There is an ever-increasing body of evidence that metallic complexes involving amphiliphic ligands do not form normal solutions in organic solvents. Instead, they form complex fluids with intricate structures. For example, the metallic complexes may aggregate into clusters, and these clusters themselves may aggregate into superclusters. To gain a deeper insight into the mechanisms at play, we have used an improved force field to conduct extensive molecular dynamics simulations of a system composed of zirconium nitrate, water, nitric acid, tri-n-butyl phosphate, and n-octane. The important new finding is that a dynamic equilibrium between the cis and trans isomers of the metal complex is likely to play a key role in the aggregation behavior. The isolated cis and trans isomers have similar energies, but simulation indicates that the clusters consist predominantly of cis isomers. With increasing metal concentration, we hypothesize that more clustering occurs and the chemical equilibrium shifts toward the cis isomer. It is possible that such isomeric effects play a role in the liquid-liquid extraction of other species and the inclusion of such effects in flow sheet modeling may lead to a better description of the process.

Liquid-liquid extraction (LLE), the go-to process for a variety of chemical separations, is limited by spontaneous organic phase splitting upon sufficient solute loading, called third phase formation. In this study we explore the applicability of critical phenomena theory to gain insight into this deleterious phase behavior with the goal of improving separations efficiency and minimizing waste. A series of samples representative of rare earth purification were constructed to include each of one light and one heavy lanthanide (cerium and lutetium) paired with one of two common malonamide extractants (DMDOHEMA and DMDBTDMA). The resulting postextraction organic phases are chemically complex and often form rich hierarchical structures whose statics and dynamics near the critical point were probed herein with small-angle X-ray scattering and high-speed X-ray photon correlation spectroscopy. Despite their different extraction behaviors, all samples show remarkably similar critical behavior with exponents well described by classical critical point theory consistent with the 3D Ising model, where the critical behavior is characterized by fluctuations with a single diverging length scale. This unexpected result indicates a significant reduction in relevant chemical parameters at the critical point, indicating that the underlying behavior of phase transitions in LLE rely on far fewer variables than are generally assumed. The obtained scalar order parameter is attributed to the extractant fraction of the extractant/diluent mixture, revealing that other solution components and their respective concentrations simply shift the critical temperature but do not affect the nature of the critical fluctuations. These findings point to an opportunity to drastically simplify studies of liquid-liquid phase separation and phase diagram development in general while providing insights into LLE process improvement.

Redox reactions of uranium (U) in aqueous environments have important impacts on the mobility and isotopic fractionation of U in the geosphere. Pentavalent U as the cationic uranyl ion, UO2+, is rarely observed in naturally occurring samples because of its limited lifetime, but it may be an important intermediate state controlling the redox kinetics between hexavalent and tetravalent U. Increasing evidence has indicated that U(V) can be stabilized under laboratory conditions. Here, we showed that U(V) is the dominant species on the magnetite (Fe3O4) surface under reducing conditions controlled by electrochemical methods. Cyclic voltammetry reveals coupled redox peaks corresponding to the U(VI)O22+/U(V)O2+ one-electron redox reaction. Magnetite electrodes polarized at a series of potentials to reduce U(VI)O22+ were characterized by X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and Auger electron mapping. The results showed that up to twice the amount of U(V) to U(VI) was present on the magnetite surface. U(V) adopted a typical uranyl-type structure, and the U coverage on the magnetite surface increased with decreasing potentials. The formation of mixed-valence U(V)/U(VI) species on the surface of magnetite may hinder the U(V) disproportionation reaction, thereby eliminating the presence of tetravalent U. These results show that U(V) can exist over short time scales as the dominant U species on mineral surfaces under selected reducing conditions by the controlled polarization of a mineral electrode.

The PUREX (Plutonium, Uranium, Reduction, EXtraction) process is the foundation of all industrial activities that involve the recycling of plutonium and uranium from used nuclear fuel. With over 70 years of research, engineering, and operations, it can be argued that little opportunity is left for further discovery and new fundamental understandings regarding PUREX, in general, and tri-n-butyl phosphate (TBP), in particular. Through use of an advanced electroanalytical approach, molecular-level insights regarding the back-extraction of Pu(IV) by reductive separation are reported. Studies of model PUREX systems, with lanthanide ions extracted into 20% TBP in n-dodecane, are used to demonstrate the approach. These studies reveal knowledge pertinent to mass-transfer aspects of the biphasic reductive stripping reaction central to the separation of U and Pu in PUREX. Two lanthanide ions - Ce(IV) and Yb(III) - were extracted from their concentrated solutions in aqueous 3 M HNO3 to expressly provide electrically conducting third phases to facilitate electrochemical data acquisition. To simplify the diabolically-complicated reductive separation aspects of PUREX, the controlled polarization of electrode surfaces was used to reduce Ce(IV) and Yb(III) in place of chemical reductants. Electroanalyses of the biphasic systems with Ce(IV) and Yb(III) using three-phase electrode differential pulse voltammetry demonstrate the effects of mass transfer across water-oil interfaces on the electrode potentials for the reduction to Ce(III) and Yb(II). These reductions trigger nitrate transfer out of the oil phase for Ce and proton transfer into the organic phase for Yb. This is in correlation with the transfer of the charge-neutral ion pairs Ce center dot 3NO(3) and Yb center dot 2NO(3) out of the oil phases. The differences in ion-transfer steps reflect significant variations in the equilibrium speciation of multinuclear Ce(IV) and mononuclear Yb(III) solvates of TBP in the oil phases and the loading of the organic phase.

Since the death of Renato Chiarizia (23 May 2019), his legacy looms large in the thoughts and the science of researchers throughout the world of chemical separations, especially for practitioners o...

Functionalized ordered mesoporous carbons facilitate the templateing of microcrystalline-like domains and multinuclear speciation under high Eu3+ loading conditions.

This work investigates the one-electron reduction of Eu(iii) to Eu(ii) by ordered mesoporous carbon (OMC), with and without tetra-n-octyl diglycolamide (TODGA) functionalization, in cavity microelectrode (CME) systems.

The advent of high-speed x-ray photon correlation spectroscopy now allows the study of critical phenomena in fluids to much smaller length scales and over a wider range of temperatures than is possible with dynamic light scattering. We present an x-ray photon correlation spectroscopy study of critical fluctuation dynamics in a complex fluid typical of those used in liquid-liquid extraction (LLE) of ions, dodecane-DMDBTDMA with extracted aqueous Ce(NO_{3})_{3}. We observe good agreement with both static and dynamic scaling without the need for significant noncritical background corrections. Critical exponents agree with 3D Ising values, and the fluctuation dynamics are described by simple exponential relaxation. The form of the dynamic master curve deviates somewhat from the Kawasaki result, with a more abrupt transition between the critical and noncritical asymptotic behavior. The concepts of critical phenomena thus provide a quantitative framework for understanding the structure and dynamics of LLE systems and a path forward to new LLE processes.

Current methods for the extraction of rhodium carry the highest carbon footprint and worst pollution metrics of all of the elements used in modern technological applications. Improving upon existing methods is made difficult by the limited understanding of the molecular-level chemistry occurring in extraction processes, particularly in the hydrometallurgical separation step. While many of the precious metals can be separated by solvent extraction, there currently exist no commercial extractants for Rh. This is due to its complicated mixed speciation upon leaching into hydrochloric acid, which gives rise to difficulties in designing effective reagents for solvent extraction. Herein we show that the diamidoamine reagent N-n-hexylbis(N-methyl-N-n-octylethylamide)amine transports Rh(III) from aqueous HCl into an organic phase as the monoaquated dianion [RhCl5(H2O)](2-) through the formation of an outer-sphere assembly; this assembly has been characterized by experimentation (slope analysis, FT-IR and NMR spectroscopy, EXAFS, SANS, and ESI-MS) and computational modeling. The paper demonstrates the importance of applying a broad range of techniques to obtain a convincing mode of action for the complex processes involved in anion recognition in the solution phase. A consistent and comprehensive understanding of how the ligand operates to achieve Rh(III) selectivity over the competitor anion Cl- has emerged. This knowledge will guide the design of extractants and thus offers promise for improving the sustainability of metal extraction from both traditional mining sources and the recycling of secondary source materials.

Keggin phosphotungstate heteropolyanions (HPA), PW12O403–, are known to interact via short‐range attraction and long‐range repulsion (SALR) at moderate and high solution concentrations and low‐pH (pH = 1). These interactions were identified through the observation of structure factor peaks in small‐angle X‐ray scattering (SAXS) data reported previously (J. Phys. Chem. C 2016, 120, 1317). Here we describe the interactions between the same heteropolyanions with extremely low concentrations at low (pH = 0) and high pH (pH = 4.7) conditions. The solution chemistry of Keggin heteropolyanions at these two pH values provides a unique way to change the charge of the cluster anions without changing the structure (from the perspective of SAXS) and, at the same time, changing the hydrogen bonding interactions. The structure factors obtained from concentration dependent SAXS data reveal that the change in pH changes the nature of SALR interactions, consistent with the presence of the plenary PW12O403− anion at pH = 0 and the monovacant lacunary PW11O397– anion at pH = 4.7. At the low pH condition, the attractive interactions dominate to provide a peak near Q = 0 Å−1 momentum transfer in the structure factors, whereas at pH = 4.7 the repulsive interactions dominate, thereby eliminating the peak near Q = 0 Å−1. The experimental results presented here are in agreement with the recent liquid state theory and computer simulation predictions on SALR interactions.

The Cover Feature shows how small-angle X-ray scattering (SAXS) of dilute solutions of Keggin heteropolyanions provides a direct probe of the short-range attractive (SA) interactions, due to hydrogen bonding, and the long-range repulsive (LR) interactions, due to Coulombic forces, between them. In low pH (0) solutions, where the plenary Keggin ions with –3 charge are stable, the SAXS structure factors reveal attractive associations. In contrast, at higher pH (4.7), where the monovacant lacunary ions with –7 charge prevail, the structure factors reveal repulsive behavior. More information can be found in the Full Paper by M. R. Antonio and M. K. Bera.

The extraction of tetravalent cerium, Ce(IV), from aqueous nitric acid with tri-n-butyl phosphate (TBP) in n-dodecane was studied by varying the aqueous, initial cerium(IV) concentration, [Ce4+](aq,init), up to and beyond the point of third phase formation, defined by the critical aqueous concentration (or CAC) and the limiting organic concentration (or LOC). The new chemistry, elaborated here for the nearly-century-old Ce(IV)-20% TBP system, focuses on the phenomena of third phase inversion and the distribution of four solutesCe(IV), HNO3, H2O, and TBPbetween the aqueous and organic phases, which are of direct relevance to the PUREX process. We demonstrate that multinuclear Ce(IV) entities are present in the organic phases.

Short- and long-range correlations between solutes in solvents can influence the macroscopic chemistry and physical properties of solutions in ways that are not fully understood. The class of liquids known as complex (structured) fluids-containing multiscale aggregates resulting from weak self-assembly-are especially important in energy-relevant systems employed for a variety of chemicaland biological-based purification, separation, and catalytic processes. In these, solute (mass) transfer across liquid-liquid (water, oil) phase boundaries is the core function. Oftentimes the operational success of phase transfer chemistry is dependent upon the bulk fluid structures for which a common functional motif and an archetype aggregate is the micelle. In particular, there is an emerging consensus that mass transfer and bulk organic phase behaviors-notably the critical phenomenon of phase splitting-are impacted by the effects of micellar-like aggregates in water-in-oil microemulsions. In this study, we elucidate the microscopic structures and mesoscopic architectures of metal-, water-, and acid-loaded organic phases using a combination of X-ray and neutron experimentation as well as density functional theory and molecular dynamics simulations. The key conclusion is that the transfer of metal ions between an aqueous phase and an organic one involves the formation of small mononuclear clusters typical of metal-ligand coordination chemistry, at one extreme, in the organic phase, and their aggregation to multinuclear primary clusters that self-assemble to form even larger superclusters typical of supramolecular chemistry, at the other. Our metrical results add an orthogonal perspective to the energetics-based view of phase splitting in chemical separations known as the micellar model-founded upon the interpretation of small-angle neutron scattering data-with respect to a more general phase-space (gas-liquid) model of soft matter self-assembly and particle growth. The structure hierarchy observed in the aggregation of our quinary (zirconium nitrate-nitric acid-water-tri-n-butyl phosphate-n-octane) system is relevant to understanding solution phase transitions, in general, and the function of engineered fluids with metalloamphiphiles, in particular, for mass transfer applications, such as demixing in separation and synthesis in catalysis science.

X-ray and electrochemical studies of organic phases obtained by the extraction of tetravalent cerium, Ce(iv), from aqueous nitric acid (3 M) with tri-n-butyl phosphate (TBP) in n-dodecane reveal a tetranuclear Ce(iv) structural motif. This finding is consistent with the results of previous liquid-liquid extraction (LLE) studies that implicate the aggregation of (Ce-O-Ce)6+ dimers into multinuclear Ce(iv)·TBP solvates. The organic solution structures elaborated here for the Ce(iv)-HNO3-20% TBP-n-C12H26 system are correlated with multiscale phenomena-from the atomic level of the cerium coordination environment to the supramolecular scale of solute aggregates-in the organic phases, which are of relevance to the PUREX (Plutonium Uranium Reduction EXtraction) process. The combination of XANES, EXAFS, and SAXS results indicate the presence of tetranuclear cerium(iv)-oxo core structures in each of the organic phases investigated. In addition to the use of X-ray spectroscopy and scattering for direct metrical details about the organic phase solute speciation, three-phase-electrode differential pulse voltammetry (DPV) of the third phase reveals a wave attributable to Ce(iv) reduction. The electrode potential is consistent with values for the reduction of Ce(iv) in (Ce-O-Ce)6+ dimers in aqueous electrolytes. The Ce(iv) coordination chemistry of the organic solvates is independent of the bulk phenomenon of phase splitting, namely third phase formation. The local, molecular environment of Ce in the organic phase before splitting is identical to those in the two organic phases (the dense third phase and the light phase) after splitting. SAXS data are consistent with the formation of small spherical reverse micelles with core diameters (approx. 6 Å) that can accommodate a tetranuclear Ce(iv) oxo-cluster solvate of TBP. Sticky sphere modeling of the SAXS data for the organic phases with low cerium concentrations (<0.14 M) is consistent with the presence of randomly- and homogenously-dispersed micelles in combination with short-range percolated, associated micelles. At high cerium concentrations (approx. 1.5 M) in the third phase, the SAXS modeling is consistent with correlated, long-range percolated micellar aggregates. The presence of strong inter-micellar interactions (-3 to -5kBT) in all organic phases of the Ce(iv)-HNO3-TBP-n-C12H26 LLE system suggests that the phenomena of phase splitting and third phase inversion are due to liquid precipitation that is dependent solely on the concentration of the tetranuclear Ce solvate.

The coordination of the trivalent 4f ions, Ln = La3+, Dy3+, and Lu3+, with neutral and acidic organophosphorus reagents, both individually and combined, was studied by use of X-ray absorption spectroscopy. These studies provide metrical information about the interatomic interactions between these cations and the ligands tri-n-butyl phosphate (TBP) and di-n-butyl phosphoric acid (HDBP), whose behavior are of practical importance to chemical separation processes that are currently used on an industrial scale. Previous studies have suggested the existence of complexes involving a mixture of ligands, accounting for extraction synergy. Through systematic variation of the aqueous phase acidity and extractant concentration and combination, we have found that complexes with Ln and TBP : HDBP at any mixture and HDBP alone involve direct Ln-O interactions involving 6 oxygen atoms and distant Ln-P interactions involving on average 3-5 phosphorus atoms per Ln ion. It was also found that Ln complexes formed by TBP alone seem to favor eight oxygen coordination, though we were unable to obtain metrical results regarding the distant Ln-P interactions due to the low signal attributed to a lower concentration of Ln ions in the organic phases. Our study does not support the existence of mixed Ln-TBP-HDBP complexes but, rather, indicates that the lanthanides are extracted as either Ln-HDBP complexes or Ln-TBP complexes and that these complexes exist in different ratios depending on the conditions of the extraction system. This fundamental structural information offers insight into the solvent extraction processes that are taking place and are of particular importance to issues arising from the separation and disposal of radioactive materials from used nuclear fuel.

We describe the synthesis and characterization of three glycine-stabilized hexanuclear Cely cluster compounds, each containing the [Ce-6(mu(3)-O)(4)(mu(3)-OH)(4)](12+) core structure. Crystallized from aqueous nitrate solutions with pH < 0, the core cluster structures exhibit variable decoration by nitrate, glycine, and water ligands depending on solution conditions, where increased nitrate and glycine decoration of the cluster core was observed for crystals synthesized at high Ce and nitrate concentrations. No other crystalline products were observed using this synthetic route. In addition to confirming the tetravalent oxidation state of cerium in one of the reported clusters, cyclic voltammetry also indicates that Ce-IV is reduced at similar to+0.60 V vs Ag/AgCl (3 M NaCl), which is significantly less than the standard electrode potential. This large decrease in the Ce-IV/Ce-III reduction potential suggests that Ce-IV is significantly stabilized relative to Ce-III within the examined cluster. These compounds are discussed in terms of their importance as small, end member, ceric oxide nanoparticles. Single-crystal structural solutions, together with voltammetry and electrolysis data, permit the decoupling of Ce-III defects and substoichiometry. In addition, Ce-Ce distances can be used to determine an "effective" CeO2-x lattice constant, providing a simple method for comparing literature descriptions. The results are discussed in terms of their potential implications for the mechanisms by which nanoparticle ceria serve as catalysts and oxygen-storage materials.