Can different counter ions and their concentration modify the structural characteristics of aqueous solutions of uranyl ions? Atomistic insights from molecular dynamics simulations

Abstract

Extensive atomistic molecular dynamics simulation results suggest that probability of counter ion occupying first solvation shell of uranyl ion depends both on its nature and concentration. In general, uranyl-counter ion complexes with pentagonal bi-pyramidal structure with five ligands in the equatorial plane of the linear UO₂²⁺ ion are observed. In case of nitrate ion, pure aqua complexes at lower concentrations and mixed mono-nitro aqua complexes are observed at higher concentrations, whereas in case of sulphate and carbonate ions, no pure aqua complexes are observed. The NO₃⁻ and SO₄²⁻ ions act as unidentate, but CO₃²⁻ acts both as uni- and bi-dentate ligand. In addition, polynuclear uranium complexes with bridging SO₄²⁻ and CO₃²⁻ ligands are observed. Relative strength of binding of counter ions with uranyl ion from PMF calculations follows the order CO₃²⁻ > SO₄²⁻ > NO₃⁻ with the contact pair free energies of about −29.0, −14.0 and −1.1 kcal/mol respectively.