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Ion-pairing and dehydration phenomena occurring in ZnBr2 aqueous solutions from normal to critical T, P conditions were investigated by x-ray absorption spectroscopy. The respective influences of temperature, pressure, and concentration were studied. The evolution of the density of solute ions, probed by the height of the absorption edge, allowed us to get information on phase diagrams and salt precipitation. The average structural evolution deduced from extended x-ray absorption fine structure was related to the formation of complexes identified from x-ray absorption near edge structure analysis. Consequently, in noncritical conditions, an increase of temperature or concentration produces dehydration and ion-pairing, while a rise of pressure destroys the ion-pairs. In contrast, concentration and pressure have weaker effects on the local order in high P, T conditions. Moreover, ion pairing formation is found not to be specifically enhanced when the fluid is close to supercritical conditions as it also occ...

Local-order evolution around ions in aqueous solutions has been investigated between normal and hydrothermal conditions. The behavior of cations and anions in aqueous ZnBr2 solution were studied by performing x-ray absorption spectroscopy experiments at both Br and Zn edges. Extended x-ray absorption fine structure analyses are made on account of anharmonic treatment and multiple scattering contributions involving H atoms at the Br edge. The extended x-ray absorption fine structure results are coupled to x-ray absorption near-edge structure simulations in order to identify the complexes formed under normal and hydrothermal conditions. It appears that both Zn and Br ions are largely hydrated under normal conditions and that Zn–Br pairs are formed in hydrothermal conditions. This is related to an octahedral-to-tetrahedral evolution of the Zn local environment, the majority of Zn atoms being surrounded by water octahedra in normal conditions and by distorted tetrahedra involving Br and O atoms in hydrotherma...