Abstract
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.
Highlights
The solution chemistry of alkaline-earth cations is extremely wide, being involved in a huge number of domains of chemistry, biology, geology, and chemical engineering.[1−3] Whereas the lighter alkaline-earth cations, Mg2+ and Ca2+, are ubiquitous in many systems and natural environments, having been deeply studied by experimental and theoretical techniques, their heavy alkaline-earth group companions are much less disseminated and their solution chemistry is more limited, that of Ra2+
This section has been split into three parts, the first of them being devoted to the building of the cation−water intermolecular potentials based on the hydrated ion model proposed by our group.[26−28] The second part gives the QM and molecular dynamics (MD) computational details
Our original statistical implementation of the concept was based on the development of a hydrated ion−water interaction potential ([M(H2O)n]m+−H2O) for stable aqua ions based on first-principles.[26,28]
Summary
The solution chemistry of alkaline-earth cations is extremely wide, being involved in a huge number of domains of chemistry, biology, geology, and chemical engineering.[1−3] Whereas the lighter alkaline-earth cations, Mg2+ and Ca2+, are ubiquitous in many systems and natural environments, having been deeply studied by experimental and theoretical techniques, their heavy alkaline-earth group companions are much less disseminated and their solution chemistry is more limited, that of Ra2+. There are not experimental studies providing the hydration number nor the average Ra−O(H2O) distance of the aqua ion These shortcomings stimulate the use of theoretical tools to undertake the estimation of physicochemical properties hardly accessible by experiments.[5,6]. Two precedent alkaline-earth cations, Sr2+ and Ba2+, are good candidates to be investigated together with Ra2+ in order to analyze the evolution of properties along the group in a systematic way, starting from a regular increase of the ionic radius. For these two ions, there is enough experimental chemical information to validate the theoretical results
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