Thermodynamic properties of some tri- and tetravalent actinide aquo ions

Abstract

The hydration model that we proposed earlier has been extended to aquo species with covalent bonds. It has been applied successfully to the majority of monatomic ions with charges from −1 to +4. An evaluation of the permittivity in the vicinity of the ion, dipole and quadrupole moments and polarisability of the solvated water molecule is presented. In the case of trivalent and tetravalent actinides, the experimental cation–oxygen distance d, accurately measured by EXAFS, is shorter at the beginning of the series than the calculated values obtained in the ionic model. The observed decrease has been used to evaluate the effective charges, Z_eff, of actinides. We discuss the evaluation of the ionic radius R_i, the coordination number N and the number of water molecules in a second hydration shell H. After defining the different parameters, which characterize the aquo ions, mainly, Z_eff, R_i, N, H and d, the general hydration and entropy models is applied to evaluate the hydration energy and the entropy of different trivalent and tetravalent actinide aquo ions. We will also re-evaluate and discuss redox potentials corresponding to some 4+/3+ couples. Finally, we will define the radius of the aquo ions and deduce the ionic conductivity of trivalent and tetravalent actinides.