Unravelling the hydration structure of ThX₄ (X = Br, Cl) water solutions by molecular dynamics simulations and X-ray absorption spectroscopy

The hydration of Th(IV) in ThCl₄ and ThBr₄ water solutions at different salt concentrations was studied in order to understand the structure of Th(IV) in liquid water and the effect of Br^- and Cl^- anions on its hydration structure. Several theoretical methods were employed: density functional theory and classical molecular dynamics based on both semiempirical polarizable potentials and ab initio derived polarizable potentials. The results of the computations were combined with extended X-ray absorption fine structure (EXAFS) experimental data. The results of this study show that in pure water the Th-O distance of 2.45 Å corresponds to a first shell coordination number between 9 and 10. In the salt solutions, while Br^- does not affect directly the hydration of Th(IV) also at relatively high concentrations, Cl^-, on the other hand, is more structured around Th(IV), in agreement with recent high-energy X-ray scattering experiments. Counterions, even at relatively high concentrations (0.8 m), do not enter in the first solvation shell of Th(IV), but they induce an increase of water molecules in the first and second hydration shells of Th(IV).