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 °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 OH ⋯ O to dynamic mixtures with OH ⋯ X− (F−, Cl−). The distribution of the “OH ⋯ X−” structures (OH ⋯ O, OH ⋯ X−) is proportional to the concentration of dissolved anions; strongly dependent, however, on ionic speciation. To this end, the OH ⋯ O structures inside the hydration shell of F− are weaker than the OH ⋯ Cl− and OH ⋯ O in Cl− bearing solutions. On the contrary, cations (Na+, Mg2 +) appear to impose a minimal effect on the Raman spectra of the OH ⋯ O structures. The thermodynamic stability of “OH ⋯ Cl−” environments shows a strong correlation with the activity of H2O, indicative of the localized effect of the Cl− electric field that decouples the hydration from the bulk water molecules. The enthalpy change required to rupture these H-bonding environments (ΔHOH ⋯ 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 “OH ⋯ 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).