Structure, Kinetics, and Thermodynamics of the Aqueous Uranyl(VI) Cation

In this work, molecular simulation techniques were employed to gain insight into the structural, kinetic, and thermodynamic properties of the uranyl(VI) cation (UO2(2+)) in aqueous solution. The simulations made use of an atomistic potential model (force field) derived in this work and based on the model of Guilbaud and Wipff [J. Mol. Struct. (THEOCHEM) 1996 , 366 , 55 - 63]. Reactive flux and thermodynamic integration calculations show that the derived potential model yields predictions for the water exchange rate and free energy of hydration, respectively, that are in agreement with experimental data. The water binding energies, hydration shell structure, and self-diffusion coefficient were also calculated and analyzed. Finally, a combination of metadynamics and transition path sampling simulations was employed to probe the mechanisms of water exchange reactions in the first hydration shell of the uranyl ion. These atomistic simulations indicate, based on two-dimensional free energy surfaces, that water exchanges follow an associative interchange mechanism. The nature and structure of the water exchange transition states were also determined. The improved potential model is expected to lead to more accurate predictions of uranyl adsorption energies at mineral surfaces using potential-based molecular dynamics simulations.