The shape – or structure – of water

Water’s structure played a key role in the development of life on Earth. But because molecules in a liquid are constantly moving, liquid structure is hard to define.

Without a rigid or regular structure, the structure of liquids can be quantified by the average number of atoms bonded to a typical central atom, known as the coordination number. Electron scattering patterns can obtain this information, but the experimental procedures are technically demanding, requiring extremely thin layers of the sample liquid. To help understand some of water’s structural intricacies, de Kock et al. developed a method of obtaining and sustaining layers of water down to 100 nanometers.

“What enabled life to evolve was water,” said author Dwayne Miller. “The dynamic structure of water has a lot of influence on simple processes that led to the assembly of living systems.”

To obtain stable, ultrathin water layers, the researchers used a liquid encapsulation device comprised of two electron transparent silicon nitride window structures separated by 10 micrometers and equipped with flow channels for circulating humid air. They also developed a method of subtracting the silicon nitride contribution to extract the desired structural information.

The group was able to directly observe the intermolecular hydrogen bonds with sufficient signal-to-noise to determine that the structure of liquid water effectively involves a full complement of hydrogen bonds with fluctuating defects, leading to additional coordination.

The study serves as a proof-of-concept of the technique’s ability to image water’s structural dynamics. Miller is optimistic it can soon be used to monitor biological processes taking place.

“You can now imagine watching DNA unwind at the atomic level to witness the physics of self- replication – the underpinnings of life,” he said.

Source: “Determining the radial distribution function of water using electron scattering: A key to solution phase chemistry,” by M. B. de Kock, S. Azim, G. H. Kassier, and R. J. D. Miller, Journal of Chemical Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0024127 .