A Stockholm University-led team of physicists has discovered two low-temperature phases of liquid water with large differences in structure and density.
When we think of ice it is most often as an ordered, crystalline phase that you get out of the ice box, but the most common form of ice is amorphous, that is disordered, and there are two forms of amorphous ice with low and high density.
The two forms can interconvert and there have been speculations that they can be related to low- and high-density forms of liquid water.
“We found that water can exist as two different liquids at low temperatures (minus 234 degrees Fahrenheit, or minus 148 degrees Celsius) where ice crystallization is slow,” said Anders Nilsson, professor in chemical physics at Stockholm University, and senior author of the paper reporting the results in the Proceedings of the National Academy of Science.
“The unique aspect of this work is the combination of two X-ray methods, where wide-angle X-ray scattering provides the evidence for the structure at the atomic level and X-ray photon-correlation spectroscopy provides insight about the motion at the nanoscale, respectively,” they explained.
The new results not only create an overall understanding of water at different temperatures and pressures, but also how water is affected by salts and biomolecules important for life.
In addition, the increased understanding of water can lead to new insights on how to purify and desalinate water in the future.
“The 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,” said co-author Lars G.M. Pettersson, professor in theoretical chemical physics at Stockholm University.
“In a nutshell: water is not a complicated liquid, but two simple liquids with a complicated relationship.”
“It is particularly exciting that the new information has been provided by X-rays since the pioneer of X-ray radiation, Wolfgang Röntgen, himself speculated that water can exist in two different forms and that the interplay between them could give rise to its strange properties,” added co-author Daniel Mariedahl, Ph.D. student in chemical physics at Stockholm University.
“I have studied amorphous ices for a long time with the goal to determine whether they can be considered a glassy state representing a frozen liquid,” said Dr. Katrin Amann-Winkel, postdoctoral researcher in chemical physics at Stockholm University.
“It is a dream come true to follow in such detail how a glassy state of water transforms into a viscous liquid which almost immediately transforms to a different, even more viscous, liquid of much lower density.”
Fivos Perakis et al. Diffusive dynamics during the high-to-low density transition in amorphous ice. PNAS, published online June 26, 2017; doi: 10.1073/pnas.1705303114