Light water, heavy water, and sodium chloride aqueous solutions under extreme conditions to shed light on water anomalies and structural properties – H2D2OX

To bring water and aqueous solutions to negative pressure, we will synthesize fluid inclusions in quartz. By heating them, the liquid occupies the entire inclusion, and when cooling at constant volume, its pressure becomes negative. Microspectroscopy gives access to thermodynamic and vibrational properties.
To bring water and aqueous solutions to pressures exceeding 10,000 times atmospheric pressure, we use diamond anvil cells. The Brownian motion of submicron spheres in suspension allows us to measure the shear viscosity, while Brillouin spectroscopy gives the bulk viscosity. Raman spectroscopy gives vibrational information.

We have developed a minimal theoretical two-state liquid model to account for the different possible cases of anomalous liquids. The first measurements of viscosity and sound attenuation were obtained on pure water up to more than 1.4 GPa.

The minimal theoretical model could be used to describe fluids other than water presenting similar anomalies. The technique for measuring viscosity can be extended to higher temperatures and to other fluids.

Publication on the minimal theoretical model :
F. Caupin and M.A. Anisimov, Minimal Microscopic Model for Liquid Polyamorphism and Waterlike Anomalies
Phys. Rev. Lett. 127, 185701 (2021).
doi.org/10.1103/PhysRevLett.127.185701
accepted version available for free at arxiv.org/abs/2104.08117

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 recently found 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 (and 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 their macroscopic properties.