X-rays reveal details of the mechanism behind the enormous density increase in highly-compressed liquid water

Water is one of the most ubiquitous substances and essential for all forms of life on Earth. Its many thermodynamic anomalies render water one of the most extraordinary liquids known to mankind. Yet, after decades of intense research, the structural details at atomic length scales underlying these anomalies remain unclear.

An example of the strange behaviour of water is its density, which is highest at 4 C. This heavily affects water’s buoyancy and impacts ocean circulation and climate patterns. Likewise, water’s low density ice phase, common ice Ih, is less dense than liquid water, a fact that is vital for aquatic life and the stability of our ecosystems. Pressure is one of the fundamental experimental parameters and is often used by researchers to observe a system’s respond to it, yielding invaluable information about the interactions between atoms and molecules at play.

Now an international team of scientists lead by the ESRF have studied pressurized water in its liquid state at atomic length scales. “There is still a lot of controversy as to how hydrogen bonding between water molecules evolves under pressure, so our study aimed to shed light on this question”, explains Christoph Sahle, scientist in charge of beamline ID20 and co-corresponding author of the publication.

The team combined X-ray Raman scattering spectroscopy at beamline ID20 of the ESRF with ab initio and path integral molecular dynamics simulations to study the local atomic and electronic structure of water under high pressure conditions. Previous studies, both experimental and theoretical, already described the collapse of the second hydration shell [Soper2000, Schwegler2000], however, it remained unclear if the created interstitial molecules form hydrogen bonds of their own or not.

In this research, the team observed spectroscopic fingerprints at the oxygen K-edge that clearly confirm the continuous collapse described in previous studies and are proof of the persistence of hydrogen bonding, even at the highest pressures. “Ultimately, the results  will help us scientists to better understand the behaviour of water when  compressed to pressures equivalent to 10000 atmospheres”, concludes Sahle.

Left - experimental oxygen K-edge spectra measured at ID20. Systematically increasing weight in the main-edge region is indicated by an arrow. Center – integrated difference spectrum with respect to the ambient pressure spectrum compared to the mean distance between two non-hydrogen-bonded nearest neighbors. Right – Snapshot from one of the molecular dynamics simulation trajectories showing a typical  pair of non-hydrogen-bonded nearest neighbours at high pressure.

Reference:

Förster, M. et al, PNAS, 16 September 2024. DOI:10.1073/pnas.2403662121

[Soper2000] A.K. Soper, M.A. Ricci, Structures of high-density and low-density water. Phys. Rev. Lett. 84, 2881 (2000).

[Schwegler2000] E Schwegler, G Galli, F Gygi, Water under pressure. Phys. Rev. Lett. 84, 2429 (2000).