Liquid interfaces related to lanthanide and actinide chemistry studied using vibrational sum frequency generation spectroscopy

Abstract

Vibrational sum frequency generation (VSFG) spectroscopy offers unique information about the molecular structure at liquid interfaces (gas/liquid, liquid/liquid, and solid/liquid) in the interfacial region, which is as thin as 1 nm. This paper presents some recent VSFG studies on liquid interfaces related to solvent extractions of lanthanide and actinide. In these studies, the organic phase of solvent extractions was excluded to prevent the transfer of lanthanide and actinide from the interface into the organic phase, and their complexes with extractants were attempted to observe at the air/aqueous solution interfaces using VSFG spectroscopy. For the Eu extraction system with the di-(2-ethylhexyl)phosphate (HDEHP) extractant, an interfacial Eu-HDEHP complex was observed, and Eu³⁺ existed between HDEHP and water molecules, whereas for U extraction with the tributyl phosphate (TBP) extractant, no U-TBP interfacial complexes were observed. These results suggest that the two ions transfer into the organic phase from the aqueous phase via different phase transfer mechanisms. This paper demonstrates that VSFG spectroscopy is a powerful technique to obtain unique information about liquid interfaces pertaining to lanthanide and actinide chemistry.