Journal of Nuclear and Radiochemical Sciences, Vol. 20, pp. 28-31, 2020

ArticlesLiquid interfaces related to lanthanide and actinide chemistry studied using vibrationalsum frequency generation spectroscopy
Ryoji Kusaka*
Nuclear Science and Engineering Center, Japan Atomic Energy Agency (JAEA), 2-4 Shirakata, Tokai, Ibaraki
319-1195, Japan
Received March 11, 2020; Accepted June 8, 2020; Published online June 24, 2020
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 1nm. 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 transferof lanthanide and actinide from the interface into the organic phase, and their complexes with extractants wereattempted 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 Eu 3+ 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.

1. Introduction
Liquid interfaces, such as gas/liquid, liquid/liquid, andsolid/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 thininterfacial region corresponds to the scale of the sizes of molecules. However, it is generally difficult to observe this verythin region of liquid interfaces by conventional methods. For
example, photoelectron spectroscopy requires a vacuum todetect 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, highbrightness 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 liquidinterfaces.1-3 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 thecell membrane. Some metal complexes have been observed at
air/aqueous interfaces using VSFG spectroscopy;4-7 however,there have been no reports on actinides because special techniques and facilities for the management and treatment ofactinides are required. Recently, we constructed an opticalexperimental 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
*Corresponding author. Email: kusaka.ryoji@jaea.go.jp

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 tothe organic phase. However, it is unknown what occurs at theinterface 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 theorganic/aqueous interface, and one reason for the difficulty isthat 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 com-Figure 1. (a) A general model of solvent extraction, and (b) a sketchof an air/aqueous-extractant interface studied by VSFG spectroscopy.

© 2020 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/24/2020

29J. Nucl. Radiochem. Sci., Vol. 20, 2020

plexes at the air/aqueous interface after the adsorption of metal
ions to the interface [Figure 1(b)], and the complexes formed at
the interface were observed by VSFG spectroscopy.8,9 Whensurface-active molecules cover interfaces, as shown in Figure
1, the air/aqueous interface and organic/aqueous interface have
similar molecular structures10 so that the air/aqueous interface
is an appropriate model for the organic/aqueous interface tostudy the molecular structure of interfacial complexes.
2. Experimental
2.1. Materials and samples. High-purity water (18.2MΩ∙cm resistivity, Millipore, Milli-Q), di-(2-ethylhexyl)phosph at e ( H DEH P, ~10 0% , Tok yo C he m ical I ndu st r y),europium(III) nitrate hexahydrate (Eu(NO3)3·6H2O, >99.95%,
Kanto Chemical), tri-n-butyl phosphate (TBP, >98.0%, KantoChemical), nitric acid (1.42) (for Ultratrace Analysis, Wako),and uranyl nitrate were used to prepare the sample aqueoussolutions. Uranyl nitrate was obtained by evaporating theaqueous solution dissolving uranium metal using the nitricacid. For the study of the solvent extraction of europium by
the HDEHP extractant, the sample aqueous solutions dissolveda small amount of HDEHP and europium nitrate. The sample
solutions were prepared by mixing an aqueous solution saturated with HDEHP and an aqueous solution to dissolve theeuropium nitrate in a 1:3 volume ratio, and the Eu3+ concentration of the solution was 10 mM. For the study of the solvent
extraction of uranium by the TBP extractant, the concentrations of TBP and nitric acid in the sample solutions were 10 −4
M and 3 M, respectively. The concentration of uranyl nitratein the samples was 0.9 M.
2.2. Vibrational sum frequency generation (VSFG) spectroscopy. The experimental setup was described previously.8,9Briefly, a Ti:Sapphire regenerative amplifier (795 nm, 5.7 mJ,95 fs, 1 kHz) was used as the light source. One part of the output was spectrally narrowed by a bandpass filter (center wavelength: 794.7 nm, bandwidth: 1.5 nm) and used as the ω1 light.
The other part of the output was converted to a tunable andbroadband infrared beam by an optical parametric amplifier(TOPAS-prime) with a difference frequency generation unitand used as the ω2 light. The ω1 and ω2 beams were focused
onto the sample surface of the aqueous solution to generate
sum frequency light (ω3) from the sample surface. When thetwo pulses were spatially and temporally overlapped at theinterface, sum frequency light was generated from the interface through the second-order nonlinear optical effect. Under
electric dipole approximation, sum frequency light is only
generated from anisotropic interfaces where the inversion
symmetry is broken and is not generated from isotropic media(i.e., bulk). The interfacial region where sum frequency generation occurs is typically considered to be ~1 nm in depth sothat interface-specific information can be obtained by detecting the sum frequency light. The incident angles and thepolarization directions were ~50° and s for ω1 and ~60° and pfor ω2, respectively. The s-polarized component of ω3 was isolated using a Glan–Taylor prism, introduced into a polychromator, and detected using a charge-coupled device. For theVSFG spectroscopy with the heterodyne detection mode for
the Eu-HDEHP study,8 the ω1 and ω2 beams were focused intoa thin y-cut quartz crystal (10-mm thickness) before the sample to generate sum frequency light (ωLO). The ωLO light waspassed through a glass plate to delay ωLO with respect to the ω1and ω2 pulses, and the ω3 and ωLO lights were introduced to apolychromator and detected by a CCD. The y-cut quartz crystal absorbs light of a wavenumber less than ~1300 cm−1; thus,
heterodyne detection cannot be performed in the i.e., region of
the P=O stretch of TBP at approximately 1250 cm−1 in the current experimental setup; thus, the P=O stretch of TBP wasKusaka
measured by homodyne detection. All VSFG measurements
were performed immediately after place a sample solution on acontainer made of Teflon. The experimental room temperature was 19°C–22°C. The intensity of sum frequency lightincreases when the wavelength of the infrared light introduced
onto an interface is resonant with the energy levels of thevibrational modes of the chemical species at the interface.
Thus, vibrational spectra are obtained by monitoring the intensity of the sum frequency light as a function of the wavenumber of the infrared light (cm−1) in VSFG spectroscopy.
3. Results and Discussion
Trivalent europium ion (Eu 3+) is extracted into an organic
phase with the extractant of di-(2-ethylhexyl)phosphate(HDEHP), and Eu3+ is extracted as Eu(DEHP ∙ HDEHP)3 in anorganic phase.11 The HDEHP extractant dissolved in aqueoussolutions spontaneously come to the surfaces of aqueous solutions and form air/aqueous-HDEHP interface; thus, the VSFG
measurement was performed on the surface. Figure 2 shows
the vibrational spectra of the air/aqueous-HDEHP interfaces
observed by VSFG spectroscopy in the CH and OH stretch frequency region.8 The vertical axis shows the imaginary part ofsecond-order nonlinear susceptibility, Imχ (2), and was measured by heterodyne-detected VSFG spectroscopy.12,13 The
absolute value of Imχ(2) increases when the IR wavenumber isresonant with the energy levels of the chemical species at theinterface, and the sign of Imχ(2) corresponds to the orientationof chemical species at the interface. The CH stretch bands ofHDEHP at approximately 2900 cm−1 show the existence ofHDEHP at the interface. When the europium ions are dissolved in the aqueous phase, a large change in the band of theOH stretching vibration of water (3000−3500 cm −1) wasobserved. The positive band was observed at a concentrationof 0 mM Eu 3+, and the negative band was observed at a concentration of 10 mM Eu 3+. This indicated that Eu 3+ adsorbedon the interface and that the structure of water at the interface
drastically changed upon Eu 3+ adsorption. Furthermore, thepositive and negative signs of Imχ(2) determine the orientationof the interfacial water.14 Because the positive and negative
signs of the OH stretching bands of water correspond to theOH-up and OH-down orientations of water, respectively, thewater molecules at the interface were found to turn their OHgroups downward when Eu3+ adsorbed to the interface, and thewater molecules were bonded to Eu3+ using their oxygen atoms

Figure 2. Vibrational spectra of the air/aqueous-HDEHP interfaces
obtained by heterodyne-detected VSFG spectroscopy. The wavenumber region corresponds to that of CH and OH stretching vibrations. The concentration shown in the inset is the Eu 3+ concentrationin the aqueous solutions. This was reproduced from Ref. 8 with permission from the Royal Society of Chemistry.

Liquid interfaces related to lanthanide and actinide chemistry studied usingFigure 3. Structure of the Eu-HDEHP complex at the air/aqueousHDEHP interface. This was reproduced from Ref. 8 with permission
from the Royal Society of Chemistry.

J. Nucl. Radiochem. Sci., Vol. 20, 2020

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Figure 4. Vibrational spectra of air/aqueous-TBP interfaces
obtained by VSFG spectroscopy. The wavenumber region corresponds to the P=O stretching region of TBP. This was reproducedfrom Ref. 9 with permission from the Royal Society of Chemistry.

(Figure 3). From this result, we proposed that Eu3+ extraction
into the organic phase occurred after the formation of the complex at the interface.
The extraction of uranyl ions by tributyl phosphate (TBP) is
one of the most significant solvent extractions in the nuclear
fuel cycle. TBP extractant dissolved in aqueous solution spontaneously comes to the surface of the aqueous solution andforms an air/aqueous-TBP interface, and a VSFG measurement was performed on this surface. In the aqueous phase ofsolvent extraction, uranyl ions exist as uranyl nitrate,
UO2(NO3)2, because of the high concentration of nitric acid.
Some molecular dynamics simulation studies have suggestedt h at m a ny t y p e s of u r a nyl-T BP complexe s, such a s
UO2(NO3)2(TBP)2 , UO2(NO3)2(H 2O)(TBP)2 , and UO2(NO3)(TBP)4, are formed at the interface before extraction into theorganic phase.15-18 However, there have not been any experimental evidence of the complex formation at the interface.
Figure 4 shows the vibrational spectra of the air/aqueous-TBP
interface, and the bands observed are due to the P=O stretch ofTBP,9 indicating the existence of TBP at the interface. The
wavenumber of P=O stretching is sensitive to whether theP=O group of TBP is bonded to uranyl; when TBP is bondedto uranyl, the band of the P=O group shifts from ~1250 cm−1 to~1190 cm−1 9,19, probably because of the weakening of the P=O
bond upon complexation. However, at high concentrations ofuranyl and nitric acid [Figure 4(a)], which is the condition thaturanyl is well extracted into an organic phase, the P=O band
was observed at ~1250 cm−1, and the band position at ~1250
cm−1 was observed in the absence of uranyl and nitric acid
[Figure 4(b)]. Therefore, uranyl was not adsorbed to the air/aqueous-TBP interface and did not form complexes with TBP
at the interface [Figure 5(a)]. From the conclusion from Figure5(a), as shown in Figure 5(b), it can be proposed that uranylnitrate [UO2(NO3)2] in the aqueous phase transfers into theorganic phase without staying at the interface. After transfer
into the organic phase, uranyl nitrate forms a complex withTBP, which is UO2(NO3)2∙2TBP, and the extraction of uranium
is completed.
4. Conclusions
VSFG spectroscopy has been applied to air/aqueousextractant interfaces, where trivalent europium ions and uranyl ions are dissolved in the aqueous phase. At the air/aqueous-HDEHP interface, the Eu-HDEHP complex is formed

Figure 5. (a) Sketch showing uranyl nitrate not adsorbed to the air/aqueous-TBP interface and (b) Proposed model of the solvent extraction of uranium by TBP.

at the interface, and Eu3+ is sandwiched between HDEHP andwater molecules. Based on this result, it can be proposed thatEu3+ forms the Eu-HDEHP interfacial complex at the interface
before phase transfer into the organic phase. On the other
hand, uranyl nitrate is not adsorbed to the air/aqueous-TBP
interface and does not form complexes with TBP at the interface. Thus, uranyl nitrate likely transfers into the organic
phase without staying at the interface, and complex formation
with TBP occurs in the organic phase. The two different
results for europium and uranyl ions suggest that the extraction mechanism that occurs at the interface will depend on thetype of solvent extraction. These studies demonstrate thatVSFG spectroscopy is a powerful technique that can obtainunique information regarding the mechanism of solvent
extractions of lanthanides and actinides. It is expected thatlanthanide and actinide chemistry that occurs at many otherinterfaces will be revealed by VSFG spectroscopy.
Acknowledgements
This work was financially supported by a Grant-in-Aid for31

J. Nucl. Radiochem. Sci., Vol. 20, 2020

Young Scientists (B) (No. 17K14919) from the JSPS. I thankTakaumi Kimura (JAEA), Masayuki Watanabe (JAEA),
Noboru Aoyagi (JAEA), and Takumi Saito (University ofTokyo) for supporting the construction of VSFG experimentalsetup in a radiation management area in JAEA.
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