On the solvation properties of supercritical electrolyte solutions

A series of hydrothermal diamond anvil cell experiments was conducted to constrain the effect of anions (Cl-, F-) and cations (Mg+2, Na+) on the molecular structure of high temperature/-pressure fluids, and to express the extent of hydration as a function of solute's speciation and concentration at temperatures ranging from 400 to 800 degrees C and pressures of 1.7 to 30.1 kbar. Results reveal that hydrogen-bonding environments for H2O molecules in the first hydration shell of anions evolve from pure O-H center dot center dot center dot O to dynamic mixtures with O-H center dot center dot center dot X- (F-, Cl-). The distribution of the "O-H center dot center dot center dot X-" structures (O-H center dot center dot center dot O, O-H center dot center dot center dot X-) is proportional to the concentration of dissolved anions; strongly dependent, however, on ionic speciation. To this end, the O-H center dot center dot center dot O structures inside the hydration shell of F- are weaker than the O-H center dot center dot center dot Cl- and O-H center dot center dot center dot O in Cl- bearing solutions. On the contrary, cations (Na+, Mg2+) appear to impose a minimal effect on the Raman spectra of the O-H center dot center dot center dot O structures. The thermodynamic stability of "O-H center dot center dot center dot Cl-" environments shows a strong correlation with the activity of H2O, indicative of the localized effect of the C-l electric field that decouples the hydration from the bulk water molecules. The enthalpy change required to rupture these H-bonding environments (Delta HO-H center dot center dot center dot Cl-) provides a measure for the extent of ion hydration and constrains the solvation properties of supercritical electrolyte solutions. For example, the dominant presence of "O-H center dot center dot center dot Cl-" environments lowers the concentration of isolated water dipoles and contributes to the decrease in the dielectric constant of supercritical electrolyte solutions, which in turn, could suppress the solubility of hydrophobic non-polar aqueous species, such as SiO2(aq). (C) 2016 Elsevier B.V. All rights reserved.