The effect of uranyl ion concentration on structure and dynamics of aqueous solutions of uranyl ions is investigated by molecular dynamics simulations. In order to get an idea about the effect of concentration of uranyl ions on local structural arrangements of water molecules around the uranyl ion, radial distribution functions of water molecules around the uranyl ion are analyzed for aqueous uranyl solutions of various concentrations. The concentration effect on translational dynamics has also been analyzed by calculating diffusion coefficients of uranyl ion, water, and nitrate ions in solution from their respective mean squared displacements. Mobility of water as well as uranyl ions has been found to decrease with increasing concentration of the uranyl ions. Orientational dynamics of water about different molecular axes of water have also been analyzed and decreasing orientational mobility of water with increasing uranyl concentration has been found. In order to get further insight into origin of slowing down of the translational mobility of water molecules with increasing uranyl ion concentration, two separate effects namely long-range effect of uranyl ions on the dynamics of water molecules beyond the solvation shell and short-range effect involving dynamics of solvation shell water have been analyzed. It is found that long-range effect is responsible for the slowing down of translational dynamics of water molecules in the presence of uranyl ions.
Investigating the characteristics of actinyl ions has been of great interest due to their direct relevance in the nuclear fuel cycle. All-atom molecular dynamics simulations have been employed to study the orientational structure and dynamics of aqueous solutions of uranyl ions of various concentrations. The orientational structure of water around a uranyl ion has been thoroughly investigated by calculating different orientational probability distributions corresponding to different molecular axes of water. The orientational distribution of water molecules in the first coordination shell of a uranyl ion is found to be markedly different from that in bulk water. Analysis of counterion distribution around the uranyl ion reveals the presence of nitrate ions along with water molecules in the first solvation shell. From the comparison of the number of coordinated water and nitrate ions at various uranyl nitrate concentrations, it is evident that these two species compete for occupying the first solvation shell of the uranyl ion. Orientational dynamics of water molecules about different molecular axes of water in the vicinity of uranyl ions have also been investigated and decreasing orientational mobility of water with increasing uranyl concentration has been found. However, it is observed that the orientational dynamics remains more or less the same whether we consider all the water molecules in the aqueous solution or only the solvation shell water molecules. The effect of temperature on the translational and orientational characteristics of the aqueous uranyl solutions has also been studied in detail.
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