Interaction of actinide ions with [NaP5W30O110]14- and [P2W18O62]6-

Stability constants (log b₁₀₁) of Th⁴⁺, UO₂ ²⁺, NpO₂ ⁺ and Am³⁺ with NaP₅W₃₀O₁₁₀]^14- were determined by solvent extraction (m = 0.1M NaCl) and found to be 6.18±0.07, 3.80±0.06, 2.98±0.04, and 5.85±0.05, respectively. The order of stability constants: Th⁴⁺>Am³⁺>UO₂ ²⁺>NpO₂ ⁺ is due to electrostatic repulsion between the actinyl oxygens and oxygens on the polyoxometalate surface. The order of stability constants for metal complexes with P₂W₁₈O₆₂]^6- is Th⁴⁺>UO₂ ²⁺>Eu³⁺>NpO₂ ⁺ because the steric repulsion between actinyl oxygens and oxygens on polyoxometalate are less important. Enthalpies of complexation were measured by calorimetric titration of Th⁴⁺, UO₂ ²⁺, Nd³⁺ with NaP₅W₃₀O₁₁₀]^14- and P₂W₁₈O₆₂]^6-. The results indicate that the conformation and charge distribution of the microscopic surface structures are important factors in the formation of pseudocolloids.