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High resolution X-ray spectroscopy on hydrothermal fluids

Abstract : Knowledge of the properties of hydrothermal fluids, i.e. aqueous solutions at high temperature (T) and high pressure (P), are essential in the fields of earth sciences, prebiotic chemistry, nuclear industry and environmental sciences. Near the critical point (Pc, Tc) and under the pseudocritic conditions (i.e. at the critical density at T-P above Tc-Pc), the fluid properties change radically. These changes take place at different scales: the macroscopic scale (density and compressibility changes), the mesoscopic scale (agglomeration process between different clusters) and finally, the molecular scale (local organization of atoms and molecules, for example hydrogen bonds, H).This study focuses on the solvation effects in different electrolytes as a function of temperature, pressure and concentration. The experimental study was carried out by X-ray absorption measurements, the technique suitable for probing both fluid density evolution and local atomic environment around the solutes. This study was carried out on pure water and salt solutions at different pressures (up to ~ 1.3Pc) and temperatures (up to ~ 2Tc), to pass from the liquid to the supercritical domain, using a cell permitting to completely decouple the effects of pressure and temperature.We have obtained experimental proofs of the displacement of the critical point and the isochore and their dependence on the salt concentration in the case of NaCl (0.3, 0.5 and 1.0 moles NaCl per kilogram of water). We have also observed a density anomaly in the supercritical region (SC) and the appearance of liquid-vapor phase separation for some aqueous solutions. The relative density increase in the critical zone is more pronounced for Cs>Rb>K>Na>Li for bromides and chlorides. The structural change in this region was followed by high-resolution XANES spectroscopy at the K-threshold of bromine for various alkali bromide. The interpretation of the XANES spectra clearly indicates a drastic changes in the fluid structure related to this anomaly, which can be interpreted by the appearance of ionic pairs.These new observations are in link with water structure and solvation properties evaluation and consequently with hydrogen bonding changes under high T-P.
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  • HAL Id : tel-01743820, version 1

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Mohammed Irar. High resolution X-ray spectroscopy on hydrothermal fluids. Fluid Dynamics [physics.flu-dyn]. Université Grenoble Alpes, 2017. English. ⟨NNT : 2017GREAY060⟩. ⟨tel-01743820⟩

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