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

Abstract

Liquid interfaces, such as gas/liquid, liquid/liquid, and solid/liquid interfaces, are ubiquitous and play important roles in chemistry. For chemical reactions at interfaces, the interfacial region at a ~1-nm depth is important because this thin interfacial region corresponds to the scale of the sizes of molecules. However, it is generally difficult to observe this very thin region of liquid interfaces by conventional methods. For example, photoelectron spectroscopy requires a vacuum to detect electrons ejected from sample surfaces; thus, it is not appropriate for liquid interfaces. X -ray scattering methods are generally used to study liquid interfaces. However, high brightness X-rays are prepared by synchrotrons, and experiments for radioactive species are difficult in such facilities. Vibrational sum frequency generation (VSFG) spectroscopy is one of the vibrational spectroscopic techniques besides FT-IR and Raman spectroscopy. VSFG spectroscopy is interface-specific and offers unique information on the molecular structure in the very thin interfacial region (~1 nm) of liquid interfaces. Although many interfacial studies by VSFG spectroscopy have been published thus far, application to lanthanides and actinides has been very limited because previous studies have paid considerable attention to interface chemistry relating to light elements, such as interface chemistry in the cell membrane. Some metal complexes have been observed at air/aqueous interfaces using VSFG spectroscopy; however, there have been no reports on actinides because special techniques and facilities for the management and treatment of actinides are required. Recently, we constructed an optical experimental setup for VSFG spectroscopy in a radiation management area in the Japan Atomic Energy Agency (JAEA), enabling us to study actinide chemistry by VSFG spectroscopy. In this paper, the focus is on liquid interfaces of solvent extraction of lanthanides and actinides studied using VSFG spectroscopy. In solvent extraction [Figure 1(a)], extractants are dissolved in an organic phase, and some extractant molecules come to the liquid/liquid interface and cover the interface because of the surface activity of the extractants. Metal ions in the aqueous phase come to the interface and form complexes with extractants (ligands) to subsequently transfer to the organic phase. However, it is unknown what occurs at the interface and how water and extractant molecules are bonded to metals at the interface to transfer into the organic phase. This is because of the experimental difficulty related to the organic/aqueous interface, and one reason for the difficulty is that metal complexes at the interface transfer into the organic phase after complex formation at the interface and are difficult to observe at the interface. Therefore, we trapped metal comLiquid interfaces related to lanthanide and actinide chemistry studied using vibrational sum frequency generation spectroscopy