The hypothesis that water has a second critical point at deeply supercooled conditions was formulated to provide a thermodynamically consistent interpretation of numerous experimental observations.
Despite a broad range of experimental observations that indirectly point out the possible existence of a liquid-liquid phase transition in profoundly supercooled water, no unambiguous experiment has shown this yet.
By Conducting computer simulations of water molecules, scientists at Princeton University and the Sapienza University of Rome provided strong evidence for this controversial hypothesis. Their study has discovered a critical point at which one liquid phase transforms into the other.
Princeton’s Dean for Research Pablo Debenedetti, the Class of 1950 Professor in Engineering and Applied Science, said, “The presence of the critical point provides a very simple explanation for water’s oddities. 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 the 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 before matter transforming from one phase into the other.
Debenedetti said, “The presence of a critical point easily explains water’s oddities. 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.”
Scientists used two different computational methods and two highly realistic computer models of water to identify 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 discovery of this critical point is a satisfactory step for scientists to determine the underlying physical explanation for water’s unusual properties.
Using computer simulations, scientists tested the hypothesis. 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, said, “You can imagine the joy when we started to see the critical fluctuations exactly behaving the way they were supposed to. Now I can sleep well, because, after 25 years, my original idea has been confirmed.”
In the simulations performed by postdoctoral specialist Gül Zerze at Princeton and Sciortino in Rome, as they brought down the temperature well beneath freezing into the supercooled range, the density of water varied fiercely similarly as anticipated.
Zerge said, “Some of the odd behaviors of water are likely to be behind water’s life-giving properties. 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.”
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 an exciting and welcome, conclusion. 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.”
- Pablo G. Debenedetti et al. Second critical point in two realistic models of water. DOI: 10.1126/science.abb9796