The binding of rare earth elements to extractants is a key step in their separation and purification by solvent extraction. Classical MD simulations are used to investigate the equilibration configuration and binding dynamics of one, two, and three DEHP- to Er3+ in both bulk water and at the water-dodecane interface. The equilibrium radial distribution function g(r) shows that Er-O and Er-P radial separations within the first and second hydration shells are essentially identical for all bulk and interface simulations, though other aspects of the spatial configuration of heteroleptic Er3+(H2O)x(DEHP-)n complexes are different. The binding dynamics was fast for all interfacial and bulk binding events. Hydration water molecules rotate about the Er3+ ion and away from the binding DEHP- upon its approach and water molecules are ejected in well-defined directions from the incoming DEHP-. Potential of mean force calculations yield the height of the reaction barrier, which provides insight into the results of calculations of the binding dynamics. Interfacial binding of DEHP- to rare earth ions is expected to dominate the solvent extraction process, and we find that the probability of interfacial binding of a third DEHP- to Er3+ is an order of magnitude lower than the probabilities of binding the first two DEHP- or of binding one, two, or three DEHP- to Er3+ in bulk water.

X-ray fluorescence near total reflection (XFNTR) is the primary technique used to measure the element-specific interfacial density of ions at liquid-liquid interfaces. Fluorescence from ions in the bulk liquids can complicate the determination of the interfacial density; consequently, measurements have been previously limited to samples without ions in the upper phase and with concentrations on the order of 10 μM or less in the lower phase. We modify the analysis of XFNTR data to account for ions in both bulk phases, then demonstrate its use in the context of rare-earth separations processes. In a model of ion stripping (i.e., back-extraction), dodecane solutions of di(2-ethylhexyl)phosphoric acid (HDEHP) loaded with Eu(III) at the 1 mM level are placed in contact with either pure water or aqueous solutions of nitric or citric acid at pH 3. XFNTR measurements of equilibrated, quiescent samples reveal that citric acid solutions produce a disproportionately large depletion of ions from the interface compared to that from the bulk organic solution, whereas stripping by pure water or nitric acid solutions is negligible. This advance in the methodology of XFNTR may have broad applicability to the investigation of metal ions in chemical and biological processes at liquid-liquid interfaces.

Ions at liquid surfaces and interfaces influence many scientific and technological areas, including molecular and nanoparticle assembly for energy and separations processes. Controlled transport of ions between interfacial and bulk liquids can lead to triggering ion-induced interfacial phenomena. Here, we show that X-ray exposure alters the competitive equilibrium of rare earth elements bound to chelating ligands in bulk water and to insoluble monolayers at the water surface. Controlling the X-ray exposure leads to reversible adsorption of rare earth trivalent ions to the liquid surface. Evidence for the exposure-induced temporal variations in the ion surface density is provided by synchrotron X-ray fluorescence near total reflection (XFNTR) measurements. Varying the X-ray penetration depth from 10 nm to 2.8 µm leads to a controlled exposure of either the surface region alone or the surface monolayer plus dissolved chelating ligands and bulk water. This separation of surface and bulk processes helped identify the role of aqueous radiolysis in the adsorption cycle. Comparison of different chelates identified amine binding sites as a contributor to the cycling mechanism. The primary molecules utilized for these studies – chelating ligand DTPA and organophosphoric acid extractant DHDP – are like those used in the separation of rare earth elements from ores and in the reprocessing of nuclear fuel. The observed reversible cycling of ion adsorption may provide an opportunity for further control over these processes and enhanced separation.

The extractant-assisted transport of metal ions from aqueous to organic environments by liquid-liquid extraction has been widely used to separate and recover critical elements on an industrial scale. While current efforts focus on designing better extractants and optimizing process conditions, the mechanism that underlies ionic transport remains poorly understood. Here, we report a nonequilibrium process in the bulk aqueous phase that influences interfacial ion transport: the formation of metastable ion-extractant precipitates away from the liquid-liquid interface, separated from it by a depletion region without precipitates. Although the precipitate is soluble in the organic phase, the depletion region separates the two and ions are sequestered in a long-lived metastable state. Since precipitation removes extractants from the aqueous phase, even extractants that are sparingly soluble in water will continue to be withdrawn from the organic phase to feed the aqueous precipitation process. Solute concentrations in both phases and the aqueous pH influence the temporal evolution of the process and ionic partitioning between the precipitate and organic phase. Aqueous ion-extractant precipitation during liquid-liquid extraction provides a reaction path that can influence the extraction kinetics, which plays an important role in designing advanced processes to separate rare earths and other minerals.

Reversible cycling of rare-earth elements between an aqueous electrolyte solution and its free surface is achieved by X-ray exposure. This exposure alters the competitive equilibrium between lanthanide ions bound to a chelating ligand, diethylenetriamine pentaacetic acid (DTPA), in the bulk solution and to insoluble monolayers of extractant di-hexadecyl phosphoric acid (DHDP) at its surface. Evidence for the exposure-induced temporal variations in the lanthanide surface density is provided by X-ray fluorescence near total reflection measurements. Comparison of results when X-rays are confined to the aqueous surface region to results when X-rays transmit into the bulk solution suggests the importance of aqueous radiolysis in the adsorption cycle. Amine binding sites in DTPA are identified as a likely target of radiolysis products. The molecules DTPA and DHDP are like those used in the separation of lanthanides from ores and in the reprocessing of nuclear fuel. These results suggest that an external source of X-rays can be used to drive rare-earth element separations. More generally, use of X-rays to controllably dose a liquid interface with lanthanides could trigger a range of interfacial processes, including enhanced metal ion extraction, catalysis, and materials synthesis.

Rare earth elements (REEs) are critical materials to modern technologies. They are obtained by selective separation from mining feedstocks consisting of mixtures of their trivalent cation. We are developing an all-aqueous, bioinspired, interfacial separation using peptides as amphiphilic molecular extractants. Lanthanide binding tags (LBTs) are amphiphilic peptide sequences based on the EF-hand metal binding loops of calcium-binding proteins which complex selectively REEs. We study LBTs optimized for coordination to Tb3+ using luminescence spectroscopy, surface tensiometry, Xray reflectivity, and X-ray fluorescence near total reflection, and find that these LBTs capture Tb3+ in bulk and adsorb the complex to the interface. Molecular dynamics show that the binding pocket remains intact upon adsorption. We find that, if the net negative charge on the peptide results in a negatively charged complex, excess cations are recruited to the interface by nonselective Coulombic interactions that compromise selective REE capture. If, however, the net negative charge on the peptide is -3, resulting in a neutral complex, a 1:1 surface ratio of cation to peptide is achieved. Surface adsorption of the neutral peptide complexes from an equimolar mixture of Tb3+ and La3+ demonstrates a switchable platform dictated by bulk and interfacial effects. The adsorption layer becomes enriched in the favored Tb3+ when the bulk peptide is saturated, but selective to La3+ for undersaturation due to a higher surface activity of the La3+ complex.

We are developing an all-aqueous, bio-inspired approach using peptides as surfactants that selectively bind rare earth element (REE) cations and adsorb at the air/water interface to enable green capture and separation. REEs are essential components in modern electronic devices and clean energy technologies which must be separated from feedstocks of aqueous mixtures. Their selective capture is particularly challenging owing to their similarity in size and charge. Lanthanide binding tags (LBTs) are amphiphilic peptide sequences based on the binding loop in the evolutionarily conserved EF-hand metal binding motif. We study LBTs optimized for coordination to Tb$^{3+}$ using a suite of experimental methods including luminescence spectroscopy, surface tensiometry, x-ray reflectivity and x-ray fluorescence near total reflection, and find that these LBTs capture Tb$^{3+}$ in bulk and adsorb at the interface. Molecular dynamics show that the binding pocket remains intact upon adsorption. We find that, if the net negative charge on the peptide results in a negatively charged complex, excess cations are recruited to the interface by non-selective Coulombic interactions that compromise selective REE capture. If, however, the net negative charge on the binding loop is -3, resulting in a neutral complex, a 1:1 surface ratio of cation to peptide is achieved. We demonstrate selective interfacial extraction from an equimolar mixture of Tb$^{3+}$ and La$^{3+}$, validating an LBT-mediated interfacial separation of REEs.

X-ray fluorescence (XRF) spectroscopy of liquid surfaces is a promising approach to elemental analysis of forensic samples. Elements in the mixture can be identified from the XRF data because chemical elements are associated with unique spectra. This report describes progress in the development of automated identification of unknown mixtures of elements from XRF spectra using a genetic algorithm (GA). The XRF spectra are measured at the Advanced Photon Source (APS) at the liquid-liquid interface during solvent extraction process. The usage of the GA is demonstrated and future implementation for related problems are then discussed.

During the solvent extraction of metal ions from an aqueous to an organic phase, organic-soluble extractants selectively target aqueous-soluble ions for transport into the organic phase. In the case of extractants that are also soluble in the aqueous phase, our recent studies of lanthanide ion-extractant complexes at the surface of aqueous solutions suggested that ion- extractant complexation in the aqueous phase can hinder the solvent extraction process. Here, we investigate a similar phenomenon relevant to the separation of Co(II), Ni(II), and Fe(III). X-ray fluorescence near total reflection and tensiometry are used to characterize ion adsorption behavior at the surface of aqueous solutions containing water-soluble extractants, either bis(2-ethylhexyl) phosphoric acid (HDEHP) or 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEHEHP), as well as adsorption to a monolayer of water-insoluble extractant dihexadecyl phosphoric acid (DHDP) at the aqueous-vapor interface. Competitive adsorption of Ni(II) and Fe(III) utilizing either HDEHP or DHDP illustrates the essential feature of the recent lanthanide studies that the ion, which is preferentially extracted in liquid-liquid extraction, Fe(III), is found preferentially adsorbed to the water-vapor interface only in the presence of the water-insoluble extractant DHDP. A more subtle competition produces comparable adsorption behavior of Co(II) and Ni(II) at the surfaces of both HDEHP-and HEHEHP-aqueous solutions in spite of the known preference for Co(II) under solvent extraction conditions. Comparison experiments with a monolayer of DHDP reveal that Co(II) is preferentially adsorbed to the surface. This preference for Co(II) is also supported by molecular dynamics simulations of the potential of mean force of ions interacting with the soluble extractants in water. These results highlight the possibility that complexation of extractants and ions in the aqueous phase can alter selectivity in the solvent extraction of critical elements.

Ion interactions with supramolecular assemblies underlie their applications in the areas of drug delivery, ion recognition, and conduction. The current challenge remains in directly characterizing the interaction of ions with supramolecular assemblies at the nanoscale. Here, we demonstrate that the micro-environment of neutral polymer micelles regulates its interaction with anions by combining element-sensitive anomalous small angle X-ray scattering (ASAXS) and theoretical calculations. ASAXS and molecular dynamics simulations reveal that monovalent ReO4− is located preferentially in the outer shell of the micelle, while the more strongly hydrated divalent SeO42– is excluded from the micelle. However, DFT calculations show that the more highly charged SeO42– should have stronger affinity for an isolated polymer chain than ReO4−. These results suggest that the confined environment created by polymer self-assembly modulates the contribution of hydrophobic and electrostatic interactions to the binding of anions with polymers. The present work sheds light on the role of crowded environments in the interaction of anions with polymers in supramolecular assemblies and offers valuable insights to optimize the design of supramolecular systems.

Solvent extraction of trivalent rare earth metal ions by organophosphorus extractants proceeds via binding of phosphoric acid headgroups to the metal ion. Water molecules in the tightly bound first hydration shell of the metal ions must be displaced by oxygen atoms from phosphoric acid headgroups. Here, we use classical molecular dynamics simulations to explore the event in which a fully hydrated Er3+ binds to its first phosphoric acid headgroup. Approach of the headgroup into the region between the first and second hydration shells leads to a fast ejection of a water molecule that is accompanied by reordering of the hydration water molecules, including discretization of their angular positions and collective rotation about the metal ion. The water molecule ejected from the first shell is located diametrically opposite from the binding oxygen. Headgroup binding places a headgroup oxygen closer to Er3+ than its first hydration shell and creates a loosely bound water that subsequently exchanges between the first shell and its environment. This second exchange of water also occurs at discrete angular positions. This geometrical aspect of binding may be of relevance to understanding the binding and transport of ion-extractant complexes that are expected to occur at the organic-aqueous liquid-liquid interface used in solvent extraction processes.

X-ray fluorescence (XRF) spectroscopy is a promising approach to elemental analysis of forensic samples. Elements in the mixture can be identified from the XRF data because chemical elements are associated with unique spectra. This report consists of preliminary assessment of automated identification of unknown mixtures of elements from XRF spectra using a genetic algorithm (GA). The XRF spectra are measured at the Advanced Photon Source (APS) at the liquid-liquid interface during solvent extraction process. Possible advantages of using GA for identifying elements at the interface from XRF data are discussed, and a path for implementation of GA is outlined.

Previous X-ray scattering measurements demonstrated the formation of a voltage-tunable 2-dimensional lattice of nanoparticles situated at the liquid-liquid interface between two immiscible electrolyte solutions (Nano Letters 2014, 14, 6816−6822). The nanoparticles had Au cores coated with trimethylammonium terminated ligands and the ITIES consisted of NaCl in water and BTPPATPFB in 1,2-dichloroethane. Here, we present additional measurements on this system which illustrate a transition that occurs at more negative potentials. The transition represents a breakup of the 2D monolayer lattice into a disordered sub-monolayer of nanoparticles that coexists with small nanoparticle clusters in the vicinity of the interface. Grazing Incidence Small Angle X-ray Scattering (GISAXS) provides evidence for the breakup of the lattice and the appearance of clusters. X-ray reflectivity (XR) characterizes the relative location along the interfacial normal of the different types of nanoparticle assemblies.

During solvent extraction of rare earth ions, an aqueous electrolyte solution is placed in contact with an immiscible organic solution of extractants to enable extractant-facilitated transport of ions into the organic solvent. Although ex-perimental methodologies such as x-ray and neutron scattering have been applied to characterize ion-extractant complexes, identifying the site of ion-extractant complexation has proven challenging. Here, we use tensiometry and surface-sensitive x-ray scattering to study the surface of aqueous solutions of lanthanide chlorides and the water-soluble extractant bis(2-ethylhexyl) phosphate (HDEHP), in the absence of a coexisting organic solvent. These studies restrict interactions of HDEHP with trivalent lanthanide ions to the aqueous phase and the liquid-vapor interface, allowing us to explore the consequences that one or the other is the site of ion-extractant complexation. Unexpectedly, we find that light lanthanides preferentially occupy the liquid-vapor interface, with an overwhelming preference for a light lanthanide, Nd, when present in a mixture with a heavy lanthanide, Er. This contradicts our expectation that heavy lanthanides should have a higher interfacial density since they are preferentially extracted by HDEHP in the presence of an organic phase. These results reveal the antagonistic role played by ion-extractant complexation within the aqueous phase and clarify the potential advantages of water-insoluble extractants that interact with ions primarily at the interface during the process of solvent extraction.

Thiol ligands bound to the metallic core of nanoparticles determine their interactions with the environment and self-assembly. Recent studies suggest that equilibrium between bound and free thiols alters the ligand coverage of the core. Here, X-ray scattering and MD simulations investigate water-supported monolayers of gold-core nanoparticles as a function of the core-ligand coverage that is varied in experiments by adjusting the concentration of total thiols (sum of free and bound thiols). Simulations demonstrate that the presence of free thiols produces a nearly symmetrical coating of ligands on the core. X-ray measurements show that above a critical value of core-ligand coverage the nanoparticle core rises above the water surface, the edge-to-edge distance between neighboring nanoparticles increases, and the nanoparticle coverage of the surface decreases. These results demonstrate the important role of free thiols: they regulate the organization of bound thiols on the core and the interactions of nanoparticles with their surroundings.

Electrochemistry at ITIES, which has been extensively studied for the oil (O)/water(W) interface, has been extended to the liquid/liquid interfaces of ionic liquids (ILs), namely, to the IL/W interface [1] and even to the IL/O interface [2,3]. One of several characteristics of ILs, compared with conventional molecular liquids like W and O, is the formation of ionic multilayers [4-6] at the interfaces due to the excluded volume and local electrostatic interactions between ions. Although such a multilayering behavior of ILs had been expected to appear in the electric double layer (EDL) at the IL/W interface, it had been challenging to detect the ionic distribution at the IL/W interface. In the present paper, we will introduce our recent study on the ionic distribution at the IL/W interface, revealed by x-ray reflectometry (XR) using the liquid surface reflectometer at NSF’s ChemMatCARS Sector 15-ID at Advanced Photon Source in Chicago [7]. We designed an ionic liquid that has both high hydrophobicity and high electron-density contrast between the cation and anion, which enabled us to widely polarize the IL/W interface and sensitively probe the cation-rich and anion-rich layers by XR, respectively. We observed the growth of alternately charged cation-rich and anion-rich layers along with a polarity reversal of the layers as the potential changed sign. We quantitatively compared the XR results with the BSK model [8], a phenomenological EDL model in ILs that takes into account both excluded volume and overscreening effects, by combining it with the Gouy-Chapman-Stern model for EDL on the W side of the IL/W interface. Our XR data reveal that the excess charge beyond the first ionic layer decays more rapidly than the model prediction, suggesting that the BSK model needs an improvement with which the layering periodicity and decay length are separately evaluated. References T. Kakiuchi and N. Nishi, Electrochemistry, 74 (2006) 942. N.E.R. Cousens and A.R. Kucernak, Electrochem. Commun., 31 (2013) 63. Y. Kuroyama, N. Nishi, T. Sakka, submitted. N. Nishi, Y. Yasui, T. Uruga, H. Tanida, T. Yamada, S. Nakayama, H. Matsuoka, T. Kakiuchi, J. Chem. Phys., 132 (2010) 164705. N. Nishi, T. Uruga, H. Tanida, T. Kakiuchi, Langmuir, 27 (2011) 7531. N. Nishi, T. Uruga, H. Tanida, J. Electroanal. Chem., 759 (2015) 129. S. Katakura, K. Amano, T. Sakka, W. Bu, B. Lin, M.L. Schlossman, N. Nishi, J. Phys. Chem. B, 124 (2020) 6412. M.Z. Bazant, B.D. Storey, A.A. Kornyshev, Phys. Rev. Lett., 106 (2011) 046102.

Solvent extraction is used widely for chemical separations and environmental remediation. Although the kinetics and efficiency of this process rely upon the formation of ion-extractant complexes, it has proven challenging to identify the location of ion-extractant complexation within the solution and its impact on the separation. Here, we use tensiometry and X-ray scattering to characterize the surface of aqueous solutions of lanthanide chlorides and the water-soluble extractant bis(2-ethylhexyl) phosphoric acid (HDEHP), in the absence of a coexisting organic solvent. These studies restrict ion-extractant interactions to the aqueous phase and its liquid-vapor interface, allowing us to explore the consequences that one or the other is the location of ion-extractant complexation. Unexpectedly, we find that light lanthanides preferentially occupy the liquid-vapor interface. This contradicts our expectation that heavy lanthanides should have a higher interfacial density since they are preferentially extracted by HDEHP in solvent extraction processes. These results reveal the antagonistic role played by ion-extractant complexation within the aqueous phase and clarify the advantages of complexation at the interface. Extractants in common use are often soluble in water, in addition to their organic phase solubility, and similar effects to those described here are expected to be relevant to a variety of separations processes.