Barium and Radium Complexation with Ethylenediaminetetraacetic Acid in Aqueous Alkaline Sodium Chloride Media

The speciation of Ra²⁺ and Ba²⁺ with EDTA was investigated at 25 °C in aqueous alkaline NaCl media as a function of ionic strength (0.2–2.5 mol·L⁻¹) in two pH regions where the EDTA⁴⁻ and HEDTA³⁻ species dominate. The stability constants for the formation of the [BaEDTA]²⁻ and [RaEDTA]²⁻ complexes were determined using an ion exchange method. Barium-133 and radium-226 were used as radiotracers and their concentrations in the aqueous phase were measured using liquid scintillation counting and gamma spectrometry, respectively. The specific ion interaction theory (SIT) was used to account for [NaEDTA]³⁻ and [NaHEDTA]²⁻ complex formation, and used to extrapolate the logarithms of the apparent stability constants (log₁₀K) to zero ionic strength (BaEDTA²⁻: 9.86 ± 0.09; RaEDTA²⁻: 9.13 ± 0.07) and obtain the Ba²⁺ and Ra²⁺ ion interaction parameters: [ε(Na⁺, BaEDTA²⁻) = − (0.03 ± 0.11); ε(Na⁺, RaEDTA²⁻) = − (0.10 ± 0.11)]. It was found that in the pH region where HEDTA³⁻ dominates, the reaction of Ba²⁺ or Ra²⁺ with the HEDTA³⁻ ligand also results in the formation of the BaEDTA²⁻ and RaEDTA²⁻ complexes (as it does in the region where the EDTA⁴⁻ ligand dominates) with the release of a proton. Comparison of the ion interaction parameters of Ba²⁺ and Ra²⁺ strongly indicates that both metal ions and their EDTA complexes have similar activity coefficients and undergo similar short-range interactions in aqueous NaCl media.

MD simulation study of the diffusion and local structure of n-alkanes in liquid and supercritical methanol at infinite dilution

The diffusion coefficients of 14 n-alkanes (ranging from methane to n-tetradecane) in liquid and supercritical methanol at infinite dilution (at a pressure of 10.5 MPa and at temperatures of 299 K and 515 K) were deduced via molecular dynamics simulations. Values for the radial distribution function, coordination number, and number of hydrogen bonds were then calculated to explore the local structure of each fluid. The flexibility of the n-alkane (as characterized by the computed dihedral distribution, end-to-end distance, and radius of gyration) was found to be a major influence and hydrogen bonding to be a minor influence on the local structure. Hydrogen bonding reduces the flexibility of the n-alkane, whereas increasing the temperature enhances its flexibility, with temperature having a greater effect than hydrogen bonding on flexibility. Graphical abstract The flexibility of the alkane is a major influence and the hydrogen bonding is a minor influence on the first solvation shell; the coordination numbers of long-chain n-alkanes in the first solvation shell are rather low.