The strangest liquid of all is even more unusual than we thought
Liquid water exists in two different forms – at least at very low temperatures. This is the conclusion drawn from X-ray experiments carried out at DESY and at the Argonne National Laboratory in the US. An international team of researchers headed by the University of Stockholm now reports its findings in the Proceedings of the National Academy of Sciences (PNAS).
The scientists led by Anders Nilsson had been studying so-called amorphous ice. This glass-like form of frozen water has been known for decades. It is quite rare on earth and does not occur in everyday life; however, most water ice in the solar system actually exists in this amorphous form. Instead of forming a solid crystal – as in an ice cube taken from the freezer – the ice takes on the form of disordered chains of molecules, more akin to the internal structure of glass. Amorphous ice can be produced, for example, by cooling liquid water so rapidly that the molecules do not have enough time to form a crystal lattice.
“Amorphous ice exists in two varieties: one with a high and one with a low density,” explains Felix Lehmkühler, a physicist from DESY who was a member of the research team. These two varieties are referred to as high-density amorphous ice (HDA) and low-density amorphous ice (LDA). “HDA ice is some 25 percent denser than LDA ice,” Lehmkühler adds. “Scientists have been wondering for some time whether these two varieties of ice might be associated with corresponding varieties of liquid water. However, that is very difficult to measure. Even if water does exist in two states, they will constantly be mixing with and turning into each other, and there is no way of separating the two.”
The scientists have now managed to overcome this obstacle at low temperatures. In the Stockholm laboratory, Katrin Amann-Winkel prepared extremely pure samples of HDA ice. At the Argonne National Laboratory in the US, the scientists noticed that the internal structure of this ice changed between temperatures of minus 150 to minus 140 degrees Celsius – turning into a lower density form.
The scientists then observed the dynamics of this phase change at the P10 beamline of DESY’s X-ray source PETRA III. They found that the transition takes place via a liquid state: first, the HDA ice turns into a liquid form of high density, then this high-density liquid (HDL) turns into a lower density form (low-density liquid, LDL). This demonstrates the existence of the two suspected varieties of liquid water – at least at very low temperatures. The extremely low-temperature water is so viscous that the two liquid phases are only transformed into each other and only mix very slowly, allowing this process to be measured.
“The new remarkable property is that we find that water can exist as two different liquids at low temperatures where ice crystallisation is slow,” says Nilsson, a professor of Chemical Physics at the University of Stockholm. “It is very exciting to be able to use X-rays to determine the relative positions between the molecules at different times,” adds Fivos Perakis of the University of Stockholm, who was one of the two principal authors of the study, along with Amann-Winkel. “We have in particular been able to follow the transformation of the sample at low temperatures between the two phases and demonstrated that there is diffusion as is typical for liquids.”
The discovery of the two varieties of water does not have any particular consequences for our everyday lives. For scientists, though, it is an important step towards understanding this extraordinary liquid. “Water may seem very simple, but it behaves very strangely compared with other liquids,” explains Lehmkühler, a member of the DESY research group for Coherent X-ray Scattering, headed by Gerhard Grübel, who is also a co-author of the study and who works for DESY as a Leading Scientist.
“Water displays so many anomalies – its density, heat capacity and thermal conductivity to name just three of several dozen properties that distinguish water from most other liquids,” notes Lehmkühler. “Many of these properties are the foundation for the existence of life, because without water and its special properties, life as we know it could not exist.” This is just one reason why the study of water is so important and a field in which DESY is increasingly involved. New X-ray sources like the soon to be opened European X-ray Free-Electron Laser (European XFEL), whose principal shareholder DESY is, and PETRA IV, the upgraded next generation of DESY’s synchrotron source PETRA III, will allow scientists to penetrate even further into the uncharted territory of water’s phase diagram.
The scientists are hoping that future experiments will help them to answer questions such as whether the two types of liquid water also exist at room temperature. In principle, there is no reason why they should only occur at low temperatures. “The new results give very strong support to a picture where water at room temperature can’t decide in which of the two forms it should be, high or low density, which results in local fluctuations between the two,” says co-author Lars G.M. Pettersson, professor in Theoretical Chemical Physics at the University of Stockholm. “In a nutshell: Water is not a complicated liquid, but two simple liquids with a complicated relationship.”
The University of Innsbruck, the KTH Royal Institute of Technology in Stockholm and the US research centre SLAC were also involved in the research.