Water is almost omnipresent on earth – and yet mysterious. Many of the unusual properties of this elixir of life have not yet been fully explained. Now researchers may have discovered one of the causes of these anomalies. In experiments in the X-ray laser, they demonstrated a new critical point of supercooled water at minus 63 degrees and a high pressure of around 1000 atmospheres. It marks the point from which the two density variants of water – one denser and one less dense – form and develop apart. According to the team, this newly discovered critical transition could explain both the density anomaly and other exotic features of the water.
Water is as omnipresent as it is mysterious. Because it has dozens of exotic peculiarities that set it apart from other liquids. These include the high heat capacity, the “lumpy” distribution of molecules in liquid water, the self-dissociation, the enormous number of different crystallization forms and, above all, the density anomaly: most other liquids become denser when crystallized – the compact crystal lattice allows the atoms to be packed more tightly than the only loosely connected liquid configuration. Not so with H2O: Water reaches its densest configuration at four degrees plus – i.e. when it is still liquid – and then expands again when it freezes. This density anomaly ensures that ice floats on the water and lakes and ponds rarely freeze completely in winter.
Two density variants
Scientists have been researching the reason for the water density anomaly for decades. There are now a few theories about this, including van der Waals bonding forces contributing to the normal hydrogen bonds between the water molecules of liquid water. Another theory, put forward in 1992, assumes that the two density variants known from amorphous water ice play a role. If you cool liquid water so quickly and so much that it doesn’t have time to form a crystal structure, a non-crystalline, glass-like ice is created. Analyzes have shown that this amorphous ice occurs in a denser (HDL) and a less dense variant (LDL). According to some hypotheses, these two density phases of water still persist in liquid water and cause some of the anomalies. If this is the case, then in extremely supercooled but unfrozen water there should be a point at which these two density anomalies arise – a critical point (LLCP).
“However, directly observing this critical point of the two liquid variants experimentally is an extreme challenge,” explain Seonju You from POSTECH University in South Korea and her colleagues. Because this point is at a point in the phase diagram of the water where it actually freezes within microseconds due to the low temperatures. But new laser techniques now make it possible to approach this suspected critical point from the other side – from the amorphous ice. “It is fascinating that this state of water, which has been so well studied, could become a gateway to the critical region for us,” says co-author Aigerim Karina from Stockholm University. For their experiment, they used an infrared laser whose nanosecond pulses quickly heated the two density variants of the ice, while at the same time an X-ray laser captured the structure and arrangement of the water molecules.
Transition at 210 Kelvin
The analyzes showed that the two ice variants changed in the temperature range around 210 Kelvin – around minus 63 degrees Celsius. “We also observed a rapid increase in heat capacity, indicating a critical divergence and increasing density fluctuations at this point,” You and her colleagues write. “Both density samples undergo a similar regime of density fluctuations at this point on their path to decompression.” According to the team, their measurement results indicate that supercooled water passes a critical point at around 210 Kelvin and a pressure of a good 1000 atmospheres. “For decades there have been speculations and various theories to explain the remarkable properties of water. One theory was the existence of a critical point,” says co-author Anders Nilsson from Stockholm University. “Now we have determined that such a point actually exists. Researchers can now use a model according to which water also has a critical point in the supercooled regime.”
Source: Seonju You (POSTECH University, Pohang, South Korea) et al., Science, doi: 10.1126/science.aec0018