Two-phased water: X-ray insights

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  • Published: Jul 1, 2017
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Two-phased water: X-ray insights

Vive la difference

Pictured is an artist's impression of the two forms of ultra-viscous liquid water with different density. On the background is depicted the x-ray speckle pattern taken from actual data of high-density amorphous ice, which is produced by pressurizing water at very low temperatures. Credit: Mattias Karlén

Liquid water exists as two distinct phases with very different structures and densities according to X-ray work by an international team. The research was led by scientists at Stockholm University, Sweden and their colleagues at the KTH Royal Institute of Technology in Stockholm, DESY in Hamburg, University of Innsbruck, Argonne National Laboratory in Chicago and SLAC National Accelerator Laboratory in California.

The common picture of liquid water is one of myriad bent H2O molecules tumbling randomly in the liquid fleetingly connecting to each other, again randomly, through hydrogen bonds. However, a new study suggests that there are actually two phases of liquid water. Results from X-ray work carried out at APS beamline 6-ID-D of the Advanced Photon Source at ANL and PETRA III beamline P10 at DESY suggest this is the case. The team reports details in the journal Proceedings of the National Academy of Science (USA).

Two-phased liquid

Water is one of the most superficially simplest of substances and yet it is also the most intriguing, not only because it is essential to life on Earth but because of its well-known anomalous properties: the proximity of the gas, liquid and solid phases on a scale of temperature compared with almost every other substance, its strangely high specific heat capacity, the fact that it expands somewhat on freezing, its ability to act as an almost universal solvent for a wide range of substances, and many other phenomenon mark out water as somehow special. It exists at the confluence of physical and chemical properties around which all other substances are outliers in at least one characteristic. There are perhaps more than 70 different properties that are so different from other substances that endow water with this unique position. And yet, just as we imagine we have listed them all, new phenomena emerge from science laboratories.

"The new remarkable property is that we find that water can exist as two different liquids at low temperatures where ice crystallization is slow", explains physicist Anders Nilsson of Stockholm University. The breakthrough in the understanding of water has been possible through a combination of studies using X-rays at ANL where the two different structures were evidence for the two different structures was obtained and at DESY where the dynamics of these phases was investigated and revealed the distinctions between the two. Fundamentally, what the team is saying is that water can exist as two different liquids.

It's complicated

"It is very exciting to be able to use X-rays to determine the relative positions between the molecules at different times", enthuses Stockholm post-doctoral researcher Fivos Perakis. "We have in particular been able to follow the transformation of the sample at low temperatures between the two phases and demonstrated that there is diffusion as is typical for liquids."

When we consider solid water, ice, we know that it exists as either an ordered crystalline form or a disordered, amorphous form. The amorphous form of ice also exists in two different phases a low and a high density phase that can interconvert. There have been suggestions that there might also exist a low- and a high-density form of water in the liquid state. However, investigating this hypothesis experimentally has been an enormous challenge to which the team has risen.

"I have studied amorphous ices for a long time with the goal to determine whether they can be considered a glassy state representing a frozen liquid", explains chemical-physicist Katrin Amann-Winkel, also at Stockholm University. "It is a dream come true to follow in such detail how a glassy state of water transforms into a viscous liquid which almost immediately transforms to a different, even more viscous, liquid of much lower density". Intriguingly enough, the discoverer of X-ray, Wolfgang Röntgen, himself speculated that water might exist in two different forms and that the interplay between them might in part explain water's anomalous properties, many of which were known about in the nineteenth century, if not before. The new study now suggests that liquid water has two different characters that have a complicated relationship with each other.

These results not only add to our overall understanding of water at different temperatures and pressures, but hint at how water is affected by salts and biomolecules important for life. Moreover, this improved understanding of water might lead to new insights into how to purify and desalinate water in the future. This will be one of the main challenges to humanity in the wake of global climate change and dwindling supplies of fresh water in many parts of the world.

Related Links

Proc Natl Acad Sci (USA) 2017, online: "Diffusive dynamics during the high-to-low density transition in amorphous ice"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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