The microscopic structure of water under extreme conditions

Abstract

Liquid water is one of the key components of life on this planet. Moreover, its unique properties and anomalies make it highly relevant for several processes in physics, biology and chemistry. The fascinating characteristics and anomalies are by no means restricted to liquid water at ambient conditions. Thus, pressurised water at high temperatures, so-called hydrothermal water, is essential for heat and mass transfer processes in Earth’s upper mantle and exerts a significant influence on geochemical processes. For instance, hydrothermal water is involved in the formation of ore deposits due to its unique dissolving properties under hydrothermal conditions. However, the transport of metal often requires an additional dissolved salt which can form a metal-bearing complex and thus enables the transport. In this context, NaCl is one of the most abundant dissolved salts in natural hydrothermal fluids. Liquid water is also of major importance when exposed to high pressures and temperatures far below 100 °C. Under these conditions, it provides a habitat for marine life in the deep sea, for instance, or is used in industrial processes. Furthermore, by changing the applied pressure, structural changes of the water structure can be induced in a controlled manner, which offers a promising opportunity to test various proposed models for liquid water. These unique properties of water are directly related to the internal arrangement of the hydrogen bond network, whose exact structure is still subject to debate. Therefore, this thesis focuses on the investigation of the local microscopic structure of water and aqueous sodium chloride solution under extreme thermodynamic conditions. For this purpose, X-ray Raman scattering spectroscopy is primarily exploited which is a powerful tool for studying absorption edges under extreme conditions. The correlation of the oxygen K-edge spectrum of water with the local arrangement of the water molecules is utilized and combined with spectral calculations based on various structural models to resolve the changes in the water network over a wide range of temperatures and pressures. This way, the exact influence of the sodium chloride ions on the water structure at hydrothermal conditions is revealed, as well as the microscopic structural changes around the anomaly at 3 kbar.