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Actinyl ions can self-assemble in aqueous solution to form closed cage clusters ranging from 1.5 to 4.0 nm in diameter. The self-assembly, stability, and behavior of the nanoclusters depend on the nature of the aqueous environment, such as the pH and cations present. In this work, a classical force field for [(UO2)20(O2)30]20- (U20) peroxide nanoclusters in aqueous solution was developed from quantum-mechanical calculations. Using molecular dynamics simulations, the preferred binding sites of six cations (Li+, Na+, K+, Rb+, Cs+, and Ca2+) to the nanocluster were determined. Replica exchange molecular dynamics was used to equilibrate the structure and determine the equilibrium distribution of cations and water with respect to the nanocluster cage. In addition, the free energy barriers associated with cations entering the cluster were computed. Finally, the association of two cages was investigated by computing the free energy as a function of intercage distance. The free energy profiles reveal that the nanoclusters prefer to be associated when neutralized with divalent cations, but do not associate when neutralized with monovalent cations. This could explain the formation of tertiary structures observed experimentally.

Actinyl ions (AnO2n+), the form in which actinides are commonly found in aqueous solution, are important species in the nuclear fuel cycle. These ions can form stable complexes with ionic ligands such as OH-, NO3-, Cl-, and even other actinyl ions in the aqueous phase. Knowledge of the relative stabilities of these complexes is important for the efficient design of separation processes used in recycling. These complexes also play a major role in the formation of actinide nanoclusters. A quantitative treatment of the stability of these actinyl ion complexes is therefore warranted. In the present work, molecular dynamics (MD) simulations have been performed to calculate the potential of mean force (PMF) between two actinyl ions (UO22+ and NpO2+) and various ligands (F-, Cl-, OH-, NO3-, SO42-, CO32-, Na+, and H2O) in explicitly-modeled aqueous solution. Equilibrium constants were calculated from the PMFs, and are consistent with experimental trends. Dication actinyls show stronger affinity for anions compared to monocation actinyls, whereas the opposite is true for cation-cation interactions between actinyls. Finally, the dynamics of actinyl-ligand contact ion pair (CIP) dissociation were characterized by calculating rate constants from transition state theory. The transmission coefficient, a dynamical correction factor used to correct for reaction barrier recrossing, was calculated for each actinyl-ligand CIP dissociation event.

The dynamics of actinyl ions (AnO2(n+)) in aqueous solutions is important not only for the design of advanced separation processes but also for understanding the fate of actinides in the environment. The hazardous nature of actinides makes it difficult to measure transport and thermodynamic properties experimentally, so predictive simulations are an attractive method for studying these systems. Here, we report the results of atomistic-level molecular dynamics simulations of actinyl ions (of U, Np, Pu, and Am) in their mono- and dication states in aqueous solution. Quantum mechanically derived force field parameters are used to compute self-diffusion coefficients of the actinyl ions, water exchange mechanisms, and residence times of water molecules in the first solvation shell of the actinyl ions. We find that monocation actinyl ions diffuse slightly faster than their dication counterparts. Our simulations suggest that there are two distinct water exchange mechanisms for mono and dications. An associative interchange pathway is observed for water exchange involving dication actinyls, while in monocation actinyls the exchange occurs via a dissociative mechanism. The residence time of water molecules in the first solvation shell depends on the water exchange mechanism. In the case of dications, a stiffer actinyl bond angle results in a longer residence time, while for monocations, a shorter water coordination distance leads to a longer residence time. The simulations predict much faster water exchange for UO2(2+) than what is observed experimentally with NMR, but other properties are consistent with experiments.

Intra- and intermolecular force field parameters for the interaction of actinyl ions (AnO2(n+), where, An = U, Np, Pu, Am and n = 1, 2) with water have been developed using quantum mechanical calculations. Water was modeled with the extended simple point charge potential (SPC/E). The resulting force field consists of a simple form in which intermolecular interactions are modeled with pairwise Lennard-Jones functions plus partial charge terms. Intramolecular bond stretching and angle bending are treated with harmonic functions. The new potentials were used to carry out extensive molecular dynamics simulations for each hydrated ion. Computed bond lengths, bond angles and coordination numbers agree well with known values and previous simulations. Hydration free energies, computed from molecular dynamics simulations as well as from quantum simulations with a solvation model, were in reasonable agreement with estimated experimental values.