Water is very unusual responds to very low temperatures. Let’s start with the fact that upon cooling, contrary to logic, water does not compress, it expands (this is why ice has a property of buoyancy). Cold water has a lower compressibility than hot. Moreover, when frozen molecules of water can strongly change their position.
All this is difficult to explain, and existing theories cause bitter controversy in the scientific community. One of them was formulated almost three decades ago and consisted in the fact that water ice can exist in two different liquid forms, one of which has a less dense structure. In other words, there are two kinds of water, each of which is a separate liquid. To prove this theory in the laboratory is difficult, but Italian scientists have been able to find evidence in her defense. The study was recently published in the journal Science.
In their study, the researchers Pablo Debenedetti and H. Zerze Gul of Princeton University and Francesco Sortino from La Sapienza in Rome suggest that the “second critical point of water” occurs at temperatures from -83 to -100 degrees and at atmospheric pressure is almost 2000 times higher than the atmospheric pressure above sea level. A critical point is a single value of temperature and pressure at which two phases of a substance become indistinguishable, and it happens just before a substance goes from one phase to another. Water, for example, has the well-known critical point at the transition from liquid to vapor.
“Just imagine our joy when we saw that the critical fluctuations occur exactly as we expected, — says Sortino, now I can sleep peacefully, because after 25 years my idea has finally found proof.”
Until now, experiments using real water molecules to test the second critical point of “supercooling” of water could not give unequivocal evidence of its existence. According to Debenedetti, it is largely due to the fact that ice water normally turns to ice.
For this reason, the researchers decided to use computer models. The process is really time consuming: despite the high power of modern supercomputers to create models, scientists 18 months doing the necessary calculations.
In simulations, when the temperature was far from freezing point, the density of water began to fluctuate. In the end, scientists were able to detect the critical point that they were looking at two different computer models of water. To find the critical point of water in both models were applied different computational approaches.
As with the transition from the liquid phase to the gas phase, ice water can go in two different phases, depending on how the rearranged its molecules. Thus, in the liquid of low density four molecules are grouped around the Central molecules, forming a tetrahedron. However, in the liquid with the higher density comes into play a sixth molecule, which leads to an increase its density.
In his article, the researchers write that “within the limits of our possible computing capabilities has been proven the existence of a metastable critical point in the phase of deep cooling of water molecules”.
Of course, now, this conclusion needs to be confirmed by other experiments “using more accurate and expensive computational resources”.