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

Stability constants (log beta(101)) of Th4+, UO22+, NpO2+ and Am3+ with [NaP5W30O110](14-) were determined by solvent extraction (mu = 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+>UO22+>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+>UO22+>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+, UO22+, 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.