Modelling of concentrated aqueous solutions of f-elements from a multi-scale approach

Classical molecular dynamics simulations with explicit polarization have been successfully used to determine the structural and thermodynamic properties of binary aqueous solutions of uranyl chloride, perchlorate and nitrate (UO₂Cl₂, UO₂(ClO₄)₂ and UO₂(NO₃)₂). From a study performed as a function of the concentration of salts, the hydration properties of solutes and the ion-ion interactions of concentrated aqueous solutions of uranyl chloride, perchlorate and nitrate have been studied. The molecular dynamics simulations allow for reproducing the solvation properties of the hydrated uranyl, chloride, perchlorate and nitrate in good agreement with the experimental data. The results point out different coordination modes of Cl^- and ClO₄^- in the UO₂²⁺ second hydration shell and the presence of NO₃^- anion in the UO₂²⁺ first coordination shell at high concentration. The ion-ion interaction properties over water configurations have been studied at different concentrations. Furthermore, we investigated the potential of mean force of ion pairs at infinite dilution as a function of the distance and the angle. The thermodynamical properties of the solutions have been calculated from these effective ion-ion pair potentials thanks to the MacMillan-Mayer theory and molecular theory. The association constant of complex UO₂Cl⁺ (Kcal UO₂Cl⁺ =2,52 L mol^-1), UO₂ClO₄⁺ (Kcal UO₂ClO₄⁺ =2,34 L mol^-1) and UO₂NO₃⁺ (Kcal UO₂NO₃⁺ =3,02 L mol^-1) has been determined and compared to the experimental ones. From the effective McMillan-Mayer potentials and using a multi-scale approach based on the MSA approximation, we also calculated the osmotic coefficients. (author)