Evidence for decades-old theory to explain the odd behaviors of water
Study detects the critical point betweenliquid forms of water

Date:
July 16, 2020
Source:
Princeton University
Summary:
A new study provides strong evidence for a controversial theory
that at very cold temperatures water can exist in two distinct
liquid forms, one being less dense and more structured than
the other. Researchers conducted computer simulations of water
molecules to discover the critical point at the transition between
the two forms.



FULL STORY ========================================================================== Water, so ordinary and so essential to life, acts in ways that are quite puzzling to scientists. For example, why is ice less dense than water,
floating rather than sinking the way other liquids do when they freeze?

==========================================================================
Now a new study provides strong evidence for a controversial theory that
at very cold temperatures water can exist in two distinct liquid forms,
one being less dense and more structured than the other.

Researchers at Princeton University and Sapienza University of Rome
conducted computer simulations of water molecules to discover the critical point at which one liquid phase transforms into the other. The study
was published this week in the journal Science.

"The presence of the critical point provides a very simple explanation for water's oddities," said Princeton's Dean for Research Pablo Debenedetti,
the Class of 1950 Professor in Engineering and Applied Science, and
professor of chemical and biological engineering. "The finding of the
critical point is equivalent to finding a good, simple explanation for the
many things that make water odd, especially at low temperatures." Water's oddities include that as water cools, it expands rather than contracting,
which is why frozen water is less dense than liquid water. Water also
becomes more squeezable -- or compressible -- at lower temperatures. There
are also at least 17 ways in which its molecules can arrange when frozen.

A critical point is a unique value of temperature and pressure at which
two phases of matter become indistinguishable, and it occurs just prior
to matter transforming from one phase into the other.



========================================================================== Water's oddities are easily explained by the presence of a critical
point, Debenedetti said. The presence of a critical point is felt
on the properties of the substance quite far away from the critical
point itself. At the critical point, the compressibility and other thermodynamic measures of how the molecules behave, such as the heat
capacity, are infinite.

Using two different computational methods and two highly realistic
computer models of water, the team identified the liquid-liquid critical
point as lying in a range of about 190 to 170 degrees Kelvin (about -117 degrees to -153 degrees Fahrenheit) at about 2,000 times the atmospheric pressure at sea level.

The detection of the critical point is a satisfying step for researchers involved in the decades-old quest to determine the underlying physical explanation for water's unusual properties. Several decades ago,
physicists theorized that cooling water to temperatures below its
freezing point while maintaining it as a liquid -- a "supercooled" state
that occurs in high- altitude clouds -- would expose water's two unique
liquid forms at sufficiently high pressures.

To test the theory, researchers turned to computer
simulations. Experiments with real-life water molecules have not so far provided unambiguous evidence of a critical point, in part due to the
tendency for supercooled water to rapidly freeze into ice.

Francesco Sciortino, a professor of physics at the Sapienza University
of Rome, conducted one of the first such modeling studies while a
postdoctoral researcher in 1992. That study, published in the journal
Nature, was the first to suggest the existence of a critical point
between the two liquid forms.



==========================================================================
The new finding is extremely satisfying for Sciortino, who is also a
co-author of the new study in Science. The new study used today's much
faster and more powerful research computers and newer and more accurate
models of water. Even with today's powerful research computers, the
simulations took roughly 1.5 years of computation time.

"You can imagine the joy when we started to see the critical fluctuations exactly behaving the way they were supposed to," Sciortino said. "Now
I can sleep well, because after 25 years, my original idea has been
confirmed." In the case of the two liquid forms of water, the two
phases coexist in uneasy equilibrium at temperatures below freezing and
at sufficiently high pressures.

As the temperature dips, the two liquid phases engage in a tug of war
until one wins out and the entire liquid becomes low- density.

In the simulations performed by postdoctoral researcher Gu"l Zerze at
Princeton and Sciortino in Rome, as they brought down the temperature
well below freezing into the supercooled range, the density of water
fluctuated wildly just as predicted.

Some of the odd behaviors of water are likely to be behind water's
life-giving properties, Zerze said. "The fluid of life is water, but
we still don't know exactly why water is not replaceable by another
liquid. We think the reason has to do with the abnormal behavior of
water. Other liquids don't show those behaviors, so this must be linked
to water as the liquid of life." The two phases of water occur because
the water molecule's shape can lead to two ways of packing together. In
the lower density liquid, four molecules cluster around a central fifth molecule in a geometric shape called a tetrahedron. In the higher density liquid, a sixth molecule squeezes in, which has the effect of increasing
the local density.

The team detected the critical point in two different computer models
of water.

For each model, the researchers subjected the water molecules to two
different computational approaches to looking for the critical point. Both approaches yielded the finding of a critical point.

Peter Poole, a professor of physics at St. Francis Xavier University in
Canada, and a graduate student when he collaborated with Sciortino and coauthored the 1992 paper in Nature, said the result was satisfying. "It's
very comforting to have this new result," he said. "It's been a long and sometimes lonely wait since 1992 to see another unambiguous case of a liquid-liquid phase transition in a realistic water model." C. Austen
Angell, Regents Professor at Arizona State University, is one of the
pioneers of experiments in the 1970s on the nature of supercooled
water. "No doubt that this is a heroic effort in the simulation of
water physics with a very interesting, and welcome, conclusion," said
Angell, who was not involved in the present study, in an email. "As
an experimentalist with access to equilibrium (long-term) physical
measurements on real water, I had always felt 'safe' from preemption by computer simulators. But the data presented in the new paper shows that
this is no longer true."

========================================================================== Story Source: Materials provided by Princeton_University. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Pablo G. Debenedetti, Francesco Sciortino, Gu"l H. Zerze. Second
critical
point in two realistic models of water. Science, 2020 DOI: 10.1126/
science.abb9796 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200716144719.htm

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