Thermodynamic constant determination for the complexation of trivalent lanthanides with polyoxometalates

Polyoxometalates (POM), which are large, water-soluble metal oxides, can be considered as small fragments of metal lattices similar to the principle components on clay or inorganic colloid surfaces. Consequently, POMs can be used to model the thermodynamic aspects of metal–clay interactions to develop an understanding of clay colloids in the migration of metal cations within an environmental setting. The binding constants for the lacunary P2W17 and the plenary P2W18 POMs with trivalent lanthanide cations have been previously reported. In this study, the thermodynamic constants for the complexation of Ln(III) cations with these polyoxometalates have been measured using both enthalpy calorimetric titrations and the entropy titration method at 25 °C and an ionic strength of 1.0 M (NaClO4). The results indicate weak binding interaction for the plenary P2W18 species, but stronger binding to the lacunary P2W17 complexes with significant entropy effects for both POMs.