Stoichiometry of Lanthanide–Phosphate Complexes at the Water Surface Studied Using Vibrational Sum Frequency Generation Spectroscopy and DFT Calculations

In the solvent extraction of metal ions, the transport mechanism of metal ions through the liquid–liquid organic/aqueous interface remains unclear. In this study, the adsorption process of trivalent lanthanide ions from the aqueous phase to the interface in the solvent extraction of lanthanides with di­(2-ethylhexyl)­phosphoric acid (HDEHP) extractant is investigated by using a model interfacewater surface covered with HDEHP (air/HDEHP/aqueous interface). As a result, symmetric POO^– stretch signals of HDEHP observed by vibrational sum frequency generation spectroscopy and density functional theory calculations show that the stoichiometric ratio of lanthanide–HDEHP complexes formed at the air/HDEHP/aqueous interface is 1:1. The formation of the interfacial 1:1 lanthanide–HDEHP complex could be an elementary chemical process occurring just before the transfer of lanthanide ions to the side of the organic phase.