Watery weirdness: X-rays reveal

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  • Published: Jan 4, 2018
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Watery weirdness: X-rays reveal


Illustration showing fluctuations between regions of two different local structures (high density as red and low density liquid as blue) of water that depend on the temperature. Maxima in the thermodynamic response and correlation functions are observed as a function of temperature, when the numbers of molecules in the two structures become equal, resulting in strong enhancement in the anomalous properties of water in the deeply supercooled regime. Credit: Stockholm University

A femtosecond X-ray laser has been used by international scientists to probe the so-called "no man's land" in the thermodynamic landscape of nanoscopic droplets of water held in a vacuum to explain some of its anomalous behaviour. The study using wide-angle X-ray scattering (WAXS) and small-angle x-ray scattering (SAXS) points to water existing as two distinct liquids at a certain critical state, the team reports in the journal Science.

Kyung Hwan Kim, Alexander Späh, Harshad Pathak, Fivos Perakis, Daniel Mariedahl, Katrin Amann-Winkel, and Anders Nilsson of the Department of Physics, at the AlbaNova University Center, Stockholm University, together with A. Sellberg of the KTH Royal Institute of Technology, Jae Hyuk Lee, Sangsoo Kim, Jaehyun Park, and Ki Hyun Nam of Pohang Accelerator Laboratory, Pohang, Gyeongbuk in the Republic of Korea, and Tetsuo Katayama of the Japan Synchrotron Radiation Research Institute, in Kouto, Japan, point out that water is well known for its deviant thermodynamic properties. This substance, vital to life on Earth and presumably elsewhere in the universe, behaves very strangely when compared to other substances: its density, specific heat, viscosity and compressibility respond to changes in pressure and temperature in the opposite way to other known liquids despite their superficial similarities in terms of molecular structure. The team explains that, "One explanation for the divergence of many of the thermodynamic properties of water is that there is a critical point in deeply supercooled water at some positive pressure. For bulk water samples, these conditions are described as “no man's land,” because ice nucleates before such temperatures can be reached."

Supercooled speculation

It is well known that water expands as it freezes unlike most other liquids. It’s the reason why pond life can survive the ice of winter and why icebergs and ice cubes float. The team has now investigated the peak of strange behaviour observed in water as it is cooled to -44 degrees Celsius (227 Kelvin). The team explains that apparently uniquely water can fluctuate between two distinct liquid states in which the bonding between water molecules is different.

"What was special was that we were able to X-ray unimaginably fast before the ice froze and could observe how it fluctuated between the two states," explains Nilsson, a chemical physicist at Stockholm University. "For decades there have been speculations and different theories to explain these remarkable properties and why they got stronger when water becomes colder. Now we have found such a maximum, which means that there should also be a critical point at higher pressures." Many unsuccessful attempts have previously been made to freeze the action at this maximum.

In addition to finding this two liquid states maximum, the study also reveals that the unusual properties of water are different between normal and heavy water and more enhanced for the lighter one. "The differences between the two isotopes, H2O and D2O shows the importance of nuclear quantum effects," explains Stockholm team member Kyung Hwan Kim.

Isothermal dependence

"The temperature dependence of the isothermal compressibility and correlation length extracted from X-ray scattering functions showed maxima at 229 K for H2O and 233 K for D2O," the team reports, "rather than diverging to infinity." This they suggest points to the existence of the Widom line, a locus of maximum correlation lengths emanating from a critical point in the supercooled domain.

"There has been an intense debate about the origin of the strange properties of water for over a century since the early work of Wolfgang Röntgen," adds Nilsson. "Researchers studying the physics of water can now settle on the model that water has a critical point in the supercooled regime. The next stage is to find the location of the critical in terms of pressure and temperature. A big challenge in the next few years."

Related Links

Science 2017, 358, 1589-1593: "Maxima in the thermodynamic response and correlation functions of deeply supercooled water"

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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