Hydration of Heavy Alkaline-Earth Cations Studied by Molecular Dynamics Simulations and X-ray Absorption Spectroscopy

The physicochemical properties of the three heaviest alkaline-earth cations, Sr²⁺, Ba²⁺, and Ra²⁺ in water have been studied by means of classical molecular dynamics (MD) simulations. A specific set of cation-water intermolecular potentials based on ab initio potential energy surfaces has been built on the basis of the hydrated ion concept. The polarizable and flexible model of water MCDHO2 was adopted. The theoretical-experimental comparison of structural, dynamical, energetic, and spectroscopical properties of Sr²⁺ and Ba²⁺ aqueous solutions is satisfactory, which supports the methodology developed. This good behavior allows a reasonable reliability for the predicted Ra²⁺ physicochemical data not experimentally determined yet. Simulated extended X-ray absorption fine-structure (EXAFS) and X-ray absorption near-edge spectroscopy spectra have been computed from the snapshots of the MD simulations and compared with the experimental information available for Sr²⁺ and Ba²⁺. For the Ra²⁺ case, the Ra L₃-edge EXAFS spectrum is proposed. Structural and dynamical properties of the aqua ions for the three cations have been obtained and analyzed. Along the [M(H₂O)_n]^m+ series, the M-O distance for the first-hydration shell is 2.57, 2.81, and 2.93 Å for Sr²⁺, Ba²⁺, and Ra²⁺, respectively. The hydration number also increases when one is going down along the group: 8.1, 9.4, and 9.8 for Sr²⁺, Ba²⁺, and Ra²⁺, respectively. Whereas [Sr(H₂O)₈]²⁺ is a typical aqua ion with a well-defined structure, the Ba²⁺ and Ra²⁺ hydration provides a picture exhibiting an average between the ennea- and the deca-hydration. These results show a similar chemical behavior of Ba²⁺ and Ra²⁺ aqueous solutions and support experimental studies on the removal of Ra-226 of aquifers by different techniques, where Ra²⁺ is replaced by Ba²⁺. A comparison of the heavy alkaline ions, Rb⁺ and Cs⁺, with the heavy alkaline-earth ions is made.