Water is one of the most studied materials on Earth and has been the focus of dedicated research for decades. Despite it being a very simple compound of two common elements, two hydrogen atoms and one oxygen atom, researchers still struggle to describe the unusual structure and dynamics of liquid water. Now, a team of scientists have used synchrotron radiation to reveal the microscopic structure of water under conditions of extreme pressure and temperature. The results were published on 11th March 2013 in the Proceedings of the National Academy of Sciences of the USA (PNAS).
When subject to extreme temperature and pressure, water exists in the so called super-critical state and exhibits a number of peculiar characteristics very different to those of the water that flows through our taps. In these extreme conditions, water becomes a very aggressive solvent, catalysing otherwise impossible chemical reactions like, for example, the oxidation of hazardous waste or the conversion of sewage into clean water and gases like hydrogen, methane, carbon dioxide and carbon monoxide.
Extremely high temperature and high pressure conditions can be found in the lower crust and upper mantle of the Earth. Here, the unique properties of super-critical water are believed to play a key role in the transfer of mass and heat as well as in the formation of ore deposits and volcanoes. Super-critical water is even thought to have contributed to the origin of life. Insight into the structural properties of water on an atomic scale are essential to understanding the role of liquid water in these processes.
The team of scientists from the Technische Universität Dortmund (Germany), the University of Helsinki (Finland), the Deutsches GeoForschungsZentrum in Potsdam, (Germany), and the European Synchrotron Radiation Facility (France), used X-ray spectroscopy on the ESRF's ID16 beamline to study the structural properties of water in the super-critical state.
The challenge that faced the research team was to study liquids and gases under extreme conditions of high temperature and high pressure. The high performance of ID16 in terms of high flux, energy resolution and highly focused beam was essential to the success of this unprecedented study.
Although conventional spectroscopic analysis techniques can provide key insights into the atomic structure of a substance, they are not well suited to studying water under super-critical conditions because of the complex environment needed to maintain water in this state.
The research exploited the ESRF's hard X-rays to access the oxygen K-edge in a complex environment finding that water evolves systematically from liquid-like to more gas-like at high temperatures and pressures.
Based on the close resemblance between the theoretical and measured data, the team was able to extract detailed information about the atomic structure and hydrogen bonding of water. They showed that according to the theoretical model, the microscopic structure of water remains spatially homogeneous throughout the range of examined temperatures and pressures.
"The unparalleled results obtained here are a good example of new possibilities open to the scientific community for experiments impossible to conduct with other techniques," says Laura Simonelli, scientist on ID20. "Recently, ID16 was upgraded and moved to ID20 where the beamline has been vastly improved in terms of intensity, beam focus, stability and energy resolution. The astounding new X-ray Raman spectrometer will give access to a novel signal to noise ratio with a very wide covering of solid angles."
Microscopic structure of water at elevated pressures and temperatures, C.J. Sahle, C. Sternemann, C. Schmidt, S. Lehtolab, S. Jahn, L. Simonelli, S. Huotari, M. Hakala, T. Pylkkänen, A. Nyrow, K. Mende, M. Tolan, K. Hämäläinen, M. Wilke, PNAS (2013);