Professor Jiang Ying of International Center of Quantum Materials (ICQM) of Peking University, in collaboration with Professors Wang Enge and Li Xinzheng from the same center, have now described a highly defective tetramer-based ice structure on the surface of a salt film, publishing their results in Nature Communications.

Over the past four years, Jiang’s group has worked on the development of ultrahigh resolution scanning tunnelling microscope (STM) for single-molecule experiments. Their present research has achieved submolecular-resolution imaging of individual water molecules adsorbed on a Au-supported NaCl(001) film. Such a technique opens up the possibility of determining the detailed topology of H-bonded networks at water/solid interfaces with atomic precision.

Ab initio theoretical calculations based on density functional theory showed that ice adopted a tetrameric structure, defying the Bernal-Fowler-Pauling ice rules. Jiang and his collaborators found that the water tetramers within the lower part of the bilayer act as the basic building blocks, which are interconnected via a novel bridging mechanism to form a regular array of defects located in the upper layer.

The authors suggest that these structures and may play a crucial role in catalyzing heterogeneous chemical reactions on water-coated salt surfaces as well as in influencing various phenomena such as heterogeneous ice nucleation, salt dissolution and caking.

The article can be found at: Chen et al. (2014) An unconventional bilayer ice structure on a NaCl(001) film.


Source: University of Peking; Photo: bollhav/Flickr/CC.
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