X‐ray laser permits insights into anomalies of water

Two liquid states of water and a hidden critical point

Using x-ray lasers, researchers at the Universities of Stockholm and Pohang (South Korea) and of the MPI for Polymer Research 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 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 with the help of ultra-short x-ray pulses at x-ray lasers 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.

For over a century, since the early work of Wolfgang Röntgen, there has been intense debate about the origin of water’s strange properties. “We now used amorphous ice as a model system and starting point for our experimental study. It is now important to understand the implications of our results in terms of their significance for physical, chemical, biological, and climate-related processes,” says Katrin Amann Winkel, Professor at Johannes Gutenberg University Mainz and group leader at the Max Planck Institute for Polymer Research.

The study was done in cooperation with Stockholm University, the POSTECH University and PAL-XFEL in South Korea and St. Francis Xavier University in Canada.