Ligth water, heavy water, and sodium chloride aqueous solutions under extreme conditions to shed light on water anomalies and structural properties

• Applicants: Dr. Christian Sternemann; Professor Dr. Max Wilke
• Subject Area: Mineralogy, Petrology and Geochemistry ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 431324570

Water and aqueous solutions are ubiquitous, being involved in countless natural phenomena and technological processes. Water stands out among all liquids because of its numerous physical anomalies related to its complex hydrogen bond network, yet it is not fully understood. The goal of our project is to combine the efforts of researchers in physics and geosciences to gain new knowledge about water and aqueous solutions under extreme conditions. On the one hand, we will explore the stretched liquid state, at negative pressures. The liquid is then metastable with respect to vapor and a bubble may nucleate at any time, bringing back the system to equilibrium. But small liquid droplets trapped in a quartz matrix reach beyond –100 MPa in the metastable state and can be studied with photons. We will use Brillouin, visible Raman and x-ray Raman spectroscopy in order to elucidate the thermodynamics and molecular structure of stretched water. Experiments will be performed not only on stretched ordinary water but also on stretched heavy water and aqueous NaCl solutions, as they are predicted to grant access to specific features, which cannot be found in ordinary water. Specifically, the line of density maxima of heavy water is predicted to reach a maximum temperature at a negative pressure accessible to experiment, while the line of compressibility maxima recentlyfound in pure, stretched water is predicted to become more pronounced with a low salt concentration. Confirming or not these features will have broad implications for our understanding of water and its phase diagram, including the intriguing (debated) possibility of liquid polyamorphism – the existence of two distinct liquid phases of water. On the other hand, we will study the stable fluid state under high pressure and temperature conditions. One key property of water in geological processes (e.g. subduction zones and hydrothermal activity) is viscosity. Yet, surprisingly, data is scarce for pure water at high pressure, and absent for salty water. Moreover, the current measurement technique (rolling ball viscometer inside the small chamber of a diamond anvil cell) has limitations and may suffer from bias. We will implement a new technique based on the Brownian motion of spheres of ca. 100 nm diameter to bypass these limitations. These results will be directly linked to the molecular structure using visible Raman spectroscopy. For further insight, we will use x-ray Raman scattering spectroscopy, particularly to address the less well known structure of heavy water and NaCl solutions at conditions up to near the liquid-vapor critical point, assessing the influence on ion hydration. Herewith, we combine our complementary expertise in order to study water, heavy water, and aqueous NaCl solutions at extreme conditions, in the stretched, supercritical and high-density regimes, giving rise to a deeper understanding of the microscopic structure of the fluids and their relation to macroscopic properties.

• DFG Programme: Research Grants
• International Connection: France
• Cooperation Partners: Professor Dr. Frédéric Caupin; Professorin Dr. Isabelle Daniel