More than 60 unique uranyl peroxide cage clusters have been reported that contain as many as 124 uranyl ions and that have overall diameters extending to 4 nm. They self-assemble in water under ambient conditions, are models for understanding structure-size-property relations as well as testing computational models for actinides, and have potential applications in nuclear fuel cycles. High-temperature drop solution calorimetry has been used to derive the enthalpies of formation of the salts of seven topologically diverse uranyl peroxide cage clusters containing from 22 to 28 uranyl ions that are bridged by various combinations of peroxide, pyrophosphate, and phosphite. The enthalpies of formation of these seven salts, as well as three salts of other uranyl peroxide clusters reported earlier, are dominated by the interactions of the alkali countercations with the clusters. There is an approximately linear relationship between the enthalpies of formation of the cluster salts and the charge density of the corresponding uranyl peroxide cluster, wherein salts containing clusters with higher charge densities have more negative enthalpies of formation.
Zippeite is a potassium uranyl sulfate mineral that forms on uranium mine wastes and that may be important in nuclear waste disposal. Little is known about its thermodynamic properties. In this study, we synthesized zippeite, K 3 (H 2 O)[(UO 2) 4 (SO 4) 2 O 3 (OH)], and measured its thermodynamic properties. The standard enthalpy of formation of zippeite from the elements, − 8655.97±12.55 kJ/mol, was calculated from the measured enthalpy of dissolution in a hightemperature oxide melt solution calorimeter. Solubility measurements were conducted from both undersaturation and supersaturation in order to constrain the equilibrium state. Using the solubility product (K sp) obtained from the solubility data, the standard Gibbs free energy of formation of zippeite at 297 K was calculated to be − 7783.44±6.87 kJ/mol. This value, coupled with the standard enthalpy of formation value determined from the calorimetric data, was then used to calculate the standard entropy of formation, − 2926.49±45.64 J/mol. Our studies yield the first complete set of thermodynamic properties of zippeite, and the results allow us to predict the conditions under which its formation is favorable relative to other uranyl minerals under a wide range of conditions of geologic and engineering interest.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.