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Tetrahedra may explain water’s uniqueness

Tetrahedra may explain water 's uniqueness
Scientists at The University of Tokyo use a two-state model based on the formation of tetrahedral structures to explain water’s anomalous properties and the surprising liquid-liquid transition of water Credit: Institute of Industrial Science, the University of Tokyo

Researchers at the Institute of Industrial Science at the University of Tokyo sifted through experimental data to probe the possibility that supercooled water has a liquid-to-liquid phase transition between disordered and tetrahedrally structured forms. They found evidence of a critical point based on the cooperative formation of tetrahedra, and show its minor role in water’s anomalies. This work shows that water’s special qualities—which are essential for life—originate predominantly from the two-state feature.

Liquid water is indispensable for life as we know it, yet many of its properties do not conform with the way other fluids behave. Some of these anomalies, such as water’s maximum density at 4°C and its large heat capacity, have important implications for living organisms. The origin of these features has sparked fierce debates in the scientific community since the time of Röntgen.

Now, researchers at The University of Tokyo have utilized a two-state model that posits the dynamical coexistence of two types of molecular structures in liquid water. These are the familiar disordered normal-liquid structure and a locally favored tetrahedral structure. As with many other phase transitions, there may be a “critical point” at which the correlation between tetrahedra takes on a power-law form, which means there will no longer be any “typical” length scale.

Using computer simulations of water molecules, along with a comprehensive analysis of experimental structural, thermodynamic, and dynamic data—including X-ray scattering, density, compressibility, and viscosity measurements—the researchers were able to narrow down where a critical point should be, if it exists.

“If the formation of tetrahedral structures in liquid water is cooperative under these conditions, then a liquid-liquid phase transition with a critical point is possible,” lead author Rui Shi says.

The team showed that this occurs around a temperature of -90°C and a pressure of about 1,700 atmospheres. Experiments in this range are exceedingly difficult: because the water is so far below its normal freezing, ice crystals can quickly form. However, samples can remain liquid in a metastable “supercooled” state at these very high pressures.

“We saw evidence that the critical point is real, but its effect is almost negligible in the experimentally accessible region of liquid water because it is too far from the critical point. This means that water’s anomalies come from the two-state feature and not from criticality,” senior author Hajime Tanaka says. The scientists anticipate that this project will lead to the convergence of the long debate on the origin of water’s anomalies and more experimental research to access the second critical point of water.

Liquid sulfur changes shape and goes critical under pressure

More information:
The anomalies and criticality of liquid water, PNAS (2020). DOI: 10.1073/pnas.2008426117
Provided by
University of Tokyo

Tetrahedra may explain water’s uniqueness (2020, October 12)
retrieved 12 October 2020


Katahdin Woods and Waters to celebrate dark sky designation in virtual event

PATTEN, Maine — A new moon over Katahdin Woods and Waters National Monument on Oct. 15 will allow the darkest skies in the Northeastern United States to be absent of moonlight, making thousands of stars and the Milky Way galaxy visible to the naked eye.

To mark the occurrence, the Friends of Katahdin Woods and Waters organization will hold its seventh annual Stars Over Katahdin event, along with a new organization, Dark Sky Maine. It won’t be held at the monument as in years past, but will be virtual, meeting the same fate of other events during the time of COVID-19.

While moving the stargazing to online is a setback for something usually held around campfires, outdoors and away from internet reception, the event is also highlighted this year by Katahdin Woods and Waters being designated as an International Dark Sky Sanctuary by the International Dark Sky Association, or IDA.

“We’re going to take the same campfire chats we had before, and we’ll bring them to people’s homes,” said Andrew Bossie, the executive director of Friends of Katahdin. “We’ll also give tips on how folks can interpret the night skies themselves.”

Founded in 1988, the IDA is a nonprofit organization that works on protecting areas with visible views of the night sky from light pollution, light from cities and other manmade structures that can obscure the brightness of stars.

Katahdin Woods and Waters’ designation marks the first such location along the East Coast of the United States.

“You have to submit a whole series of night sky readings showing how dark it is,” Tim Hudson, superintendent of the monument, said. “You have to be below certain numbers to be able to do this. So people have been taking readings out there over the years, and that’s how you build it up. It isn’t a one-shot deal.”

To measure the brightness of the night sky and lack of light pollution, astronomers use the Bortle Scale, which ranks the brightness of the sky on a scale from one to nine, with one being the most visible night sky and nine having the most light pollution.

Katahdin Woods and Waters ranks a two on the Bortle Scale, the second best ranking for viewing the stars. The only places that rank at one are located along Antarctica and the North Pole, Bossie said, making this part of northern Maine among the highest quality of stargazing for most ordinary citizens.

“It only gets darker if you go literally to the edge of the Earth,” said Bossie. “A lot of volunteers that are part of our community worked very hard to get this [IDA] designation. And it does mean those night skies are protected now.”

In addition to celebrating the designation, the virtual event will also feature several guest speakers, such as Kelley Beatty, a former board member of the IDA, and John Dennis of the Aroostook Band of Micmacs, who will explain the importance of the night skies among the culture of the various Wabanaki