Effect of hydration of a room temperature ionic liquid on the cation–cation interaction of UO22+ and NpO2+ ions

Abstract

Though the literature on the unique cation–cation interaction (CCI) behaviour of actinyl ions is mainly dominated by Np(V) in solution, but it can also alter the structural motifs in solids and influence the chemical or redox properties of the involved actinyl ions in solutions. Although some of the recent studies deal with the extraction of Np(V) in a room temperature ionic liquid (RTIL) and dissolution of spent nuclear fuel containing both U(VI) and Np(V) in RTIL, no specific study is available for the behaviour of Np(V) and its CCI with other actinyls in hydrophobic ionic liquids. In the present work, we have studied the absorption spectroscopy of Np(V) and its CCI with U(VI) in a hydrophobic ionic liquid, i.e., C₄mim.NTf₂, at different hydration levels to understand the coordination environment of Np(V). The f–f transition band of the Np(V) ion was found to be blue-shifted at lower hydration levels of C₄mim.NTf₂. Decreasing strength of the CCI between Np(V) and U(VI) with increasing water activity in C₄mim.NTf₂ is shown using vis-NIR studies. The experimental observations were also rationalized using computational studies. The CCI between Np(V) and U(VI) was found to be exothermic in nature.