Effect of temperature on the complexation of NpO2+ with benzoic acid: Spectrophotometric and calorimetric studies

Abstract

The equilibrium constants of the 1:1 NpO2+/benzoate complex were determined by spectrophotometric titrations at variable temperatures (T=283 to 343K) and the ionic strength of 1.05mol·kg−1. The enthalpy of complexation at T=298K was determined by microcalorimetric titrations. Similar to other monocarboxylates, benzoate forms a weak complex with NpO2+ and the complexation is strengthened as the temperature is increased. The complexation is endothermic and is entropy-driven. The enhancement of the complexation at elevated temperatures is primarily attributed to the increasingly larger entropy gain when the water molecules are released from the highly-ordered solvation spheres of NpO2+ and benzoate to the bulk solvent where the degree of disorder is higher at higher temperatures. The spectroscopic features of the Np(V)/benzoate system, including the effect of temperature on the absorption bands, are discussed in terms of ligand field splitting and a thermal expansion mechanism.