Using x-ray lasers, researchers at Stockholm University have been able to determine the existence of a critical point in supercooled water at around -63 °C and 1000 atmosphere. Ordinary water at higher temperatures and lower pressures is strongly affected by the presence of this critical point, causing the origin of its strange properties. The findings are being published in the journal Science.
Water, both omnipresent and essential for life on earth, behaves very strangely in comparison with other substances. How water's density, specific heat, viscosity and compressibility respond to changes in pressure and temperature is completely opposite to other liquids that we know.
All matter shrinks when it is cooled resulting in an increase in its density. One would therefore expect that water would have high density at the freezing point. However, looking at a glass of ice water, everything is upside down since ,as we all know, ice cubes float. Strangely enough for the liquid state, water is the densest at 4 degrees C, and therefore it stays on the bottom whether it's in a glass or in an ocean.
If you chill water below 4 degrees, it starts to expand again. If you continue to cool pure water (where the rate of crystallization is low) to below 0 degrees, it continues to expand – the expansion even speeds up when it gets colder. Many more properties such as compressibility and heat capacity become increasingly strange as water is cooled. Now researchers at Stockholm University, with the help of ultra-short x-ray pulses at x-ray lasers in South Korea, have succeeded in determining that water has a critical point upon deep supercooling and that critical point is the source of the strange properties.
"What was special was that we were able to X-ray unimaginably fast before the ice froze and could observe how the liquid-liquid transition vanishes and a new critical state emerges", says Anders Nilsson, Professor of Chemical Physics at the Department of Physics at Stockholm University. "For decades there has been speculations and different theories to explain these remarkable properties and one theory has been the existence of a critical point. Now we have found that such a point exists".
Water is unique, as it can exist in two liquid macroscopic phases that have different ways of bonding the water molecules together at low temperature and high pressure. When the temperature increases and pressure decreases there is a state where distinction between the two liquid phases vanishes and only one phase is present. It is a point of large instability, causing fluctuations in a large temperature and pressure region all the way up to ambient conditions. The water fluctuates between the two liquid states and mixtures of the two as if it can't make up its mind. It is these fluctuations that give water its unusual properties. The state beyond a critical point is called supercritical and ambient water is in that state.
Another remarkable finding of the study is that that the dynamics of the system slows down as it enters the critical point. "It looks almost that you cannot escape the critical point if you entered it, almost like a Black Hole", says Robin Tyburski, researcher in Chemical Physics at Stockholm University.
"It's amazing how amorphous ices, such an extensively studied state of water, happened to become our entrance to the critical region. It's a great inspiration for my further studies and a reminder of the possibilities of making discoveries in much-studied topics such as water", says Aigerim Karina, Postdoc in Chemical Physics at Stockholm University.
"It was a dream come true to be able to measure water under such low temperature condition without freezing" says Iason Andronis, PhD student in Chemical Physics at Stockholm University. "Many have dreamt about finding this critical point but the means have not been available before the development of the x-ray lasers".
"I find it very exciting that water is the only supercritical liquid at ambient conditions where life exists and we also know there is no life without water. Is this a pure coincidence or is there some essential knowledge for us to gain in the future?", says Fivos Perakis, an associate professor in Chemical Physics at Stockholm University.
"There has been an intense debate about the origin of the strange properties of water for over a century since the early work of Wolfgang Röntgen", explains Anders Nilsson. "Researchers studying the physics of water can now settle on the model that water has a critical point in the supercooled regime. The next stage is to find the implications of these findings on waters importance in physical, chemical, biological, geological and climate related processes. A big challenge in the next few years."
The study was done in cooperation with the POSTECH University and PAL-XFEL in South Korea, Max Planck Society and Johannes Gutenberg University in Germany, and St. Francis Xavier University in Candada. People from Stockholm University contributing to the study include Aigerim Karina, Robin Tyburski, Iason Andronis and Fivos Perakis. Other people that contributed to the study include previous members of the Chemical Physics group at Stockholm University - Kyung Hwan Kim, Marjorie Ladd-Parada and Katrin Amann-Winkel.
Further reading in Science: Experimental evidence of a liquid-liquid critical point in supercooled water by Seonju You and Marjorie Ladd Parada et al.
Science DOI: https://doi.org/10.1126/science.aec0018
