Discovered water exists of two liquids

Breathtaking discovery: Scientists were able to prove that liquid water consists of two different liquids. These would interact with each other. So far, the two types of water have been detected at very low temperatures. Physicists think it very likely that they exist as room temperature.

Liquid water exists in two variants: High Density Liquid (HDL) and Low Density Liquid (LDL), which have now been detected at very low temperatures - but can not be bottled. This is shown by X-ray examinations at DESY and the Argonne National Laboratory in the USA. An international research team led by the University of Stockholm is now presenting its discovery in the Proceedings of the US Academy of Sciences (PNAS).

Water consists of two different liquids (Image: GianlucaCiroTancredi / fotolia.com)

The researchers around Anders Nilsson had investigated so-called amorphous ice. This glass-like form of water ice has been known for decades. It is rare on earth and does not occur in everyday life, but most of the water ice in the solar system exists in this amorphous form. Instead of a solid crystal - such as an ice cube from the freezer compartment - the ice is in the form of disordered molecular chains, which corresponds more to the internal structure of a glass. For example, amorphous ice can be made by cooling liquid water so fast that the molecules do not have time to form an ordered crystal structure.

"Amorphous ice exists in two variants, one with high density and one with lower density," explains DESY physicist Felix Lehmkühler from the research team. The two variants are called High Density Amorphous Ice (HDA) and Low Density Amorphous Ice (LDA). "HDA ice has a density about 25 percent higher than LDA ice," says Lehmkühler. "Scientists have long wondered whether these two types of ice cream do not have corresponding variants in liquid water. This is very difficult to measure. Even if there are both variants in liquid water, they are constantly mixing, turning into each other, and there is no way to separate the two. "

When converting HDA ice into LDA ice, the ice volume spontaneously increases by about a quarter. This could already be observed before the current investigation. Photos: Katrin Amann-Winkel / Filippo Cavalca, University of Stockholm
The researchers have now overcome this hurdle at low temperatures. In the Stockholm lab, Katrin Amann-Winkel prepared especially pure samples of HDA ice. At the Argonne National Laboratory in the US, scientists observed that the internal structure of this ice changes when heated between minus 150 degrees and minus 140 degrees Celsius, transforming it into a lower density form.

At the measuring station P10 of DESY's X-ray source PETRA III, the researchers were able to follow the dynamics of this phase transformation. It showed that the conversion takes place via a liquid: First, the HDA ice is converted into a liquid form of high density, then this "High Density Liquid" (HDL) transforms into a form of lower density ("Low Density Liquid", LDL). This proves the existence of the two suspected variants of liquid water - at least at very low temperatures. The extremely deep-frozen water is so viscous that the two liquid phases only very slowly transform into each other and mix and thus be measured.

"The new noteworthy feature we have observed is that water can exist as two different liquids, at low temperatures where ice crystallization is slow," explains Research Director Nilsson, Professor of Chemical Physics at the University of Stockholm. "It is very exciting that with X-radiation we are able to determine the relative positions of the molecules at different times," adds Fivos Perakis from Stockholm University, along with Amann-Winkel's lead author of the study. "In particular, we were able to follow the transformation of the sample between the two phases at low temperatures and show that diffusion begins as is typical for liquids."

For everyday life, the discovery of the two variants of liquid water changes nothing. For science, however, it is an important step in understanding this extraordinary fluid. "As simple as water appears, it behaves strangely in comparison to other liquids," explains Lehmkühler from the DESY research group "Coherent X-ray Scattering" by Professor Gerhard Grübel, who is also co-author of the study and works as a senior scientist at DESY.

"Water shows so many anomalies - density, heat capacity and thermal conductivity are just three of several dozen properties that are different with water than most other liquids," says Lehmkühler. "Many of these qualities are the basis for the existence of life, because without water and its special qualities, life, as we know it, is not possible." The study of water is therefore not only of great importance and is an area in which also DESY strengthens its commitment. New X-ray sources, such as the recently launched European XFEL European XFEL, whose principal shareholder is DESY, or the expansion of DESY's next-generation PETRA III synchrotron source, PETRA IV, will allow researchers to push even further into the unmapped terrain of the water-phase diagram.

With future investigations, the scientists hope to answer the question, among other things, whether the two types of liquid water also exist at room temperature. There is no fundamental reason that they should only exist at low temperatures. "The new results strongly support the picture in which water at room temperature can not decide which of the two forms to assume, high or low density, which leads to local fluctuations between the two," said co-author Lars Pettersson, professor of Theoretical Chemical Physics at the University of Stockholm. "In a nutshell, water is not a complicated liquid, but two simple liquids in a complicated relationship."

Also involved were the University of Innsbruck, the Royal Institute of Technology (KTH) Stockholm and the US Research Center SLAC. (Sb, pm)

Original work:
"Diffusive dynamics during the high-to-low density transition in amorphous ice"; Fivos Perakis, Katrin Amann-Winkel, Felix Lehmkühler, Michael Sprung, Daniel Mariedahl, Jonas A. Sellberg, Harshad Pathak, Alexander Späh, Filippo Cavalca, Daniel Schlesinger, Alessandro Ricci, Avni Jain, Bernhard Massani, Flora Aubree, Chris J. Benmore , Thomas Loerting, Gerhard Grübel, Lars GM Pettersson and Anders Nilsson; Proceedings of the National Academy of Sciences, 2017; DOI: 10.1073 / pnas.1705303114