The Development of New Calorimetric Methods Aid the Understanding of Uranyl Peroxide Nanocluster Solution Behavior

Herein, we developed a calorimetric method to obtain new thermodynamic values for polyoxometalates (POMs) that helped elucidate their thermodynamic landscape. The studies were primarily conducted on uranyl peroxide nanoclusters (UPCs). In the 16 years since the discovery of UPCs, over 60 have been reported, but their assembly and solution behavior is still perplexing. Thermodynamics can provide a quantitative analysis to predict and study UPC stability and solubility, and chemical processes in solution.

In total, the energy landscapes for 10 UPCs, two Mo POMs, and one uranyl peroxide monomer (building block of UPCs) were analyzed by obtaining values for enthalpy of formation of POM crystals (ΔH_f,(c)), enthalpy of formation of aqueous POMs (ΔH_f,(aq)), and enthalpy of dissolution (ΔH_diss). The results allowed us to predict the conversion of LiNa-U₂₄Pp₁₂ to LiNa-U₂₄ and Li-U₂₈ to LiCa-U₂₄ and elucidate their transformations; and the enthalpy of conversion from lithium uranyl triperoxide monomer to U₂₄ and U₂₈ was determined to be negative, which is in agreement with literature observations. Additionally, our results reveal that ΔH_diss is highly dependent on the entity of the counter cations and their affinities with the topological features of the UPCs, and a complex but linear relationship between ΔH_diss and the cluster solubility exists.