Liquids are more difficult to describe than gases or crystalline solids. The HZB team has now mapped for the first time the potential energy surface of water molecules in liquid water under ambient conditions at the Swiss Light Source SLS at the Paul Scherrer Institute in Switzerland. This helps to better understand the chemical properties of water and aqueous solutions. These investigations will soon continue at the new METRIXS station on the X-ray source BESSY II.
Water is undoubtedly the most famous liquid in the world. Water plays a vital role in all living things and in many chemical processes. The water molecule itself has almost no secrets. In school, we already knew that water consists of one oxygen atom and two hydrogen atoms. We even know the typical obtuse angle that two OH legs form with each other. Additionally, we know when water boils or freezes, and how these phase transitions relate to pressure. But between the facts about individual molecules and a deeper understanding of macroscopic phenomena, there is a great deal of uncertainty: about the behavior of individual molecules in normal liquid water, only statistical information is known: water in the liquid phase Molecules form a network of fluctuating hydrogen bonds, disordered and dense, and their interactions are simply not understood as in the gaseous state.
Check for pure liquid water
Now, a team led by HZB physicist Dr. Annette Pietzsch has taken a closer look at pure liquid water at room temperature and pressure. Using X-ray analysis and statistical modeling at the Swiss Light Source of the Paul Scherrer Institute, the scientists succeeded in mapping the so-called potential surface of individual water molecules in the ground state, which have a wide variety of shapes depending on their environment.
Measure oscillations and vibrations
“What’s special here is the method: we used resonant inelastic X-ray scattering to study water molecules on the ADRESS beamline. Simply put, we nudge individual molecules very carefully and then measure how they return to the ground state,” Pietzsch said . Low-energy excitations lead to tensile oscillations and other vibrations, which, combined with model calculations, yield detailed pictures of the underlying surface.
“This gave us a way to experimentally determine how a molecule’s energy changes with its structure,” explains Pietzsch. “These results help shed light on the chemistry of water, such as better understanding how water behaves as a solvent.”
Outlook: METRIXS at BESSY II
The next experiment is already planned at HZB’s BESSY II X-ray source. There, Annette Pietzsch and her team set up the METRIXS measuring station, which is specially designed for the study of liquid samples through RIXS experiments. “After the summer shutdown due to maintenance work on BESSY II, we will begin the first tests of our instrument. Then we can move on.”
Material Provided by the Helmholtz-Zentrum Berlin für Materialien und Energie. Note: Content may be edited for style and length.