Water breaks the rules: An ice-like overlayer stabilised by Bjerrum defects

On many wet surfaces the first contact layer of water is not comprised of pure water but is instead a mixture of water and hydroxyl molecules, often caused by the spontaneous dissociation of water at the interface. Understanding the composition and stability of these layers is a key step in describing both wetting and how surfaces respond to redox processes (gain and loss of electrons) of importance in a variety of fields such as electrochemistry, geology, and biology.

In collaboration with Matthew Forster, Rasmita Raval and Andrew Hodgson (University of Liverpool), Javier Carrasco and Angelos Michaelides (LCN and TYC) have shown that structures containing an excess of water over hydroxyl are stabilized on a copper surface by forming a distorted hexagonal network containing Bjerrum defects (in which two H atoms sit between two adjacent O atoms). This novel defect-rich structure is contrary to what would the so-called "ice rules" (which specify exactly one H atom between each pair of O atoms) predict , and was identified through a combination of scanning tunneling microscopy (STM) experiments in Liverpool and density functional theory (DFT) calculations in London.

Careful analysis reveals that the overlayer forms because it maximizes the number of strong bonds formed by water donation to OH. As a result this work also emphasizes that maximizing the number of hydrogen bonds per molecule is not necessarily a useful way to identify stable structures as previously assumed.

It is also shown in the paper that the Bjerrum defects provide uncoordinated OH groups able to hydrogen-bond multilayer water and to nucleate growth. This suggests that defects will facilitate ice nucleation, one of the most important everyday processes of particular relevance to atmospheric chemistry and cloud formation.

For more information see Forster et al. Phys. Rev. Lett. 106, 046103 (2011)

STM image (showing a distorted hexagonal arrangement of overlayer molecules

Figure: On the left is an STM image (Hodgson and co-workers, University of Liverpool) showing a distorted hexagonal arrangement of overlayer molecules. The theoretical (DFT) structure (right) show the precise geometric arrangement of this overlayer with the Bjerrum defects highlighted.

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