Researchers Analyze Two Liquid States of Water Using X-Ray Lasers

Scientists at Stockholm University have used X-ray lasers and successfully studied the transformation between two different liquid states of water (H₂O), both being composed of H₂O molecules.

At a temperature of about −63 °C, both the liquids occur at distinct pressure regimes with a density variance of 20%. The team quickly changed the pressure before the sample froze and could thus see one liquid turning into the other in real time. Their study results have been reported in the Science journal.

Unlike other substances, Water, which is common and essential for the existence of life on Earth, acts very odd. The way properties of H₂O—such as density, viscosity, specific heat, and compressibility—change in response to variations in temperature and pressure is totally different from that of other common liquids. Therefore, H₂O is often referred to as “anomalous.”

If water would have acted like a “normal liquid,” man would not be able to exist, as marine life could not have formed. But what triggers these anomalies is still unresolved.

There have been several explanations for the odd properties of H₂O, where one of them suggests that H₂O has the capability to occur as two distinct liquids at different pressures and at low temperatures. If both the liquids could be placed in a glass, they would separate with a distinct interface in between, as for oil and water.

Normal water at ambient conditions is just one liquid and no interface can be observed in a glass—but on a molecular scale, it fluctuates and forms small local areas of similar density as the two liquids, resulting in water’s odd behavior.

The difficulty faced by researchers is that no experiment has been feasible at the temperatures where the two liquids would co-occur since ice would form virtually immediately. So far, it has only been feasible to examine water at these conditions using various types of computer simulations, which has resulted in a lot of contradicting results based on the model employed.

What was special was that we were able to X-ray unimaginably fast, before the water froze, and could observe how one liquid transformed to the other. For decades, there has been speculations and different theories to explain these anomalous properties and why they get stronger when water becomes colder. Now we have found that the two liquid states are real and can explain the water strangeness.

Anders Nilsson, Professor of Chemical Physics, Stockholm University

“I have studied several forms of disordered ices for a long time with the goal to determine whether they can be considered a glassy state representing a frozen liquid,” says Katrin Amann-Winkel, Senior Researcher in Chemical Physics at Stockholm University. “It is a dream come true to see that indeed they represent real liquids and we see the transformation between them.”

We worked so hard for several years to conduct measurements of water under such low temperature conditions without freezing and it is so rewarding to see the outcome. Many attempts over the world have been made to look for the two liquids by putting water in tiny compartments or mixing it with other compounds but here we could follow it as simple pure water.

Harshad Pathak, Researcher in Chemical Physics, Stockholm University

“I wonder if the two liquid states as fluctuations could be an important ingredient to the biological processes in living cells,” says Fivos Perakis, Assistant Professor in Chemical Physics at Stockholm University. “The new result can open up many new research directions also about water in biological sciences.”

Maybe one of the liquid forms is more prominent for water in small pores inside membranes used to desalinate water. I think the access to clean water will be one of the major challenges with climate change.

Marjorie Ladd Parada, Postdoc, 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. Researchers studying the physics of water can now settle on the model that water can exist as two liquids in the supercooled regime,” explains Anders Nilsson.

“The next stage is to find if there is a critical point when the two liquids cross over to become only one liquid, as the pressure and temperature changes. A big challenge for the next few years,” concluded Nilsson.

Journal Reference:

Kim, K. H., et al. (2020) Experimental observation of the liquid-liquid transition in bulk supercooled water under pressure. Science. doi.org/10.1126/science.abb9385.