Coordination Structures and Supramolecular Architectures in a

Mar 15, 2012 - Stability of reverse micelles in rare-earth separation: a chemical model based on a molecular approach. Yushu Chen , Magali Duvail , Ph...
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Coordination Structures and Supramolecular Architectures in a Cerium(III)−Malonamide Solvent Extraction System Ross J. Ellis and Mark R. Antonio* Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: The process chemistry and solution structures investigated in the title system bridge the three ostensibly disparate fields of separation sciences, soft matter research, and coordination chemistry. We have explored this subject with synchrotron radiation research and advanced analyses leading to original insights into aggregation phenomena in solvent extraction. Herein we present findings showing the coagulation of reverse micelles into wormlike aggregates in organic phases (N,N′-dimethyl-N,N′-dibutyltetradecylmalonamideabbreviated as DMDBTDMAin n-dodecane) obtained by liquid− liquid extraction following contact with acidic and neutral aqueous media containing trivalent cerium. The growth of solute architectures was shown to prelude phase transition (i.e., the formation of a “third phase”). The presence of acid was shown to promote the growth of these micellar chains and, therefore, promoted third-phase formation. Acid was also shown to hydrate and swell the reverse micelle units, preorganizing them to allow for incorporation of cerium, leading to different coordination structures and enhanced metal extraction. The approach of linking both the coordination environment and supramolecular structures to the process properties of a solvent extraction system in a single study provides perspectives that are not available from independent, uncorrelated experimentation. Moreover, the analysis of small-angle X-ray scattering data from a solvent extraction system using the generalized indirect Fourier transform method to gain real-space information led to insights not otherwise available, showing that micellar assemblies are larger and more ordered than previously thought. This multipronged and multidisciplinary investigation opens new avenues in the evolving understanding of solute architectures in organic phases of practical relevance to solvent extraction and, simultaneously, of fundamental relevance to structured fluids and, in particular, phase transition phenomena.

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INTRODUCTION Solvent extraction is used in myriad metal separations systems including precious,1 rare-earth,2 and base metal3 refining and in the treatment of used nuclear fuel.4 The process involves contacting an aqueous phase, in which is dissolved a mixture of metal ions, with an organic phase made up from an amphiphilic molecule (the extractant) dissolved in a suitable diluent. The extractant that is the subject of the present study is the malonamide N,N′-dimethyl-N,N′-dibutyltetradecylmalonamide, abbreviated DMDBTDMA (Figure 1). This reagent is of

that are of similar size (with coordination number 8; ionic radii (IR) Ce3+ = 1.143 Å, Am3+ = 1.09 Å)8 and are the focus of the DIAMEX process. We sought to investigate the effect that acid has on the extractive properties and solution structures of this system. The DIAMEX process is conducted with an aqueous phase of 3 M nitric acid, which is ideal for the extraction of trivalent lanthanides (Ln) and actinides using an aliphatic malonamide (MA), like DMDBTDMA.9 Musikas et al. explained this optimum aqueous condition by referring to a trade-off between the favorable effect of high nitrate activity, promoting the formation of the extracted neutral nitrate salt (eq 1), and the disfavorable effect of acid that is in competition with the metal for the coordination sites (eq 2).9 An aqueous phase of 3 M HNO3 therefore contains enough nitrate to favor extraction,

Figure 1. Structure of the extractant N,N′-dimethyl-N,N′-dibutyltetradecylmalonamide (DMDBTDMA).

historical relevance to the contemporary DIAMEX (diamide extraction) process5−7 for the separation of trivalent actinides in used nuclear fuel. Cerium(III) was investigated as the target metal ion due to its stable trivalent state in nitrate media, making it a good model for the trivalent radioactive actinides © 2012 American Chemical Society

Received: January 20, 2012 Revised: March 14, 2012 Published: March 15, 2012 5987

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Q region tends to deviate from it.15,20,25−27 The reason for this is probably that the Baxter model accurately quantifies the strength of attraction between micelle cores (giving rise to the scattering at low-Q) but is not sensitive to the changes in micelle form (influencing the scattering at high-Q) as higherorder assemblies develop as a result of this interaction. Problems with the application of the Baxter model to solvent extraction systems stem from two assumptions: (1) The model assumes micelles are sticky hard spheres that interact with each other accordingly. Reverse micelles are not hard; they are in fact fluid and can change shape and conformation easily.28 Neither are the reverse micelles limited to spherical shapes (their extensive aliphatic flora makes sure of this). (2) According to the Baxter sticky sphere model, the “hard spheres” must come into close proximity in order to interact, i.e., within 10% of the diameter of the micelle core (ca. 2 Å). It is difficult to envision, considering the steric requirements of the aforementioned aliphatic functionalities and the fluid nature of the aggregates, that the micelles would approach each other in a purely random fashion (as with hard sticky spheres) without rearranging. Erlinger et al. addressed the latter issue by hypothesizing significant interdigitation of micellar aliphatic chains to allow micelles to adhere together randomly.13 An alternative explanation could be micellar stacking into chains or “worms” similar to those that are observed in some organic phase surfactant reverse micelle systems.29−32 This arrangement would be thermodynamically favorable as the micelles “share” their aliphatic shells in forming higher-order assemblies so that hydrophobic/lipophilic effects are better balanced than in randomly oriented assemblies. These two scenarios are illustrated in Figure 2.

but with low enough acidity so that competition from acid does not dominate. 4MA(org) + Ln3 + + 3NO3 ⇌ (MA)4 Ln(NO3)3 (org) (1)

MA(org) + HNO3 ⇌ MA(HNO3)(org)

(2)

Extraction takes place via the interaction of the amphiphilic molecule with the metal ion in the aqueous phase, thus drawing the metal into the organic phase. One (or more) of the polar headgroup(s) of the amphiphile are designed to present coordination sites that complement the geometry of the aquated target metal ion in order to achieve good separation, selectivity and efficiency. Thus, coordination chemistry is the predominant platform for developing a fundamental understanding of the organic phase complex structure that underpins solvent extraction system properties.10 Lanthanide extraction with DMDBTDMA proceeds via the interaction of four extractant molecules with a neutral Ln(NO3)3 complex in the organic phase, as outlined in eq 1.11 This mechanism is, however, an oversimplification, as recognized by Musikas et al., who acknowledged the presence of weak dipole−dipole interactions between the malonamide Nδ+−COδ− units with each other and the polar solutes in the organic phase (such as nitric acid or metal nitrate) so that both ion−dipole (metal−ligand coordination) and dipole−dipole interactions are present.9 In a ground-breaking study, Osseo-Asare proposed that, in solvent extraction systems utilizing nonpolar diluents, extractant−acid/metal ion complexes self-assemble into higher-order architectures where the extractants and solutes are aggregated in the form of reverse micelles.12 Dipole−dipole interactions between the extractant head-groups, in combination with hydrophobic effects, favor the formation of reverse micelles that, upon contact with the metal-bearing aqueous phase, incorporate hydrophilic solutes (acid/water/metal ions) into their polar cores. The micellar model was further developed by Erlinger et al.13,14 and Chiarizia et al.,15−21 who interpreted small-angle scattering data using the Baxter model for sticky hard spheres.22,23 The micelles, they proposed, behave like small adhesive spheres that, upon incorporation of metal ions and/or acid, attract each other. When the strength of the interaction exceeds 2kBT, the micelles “condense” and form a discrete “third” phase. Third-phase formation is the scourge of solvent extraction systems, as it limits processing capacity and can be dangerous in nuclear fuel reprocessing due to criticality issues.24 The application of the Baxter model to solvent extraction systems represented a major step forward in understanding the organic phase behaviors involved in thirdphase formation. Although helpful in understanding the physical reasons behind third-phase formation, the Baxter model is an oversimplification of the dynamic, fluid properties of solvent extraction supramolecular structures. This is evident from small-angle X-ray (SAXS) and neutron (SANS) scattering experiments, which give scattering patterns that are dependent on the structure of the solution. The small-angle scattering data are presented in terms of scattering intensity, I(Q), in absolute units (cm−1) versus the momentum transfer (Q), which is defined by the wavelength, λ, and scattering angle, ϑ, of the Xrays or neutrons according to Q = (4π/λ) sin ϑ. The Baxter model for hard sticky spheres often closely approximates the experimental scattering data in the low-Q region, but the high-

Figure 2. Spherical reverse micelles attract each other and their cores (blue circles) approach to form superaggregates of (1) randomly adhered spheres as described by the Baxter model (steric requirements of aliphatic functionalities and close proximity of cores are satisfied by interdigitation of alkyl chains, as suggested by Erlinger et al.13) and (2) chains or worms, like those reported in previous studies of organic phase surfactant reverse micelle systems29−33 (steric requirements of aliphatic functionalities and close proximity of micelle cores are satisfied by shifting alkyl chains to facilitate micelle stacking where adjacent cores share an aliphatic shell, as proposed in this study). 5988

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be covered for the acidic and neutral systems (corresponding to 0.0065, 0.015, 0.02, 0.0265, 0.04, 0.055, and 0.1 M and 0.014, 0.0273, 0.041, 0.0546, 0.0683, 0.0819, 0.0956 M, respectively). The compositions of each of the different phases (aqueous and organic alike, including third phases) were determined with respect to concentrations of acid, Li, Ce, water, and DMDBTDMA. The acid, Li, and Ce concentrations in the organic phases following equilibration (denoted org,eq) were determined after stripping with three contacts of 0.1 M HNO3 at a 1:2 organic to aqueous phase ratio and analyses of the aqueous strip solutions (complete stripping was confirmed by mass balance with concentrations in the initial aqueous − raffinate). The cerium distribution ratio, DCe, was obtained as [Ce]org,eq/[Ce]aq,eq. The third phases were stripped as described above after appropriate dilution in n-dodecane. Acid concentrations were determined by potentiometric titration with 0.1 or 1 M NaOH solutions. Li and Ce concentrations were determined by Galbraith Laboratories (Knoxville, TN) using ICP-OES. Water in the organic phase was determined by Karl Fischer titrations using a Metrohm 756/831 KF Coulometer (Riverview, FL). The malonamide concentration in the organic phases was determined by potentiometric titration with HClO4 in acetic anhydride. FT-IR. A Nicolet Nexus 870 FT-IR spectrometer equipped with an HATR attenuated reflectance accessory containing a diamond resolution element was used to collect IR spectra. Two drops of organic solution were deposited on the diamond surface and covered with a glass adapter to prevent evaporation. Infrared spectra were obtained in the 4000−600 cm−1 region by collecting 25 scans at 2 cm−1 resolution. As a reference, the empty diamond ATR accessory or 0.5 M DMDBTDMA in n-dodecane after contact with either 3 M HNO3 (for the acidic system) or 3 M LiNO3 (for the neutral) was used. X-ray Absorption Spectroscopy. The Ce L3-edge data were obtained for solutions contained in micro X-ray cells (SPEX 3577) with Kapton film windows, (0.0003″, SPEX 3511). All measurements were made at the Advanced Photon Source (APS) beamline 12-BM-B (Argonne National Laboratory) using a 13-element fluorescence detector (Canberra). The EXAFS (extended X-ray absorption fine structure) was analyzed in consistent fashion in the usual manner with EXAFSPAK.37 The conventional metrical analysis of the k3χ(k) EXAFS was performed with a fixed scale factor (S02 = 0.9) and theoretical phase and amplitude functions calculated with FEFF8.0.38 Each of the EXAFS spectra were fit with a four-parameter, single-O shell model including the average Ce−O interatomic distance, r; O coordination number, CN; Ce−O Debye−Waller factor, σ2; energy threshold parameter, ΔE0. The fits show good correspondence with the experimental data; see the Supporting Information. Small-Angle X-ray Scattering (SAXS). Data Collection. SAXS measurements were made at the APS using beamline 12-ID-B at which the 12 keV incident X-ray beam is monochromatized with a Si⟨220⟩ double crystal monochromator and double focused by flat horizontal and vertical Pd-coated mirrors down to a spot size of 200 (H) × 50 (V) μm2. All solution samples were contained in 2 mm diameter quartz capillary tubes (Charles Supper Co., 20-QZ) and held at ambient conditions. At the incident photon energy, the 1/e attenuation length of the pure diluent, n-dodecane, is 12 mm, which is 6 times longer than the path length of the capillary tubes. The 2-D scattering profiles were acquired in 1−5 s exposures with a Pilates 2M counting detector, which has a pixel size of 0.172 mm and dynamic range of 2.20 The sample-to-detector distance was 2200 mm, providing a range for momentum transfer of 0.004 ≤ Q (Å−1) ≤ 0.65. The scattering vector, Q, was calibrated using a silver behenate standard.39 The 2-D scattering images were radially averaged to produce plots of scattered intensity, I(Q) vs Q, where Q = (4π/λ) sin ϑ/Å−1, in which 2ϑ is the scattering angle and λ is the wavelength of the X-rays, following standard procedures.40 The I(Q) data were put on an absolute scale (cm−1) by calibration with water (18.2 MΩ cm−1) scattering. The background subtraction was done with diligence; a single capillary tube was used to obtain responses that were free of

In previous studies using the Baxter model to interpret SAXS data, the low-Q data are closely approximated and, as this is the scattering region dominated by interparticle scattering, the concept of a system driven by small interacting polar micelle cores is a sound one. However, the Baxter model provides little information about the higher-order structures of superaggregates that might form as a result of the polar cores sticking together. In the present study, SAXS data were interpreted in a way not fully realized beforehand for solvent extraction systems, and the shapes and sizes of aggregates that form as a result of micellar attraction are revealed in an original manner. For this we used the generalized indirect Fourier transformation (GIFT) method developed recently by Glatter et al.34−36 to provide information on the shape of the particles in real space, without making any assumptions as to the shape or size of the aggregates. Small-angle scattering studies by Erlinger et al. showed, using the Baxter model, that DMDBTDMA in apolar aliphatic diluents is aggregated into reverse micelles with aggregation numbers between 3 and 8, and these micelles attract as acid is extracted.13,14 The effect that acid has on the behavior of the organic phase inspired us to conduct a study to demonstrate how HNO3 influences the extractive properties of the Ce(III)− malonamide system and to couple this to an investigation of the supramolecular and coordination structures in the organic phase. The findings in this study not only give new insights into the rich and dynamic structural chemistry underpinning solvent extraction systems but will also resonate in the general field of soft-matter chemistry and the fundamental science behind phase transition phenomena.

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EXPERIMENTAL SECTION

Materials. DMDBTDMA was obtained from Panchim with a purity higher than 99% as determined by gas chromatography−mass spectrometry (GC−MS). The cerium(III) nitrate hexahydrate, ndodecane (99%), and lithium nitrate were supplied by Sigma-Aldrich (Milwaukee, WI) and the HNO3 was of Optima grade from Fisher Scientific (Pittsburgh, PA). All other reagents were of analytical grade and used as received. Aqueous solutions of 3 M HNO3 and 3 M LiNO3 were prepared using ultra-high-purity water (18.2 MΩ cm). The high ionic strength of the 3 M LiNO3 solution of measured pH = 8 suppressed hydrolysis of the dilute solutions of Ce(III). The integrity of the initial aqueous solutions was demonstrated by X-ray absorption spectroscopy (vide infra); the Ce coordination in 3 M LiNO3 and 3 M HNO3 was equivalent. Solvent Extraction Procedure. Aliquots of aqueous phases containing 3 M HNO3 or 3 M LiNO3 and varying Ce(III) concentrations were equilibrated with equal aliquots of organic phases (0.5 M DMDBTDMA in n-dodecane) at 23 ± 0.5 °C by vortexing in screw-cap glass test tubes for 30 min. For aqueous Ce concentrations lower than that corresponding to the limiting organic concentration [LOC; i.e., the highest Ce(III) concentration attainable in the organic phase without phase splitting], 2 mL of each phase was contacted. After equilibration, the phases were separated by centrifugation (5 min at 3300 rpm). Aliquots of the various phases were withdrawn for analysis. For the LOC value determination, 1.5 mL of each phase was contacted under conditions where a third phase was formed. At this point, the LOC condition was determined by adding very small volumes (10 to 20 μL) of fresh organic phase and aqueous diluent (3 M HNO3 or 3 M LiNO3) until the third phase disappeared as indicated by the absence of turbidity in the organic phase after further equilibration and centrifugation. Beyond the LOC value, larger volumes (5 mL of each phase) were contacted to get a sufficiently large volume of third phase for further analyses. Initial aqueous cerium concentrations, [Ce]aq,init, ranging from 0 to 0.1 M were explored, allowing for three pre-LOC, three post-LOC, and the LOC points to 5989

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capillary-to-capillary variation, and each of the diluent volume subtractions was evaluated to reveal shot-to-shot agreement of I(Q) with less than ±2% differences over all Q for each solution. Data Interpretation. The GIFT method34−36 was used to obtain pair−distance distribution functions (PDDFs; structure information in real space) from scattering data in Q-space. GIFT interprets the globular particle system, I(Q) = nP(Q) S(Q), where P(Q) is the average form factor (corresponding to the shape and size of the scattering particles), S(Q) is the average structure factor (from interparticle interactions), and n is the number of particles per unit volume. P(Q) is the Fourier transformation of its real space counterpart, p(r), according to eq 3: P(Q ) = 4πr

∫0

∞

p(r )

sin Qr dr Qr

(3)

This means that, to deduce p(r), the inverse Fourier transformation (IFT) of an experimental P(Q) must be calculated. In concentrated interacting systems, such as those involved in solvent extraction, the structure factor, S(Q), must be modeled and subtracted from the scattering data, I(Q), to give P(Q) from which p(r) is derived. The selection of the appropriate structure factor model is key to achieve coherent PDDFs from GIFT, and the model selected in the present study was the Percus−Yevick (PY) closure relation41 that has been shown to closely approximate the interaction effects of micelles in the Baxter model studies (see above). More precedence for the applicability of the PY closure relation structure factor model to the present study is given by a string of recent publications by Glatter et al. and Shrestha et al., who used the GIFT in combination with the PY closure relation to interpret SAXS data from interacting nonionic surfactant reverse micelle systems in nonpolar media (similar to solvent extraction organic phases), where they observed the selfassembly of reverse micelles into higher-ordered architectures.29−33,42,43

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RESULTS AND DISCUSSION Solvent Extraction. To assess the effect of acid on the Ce− malonamide system, extraction experiments were performed from neutral (3 M LiNO3) and acidic (3 M HNO3) aqueous nitrate media and the results directly compared. Nitrate concentration was constant, while Li + was considered “innocent” as its hydration enthalpy is high enough to facilitate virtually complete dissociation from nitrate at this concentration,44 and its extraction by the malonamide is negligible.45 Large differences in the distribution of active species between phases for the two systems were therefore attributed to the presence of acid. Figure 3 shows the extraction of cerium, water, and HNO3 in the acidic and neutral systems, tracking the organic phase concentrations as a function of initial aqueous cerium concentration, [Ce]aq,init. The branching of the curves with increasing [Ce]aq,init illustrates the transition from biphasic to triphasic system chemistry wherein, for the latter, the organic phase splits in two for both the neutral and acidic media. In the simplified phase diagrams of Figure 3, the lower branches beyond the LOC points describe the light organic phases and the upper ones describe the heavy organic phases (third phases). In Figure 3a, the pre-LOC Ce distribution shows that extraction is more favorable from acidic than from neutral media, so that DCe is higher (1.7−2.6 in the acidic versus 0.5− 1.3 in the neutral). The LOC is lower in the acidic system, with phase splitting occurring at an organic Ce concentration of 20.0 versus 23.3 mM in the neutral system. These two factors are reflected in the large difference in initial aqueous Ce(III) concentration needed to induce phase separation in the two systems (26.5 mM for acidic, 54.6 mM for neutral).

Figure 3. Effect of increasing initial aqueous Ce concentration, [Ce]aq,init (M), on extraction by 0.5 M DMDBTDMA in n-dodecane at 23 ± 0.5 °C. (a) Extraction of Ce in the acidic system (red diamonds and lines) and neutral system (blue squares and lines); LOC points are indicated by dashed lines. (b) Extraction of water in the acidic system (red diamonds and lines) and neutral system (blue squares and lines). (c) Extraction of water (cyan diamonds and line) and HNO3 (purple triangles and line) in the acidic system. All ordinates are on logarithmic scales. Lines between data points serve as a guide for the eye.

In terms of the simplistic equilibria-based extraction model for trivalent lanthanides understood by eq 1, the larger DCe values for the acidic system relative to the neutral system are counterintuitive. Acid extraction would be expected to compete with Ce(III) for the basic electron-donor sites on the extractant via protonation of the malonamide according to eq 2 and should therefore retard Ce(III) loading. Indeed, significant protonation of the DMDBTDMA carbonyl oxygens by nitric acid has been shown to occur by Lefrancois et al.46 The increase in DCe caused by the presence of acid must therefore be understood in wider terms than just short-range phenomenon wherein the active electrophilic entities (acid and cerium) compete for the nucleophilic sites of the extractant. Figure 3b compares the extraction of water at various Ce concentrations in the acidic and neutral systems. The presence 5990

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Figure 4. Ce L3-edge k3χ(k) EXAFS (left) for trivalent Ce(III) in the neutral and acidic media and (right) their corresponding FT data: (a, b) aqueous phase and (c, d) organic phase. Acidic system (red lines): aqueous = 20 mM Ce(III) in 3 M HNO3; organic = 16 mM Ce(III) in 0.5 M DMDBTDMA−n-dodecane. Neutral system (blue lines): aqueous = 40 mM Ce(III) in 3 M LiNO3; organic = 18 mM Ce(III) in 0.5 M DMDBTDMA−n-dodecane.

in the organic phase as [Ce]aq,init increases (Figure 3c). In the neutral system, the micelles are not complementary for Ce accommodation so that substantial reorganization and hydration of the organic phase is needed to incorporate the metal ion, a process that would be disfavorable to extraction. Structural Studies. As selected chemical phenomena relevant to third-phase behaviors in this system have already been explored,47,48 the scope of the present study was to investigate the micro- and macromolecular structural environments in the reverse micellar region and to link them to the extractive properties. Microscopic Perspectives. EXAFS and FT-IR spectroscopies were employed to explore the micromolecular structures of extracted moieties, focusing on the coordination environment of cerium in the reverse micellar phases formed upon extraction from both neutral and acidic aqueous phases as well as a comparison with the initial aqueous speciation. The k3χ(k) EXAFS data and the corresponding Fourier transforms (FTs) for the acidic and neutral phases with Ce(III) concentrations close to the LOC are plotted in Figure 4a−d. The oxygen-phase-shift-corrected FTs (Figure 4b,d) all show one intense peak at 2.52−2.55 Å of physical significance attributed to the nearest O neighbors.49 Whereas the FT data for the acidic and neutral aqueous solutions are identical in all respects (even the two weak distant peaks at ca. 3.7 and 4.8 Å are superimposableconsistent with the predominant solution species, [Ce(OH2)9]3+),50−52 the FTs of the organic phases are significantly different from one another. The principal Ce−O peaks do not match and a new peak at ca. 4.2 Å (indicated by a dashed arrow) is evident between the weak distant features that are common to the aqueous FT data. The position of this distant peak is consistent with distal O scattering from bidentate coordinated nitrate ions,53 and the peak intensity is significantly higher and of greater physical significance for the neutral system than for the acidic one. Because of the limited k-

of acid profoundly increases the ability of the solvent to extract water. When extracting from acid, the organic water content remains constant at ca. 0.24 M as [Ce]aq,init is increased to the LOC, whereas a significant increase in water concentration from 0.05 to 0.1 M is observed when the system is neutral. The connection between acid and water extraction may be explained by inferring the extraction of hydronium nitrate ion pairs, suggested in a previous publication as the proton-bearing species in DMDBTDMA organic solvents.46 Another hypothesis has its roots in supramolecular chemistry, where improved water extraction in the acidic DMDBTDMA system may be due to the formation in the organic phase of reverse micelles with HNO 3 having a polar core in which water is accommodated; i.e., upon the extraction of acid the micelles swell and more water is incorporated. In Figure 3c, the water and HNO3 isotherms for the acidic system are compared. Increasing [Ce]aq,init leads to decreasing acid concentration in the pre-LOC organic phase, while water concentration remains constant. That the water concentration remains constant whereas HNO3 is removed from the organic phase upon Ce extraction indicates that the water is likely contained in the polar core of reverse micelles and is not necessarily bound to the acid. Comparing the change in organic concentrations of Ce (Δ[Ce]) with acid (Δ[H+]) shows that between 1 and 2 mol of acid are displaced per mole of Ce extracted. The solvent extraction data can be understood by invoking the micellar model. According to this interpretation, the organic phase after contact with acidic media preorganizes into hydrated reverse micelles that are complementary for Ce accommodation. By displacing acid from the polar core of the reverse micelles, Ce is incorporated into the micellar structure of the organic phase without the need for supramolecular rearrangement and water extraction. This hypothesis is supported by the observed decrease in nitric acid concentration 5991

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obtained therein, in organic media, especially in nonpolar aliphatic diluents (e.g., n-dodecane) employed for solvent extraction, the monodentate and bidentate modes of nitrate coordination become of similar energy.62 In this regard, monodentate nitrate coordination is found in a variety of solid-state structures;63−66 an archetype is molecular complex [Ce(NO3)5(H2O)2]2‑, in which there are two monodentate nitrates and three bidentate nitrates.67 In this study with DMDBTDMA, the acidity of the aqueous phase was found to influence nitrate denticity in the organic phase so that neutral media gave more bidentate nitrate character than the acidic media in the loaded organic solvent. Two lines of evidence support this: (1) the long Ce−O distances and (2) the characteristic peak at ca. 4.2 Å in the FT data for the neutral system (blue line, Figure 4d) that is attributed to the distal oxygen atom of nitrate. This distal oxygen backscattering for the acid media extraction system (red line, Figure 4d) is weak and near the background noise level in the data, suggesting that the nitrate coordination is largely of monodentate nature, a result that is corroborated by vibrational spectroscopy (vide infra). FT-IR spectroscopy was used to further explore the micromolecular interactions and organic phase structures. Table 2 lists key vibrations and their assignments (see

range available with Ce L3-edge EXAFS, it is not practical to fit all the high r features in the FT data with a multishell, multiparameter model. Instead, we turn our attention to the details obtained from the conservative one-shell nonlinear leastsquares curve fitting analyses of the large and intense O backscattering peaks. The fits are shown in the Supporting Information (Figures S1−S4). The results of the metrical analyses are collected in Table 1. Table 1. One-Shell (Ce−O) Fits to the k3χ(k) EXAFS (Figure 4a,c) for the Ce(III) Complexes with DMDBTDMAa solution

[Ce(III)]/ mM

r /Å

CN

σ /Å

ΔE /eV

aq neutral aq acid org neutral org acid

40 20 18 16

2.54(1) 2.54(1) 2.53(1) 2.51(1)

8.6(8) 9.2(9) 7.9(11) 8.3(8)

0.011(2) 0.011(2) 0.011(2) 0.011(2)

−1.5(7) −1.7(7) −2.0(9) −2.3(8)

a

Values shown in parentheses are the esds at the 95% (3σ) confidence level. Interatomic distances = r. Coordination numbers = CN. Debye− Waller factors = σ. Energy threshold values = ΔE.

In all of the samples in Table 1, Ce(III) is between eight and nine coordinate with O. In the aqueous phase, the average EXAFS-determined Ce−O bond lengths observed in the neutral and acidic systems are equivalent at 2.54(1) Å. The predicted Ce−O distances, based upon the sum of ionic radii determined by Shannon8 for O2‑ (IR = 1.35 Å) and Ce(III) at eight (IR = 1.143 Å) or nine (IR = 1.196 Å) coordinate, are 2.493 and 2.546 Å, respectively. The measured Ce−O distance in the aqueous phase in Table 1 is within error of the predicted distance for nine-coordinate Ce, whereas the predicted distance for eight-coordinate Ce significantly shorter. This lends precedent to the nine-coordination aqueous Ce(III) model recently proposed by D’Angelo et al.54 The equivalent Ce−O distance suggests that the Ce(III) speciation in the aqueous phase is not affected by acid, so that differences in extractive properties are likely due to organic phase behaviors and not aqueous metal complex speciation. In the neutral organic phase, the average EXAFS-determined Ce− O bond lengths are slightly shorter than in the aqueous phase at 2.53(1) Å, and the acidic organic phase is shorter still at 2.51(1) Å. This shortening of the Ce−O distance in the organic phase with respect to the aqueous is consistent with the replacement of water with amide oxygen donors, which are stronger Lewis bases and therefore form stronger and shorter dative bonds.55 Support for this structural motif comes also from crystallography literature that, for 8−10-coordinate Ce(III), gives typical Ce−O distances of ca. 2.50 Å for coordinated water56−60 compared to 2.40−2.46 Å for diamide.61 The different EXAFS determined Ce−O distances in the acidic and neutral organic phases indicate that the organic Ce(III) coordination environment is influenced by the presence of acid. Upon extraction of the nonaaqua cation, the species that obtain in the organic phases are charge neutral complexes containing bound nitrate as well as extractant and water with the general composition [Ce(NO3)3(MA)2(H2O)n]. From previous work with DMDBTDMA11,48 and its congener DMDOHEMA,49 the inner-sphere oxygen bonding to Ce(III) arises from a mixture of mono- and bidentate nitrates, one−two water molecules, and two bidentate MAs. Whereas bidentate nitrate coordination is preferred in aqueous media and salts

Table 2. Key Vibrational Bands (cm−1) of 0.5 M DMDBTDMA in n-Dodecane after Contact with Aqueous Solutions Containing 20 and 40 mM Ce(NO3)3·6H2O in 3 M HNO3 and 3 M LiNO3, Respectivelya acidic system 1637 (m) 1612 (m) 1436 (w) 1324 (w) 1030 (w) 950 (m)

neutral system 1637 1612 1476 1304 1030

(m) (m) (w) (w) (w)

assignment unbound CO stretch bound CO stretch bound O−N−O asym stretch bound O−N−O sym stretch bound N−O stretch NO−H stretch

a

Letters shown in parentheses provide a qualitative indication the peak magnitude: weak (w) and medium (m).

Supporting Information for vibrational spectra, Figures S5 and S6) for the same near-LOC organic phases that were analyzed using EXAFS. In both the neutral and acidic systems, a sharp peak at 1612 cm−1 appears after contact; it was assigned to the Ce(III)bound malonamide CO stretch (protonation of the carbonyl oxygen also results in a CO stretch peak at lower wavenumbers; however, this peak is very broad and not easily distinguished). This supports the hypothesis that one (or both) of the malonamide carbonyl oxygen(s) is directly bound to Ce in an inner-sphere manner, consistent with the shortened average Ce−O distance determined in the organic phase EXAFS relative to the aqueous phase. Replacement of water from the inner-coordination sphere of Ce(III) with the more basic malonamide oxygens drives extraction in this system, underpinning the micromolecular structure in the organic phase. Malonamides are bidentate ligands and both carbonyl oxygens are involved in metal coordination during extraction.68 However, not all of the malonamide molecules involved in extraction coordinate to Ce(III) in an inner-sphere fashion.11 Before Osseo-Asare introduced the micellar model to solvent extraction systems, Musikas and co-workers already recognized the importance of aggregation in solvent extraction systems and 5992

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solvent and the aliphatic hydrocarbon chains of the extractant are almost identical,76 SAXS only detects the polar core of the reverse micelles.42 Figure 5 shows the background-subtracted

posited that the mechanism of extraction of f-block elements with aliphatic malonamide involves both inner-sphere and outersphere extractant−metal complex interactions.9,69 Three FT-IR peaks are listed in Table 2 that correspond to Ce(III)-bound nitrate. Of particular relevance is the O−N−O asymmetric and symmetric stretches, as these are diagnostic of nitrate denticity.70−72 A large O−N−O asymmetric/symmetric stretch peak separation, Δν, indicates bidentate nitrate character;49 Δν for the neutral system (172 cm−1) is greater than Δν for the acidic one (112 cm−1), supporting the EXAFS result that suggests bidentate nitrate coordination in the neutral system and essentially monodentate coordination in the acidic one. Raman studies of aqueous Ce(III)−nitrate systems by Nelson et al. report the O−N−O symmetric stretching band at 1325 cm−1 for purely monodentate nitrate, which shifts to 1300 cm−1 when one nitrate becomes bidentate.73 These peak positions are remarkably close to those found for the acidic and neutral organic phases for the O−N−O symmetric stretch in Table 2 so that one might infer purely monodentate nitrate coordination in the acidic system and mixed denticity in the neutral. The effect that acid has on the denticity of nitrate can be understood in terms of the micellar model. The solvent extraction data (Figure 3) shows that the presence of acid significantly increases the hydration of the organic phase so that much more water is incorporated into the micelle cores. Coexistence of water and/or nitric acid in the same local micellar environment as the extracted Ce(III) complex could affect the denticity of nitrate in two different ways: (i) by occupying metal coordination sites and (ii) by hydrogen bonding to the metal nitrate in an outer sphere manner. Both of these interactions would favor monodentate nitrate coordination as water and nitric acid would either compete with nitrate for metal coordination sites or compete with the metal for nitrate coordination sites by H-bonding to the nitrate oxygens of the outer sphere. With less H2O and no HNO3 in the organic phase after extraction with 3 M LiNO3, the Ce(III) ions are starved for electron density in forms other than NO3−. In Table 2, the last IR absorption band is at 950 cm−1, and it is observed only in the acidic system. According to a study by Ferraro and Chiarizia74 on the extraction of nitric acid by TBP, this band corresponds to the H−NO3 stretch for associated nitric acid. In the discussion on the solvent extraction properties in the previous section, two hypotheses were put forward for the increased extraction of water with acid: (i) water and acid are extracted as a hydronium nitrate ion pair and (ii) incorporation of acid into the micellar core causes the micelles to swell and absorb more water. In other words, water could be an associated adduct that extracts with acid or, alternatively, is free from acid coexisting in the organic phase micellar structure. The prominent band at 950 cm−1 supports the latter view, as a hydronium nitrate ion pair would be expected either not to have an NO−H stretching band or the band would be significantly shifted as the covalently bound proton becomes hydrogen bonded. Macroscopic Perspectives. The macromolecular structures formed in solvent extraction systems are dynamic and delicate, being profoundly affected by small temperature variations,28 type of alkane diluent,75 and concentrations of extractant and solute.45 Small-angle X-ray scattering is a useful technique to analyze such systems, and measurements were performed to probe the higher-order structures in the organic phase as a function of Ce(III) concentration in the acidic and neutral systems. As the scattering length density of the n-dodecane

Figure 5. X-ray scattered intensities, I(Q), of the neutral (a) and acidic (b) 0.5 M DMDBTDMA−n-dodecane−Ce(III) extraction systems at various Ce(III) concentrations. The organic phase after contact with 3 M LiNO3 or 3 M HNO3 containing 0 mM Ce(III) is marked by red squares and 10 mM Ce(III) is marked by blue diamonds and the LOC [50 mM Ce(III) for the neutral system and 25 mM Ce(III) for the acidic one] by green triangles.

SAXS results in absolute intensity units for the organic phases before and at the LOC for the neutral and the acidic systems, with varying Ce(III) concentrations. Initial inspection and comparison of the scattering data in Figure 5 gives qualitative information on the structures in the systems. In the acidic regime (Figure 5b), all of the scattering waves converge at high Q, indicating that the size of the scattering polar cores of the micellar units are about the same at all three Ce(III) concentrations.13 This means that, when Ce(III) is extracted from acidic media, the size of the micelle cores does not change significantly. In the neutral regime (Figure 5a), the scattering curves do not converge at high Q, indicating that the cores of the micellar units change in size as Ce(III) is extracted from neutral media. Expanded views of the high-Q region of Figure 5 are provided as Supporting Information (Figure S7). Siphoning quantitative information on the form of the micellar units in interacting systems from SAXS data can be difficult, because the structure factor (the scattering caused by micelle cores coming into close proximity) interferes with the scattering response produced by the size and shape of the aggregates (form factor). This effect is amplified in the low-Q region; the high-Q response is dominated by scattering from the small cores of the individual micelles.77 Therefore, the IFT 5993

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waves tend toward power-law behavior (straight line in the low to mid-Q region on the log−log scale), as this gives qualitative information on the shape of the extended structures.80 The scattering function for the neutral system in the absence of Ce(III) (red markers, Figure 5a) shows that the low-Q region tends to a flat line as Q = 0 is approached (the slight decrease in intensity at 0.01 Å−1 is likely the result of imperfect background subtraction). This indicates Q0 behavior and is suggestive of globular micelles. As the Ce(III) concentration is increased to the LOC, the SAXS waves for both the acidic and neutral systems approach a straight line of gradient −1.7 in the medium to low-Q range of the log−log plots (green markers, Figure 5). This corresponds to the scattering intensity obeying a Q−1.7 power law, which is observed for solutions of flexible polymer chains.81 Therefore, judging by the power-law behavior, as Ce(III) is extracted, small globular units attract and assemble into chainlike structures. The nearly exact power-law behavior for the LOC of the acid system over a wide Q-range (green triangles, Figure 5b) may further indicate a structural ripening into sheetlike arrangements of cylinders.36,82 This contrasts with the mechanism implied by the Baxter model, where micelle units randomly interact according to the behavior expected from uniform hard sticky spheres. The growth of micellar aggregates with Ce(III) extraction is better visualized in real space using the GIFT PDDF plots (shown in Figure 6). The GIFT line fits to the experimental data and the corresponding structure and form factors are presented in the Supporting Information (Figures S8−S10). The PDDF for the neutral system without Ce(III) (red curve in Figure 6a) is bell-shaped with an extension of the tail up to r

of this latter region gives real space information selective to the micellar units, ignoring the extended structure (the structure formed from adhesion of micelle cores) carried in the low-Q data. Precedence for this is given by Glatter et al., who used the “cutoff” IFT technique (selecting only the data at high-Q for IFT) to gain information on the micelle form in interacting surfactant systems.34,78 In SX systems, the unit structures are small globular micelles that interact. By taking the IFT of the high Q region from the point at which the PDDF converges toward that of globular micelles,79 the radius of gyration, Rg, corresponding to the micelle unit size is obtained, as shown in Table 3. Table 3. Rg Values for the Unit Micelles in the Acidic and Neutral Organic Phases at Various Ce(III) Concentrations Determined Using the Cutoff IFT Technique of Glatter et al.34,78 (for 0.1 Å−1 ≤ Q ≤ 0.6 Å−1) along with the Metrical Results from Fits with a Cylinder Form Factor (in terms of radius, R, and length, L) to the High-Q Data Onlya,b system acid

neutral

[Ce]aq,init/mM

Rg/Å

R

L

0 10 25 0 10 50

10.6(1) 11.2(1) 11.3(1) 7.3(1) 8.0(1) 10.1(3)

6.8 7.2 7.3 5.8 6.0 6.8

125 293 607 53 92 294

a

See Supporting Information, Figure S11. bValues shown in parentheses are esds for Rg; the errors on R and L are ±2% relative.

The micelles formed in the neutral system without any Ce(III) have an Rg of 7.3 Å. Using the Baxter model to interpret SAXS data from solutions of 0.5 M DMDBTDMA in n-dodecane after contact with water, Erlinger et al. found micelles with a polar core radius of 6.5 Å, which corresponds to an Rg of 5.0 Å for spheres.13,14 The larger Rg determined in the present study using the GIFT method could be an indication that the micelles cannot be described as spheres but are more globular, covering a range of elliptical forms, which is more befitting of soft, fluid systems. As the Ce(III) concentration is increased to the LOC, the Rg increases by nearly 3 Å, corresponding to a 39% growth in polar core radius for the neutral system. In contrast, the acidic system micelles are already swollen even without Ce(III), giving an Rg of 10.6 Å, and increasing the Ce(III) concentration up to the LOC results only in a modest 7% micelle core growth. The observed micelle growth in the neutral system versus the larger micelles of comparatively constant size in the acidic system is linked to the solvent extraction data where contact with acid caused hydration of micelles that was independent of Ce extraction. Acidification and subsequent hydration of the organic phase causes swollen micelles, which are associated with increased Ce(III) extraction. The size of micelles in the neutral system is always smaller than those observed in the acidic system. In the micromolecular structural study, the difference in nitrate denticity was attributed to the presence of more water in the micellar structure in the acid system, and incorporation of more water into the micelle core is consistent with larger micelles.45 The SAXS data shown in Figure 5a,b also give an indication of the extended structures of the aggregates formed from the attraction and adhesion of micellar cores. The first step is to inspect the scattering curves in Figure 5 to judge whether the

Figure 6. Pair distance distribution functions (PDDFs) determined by GIFT for the neutral (a) and acidic (b) 0.5 M DMDBTDMA−ndodecane−Ce(III) extraction system at various Ce(III) concentrations. The organic phase after contact with 3 M LiNO3 or 3 M HNO3 containing 0 M Ce(III) is red and 0.01 M Ce(III) is blue and the LOC (0.05 M Ce(III) for the neutral system and 0.025 M Ce(III) for the acidic) is green. 5994

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Figure 7. Graphic summary of the effect of acid on the extractive properties and micro- and macro-molecular organic phase structures of the Ce(III)−malonamide system (neutral system is blue, acidic system is red). Each step, 1N to 3N and 1A to 3A, is discussed in the text.

and tail similar to the PDDF of the acidic system without Ce(III) (red curve in Figure 6b). As the critical point was approached, a second peak began to develop, similar to that observed at the neutral LOC (green curve in Figure 6a) or the acidic intermediate (blue curve in Figure 6b) PDDFs. This second peak was attributed to interaction between cylindrical aggregates so that not just the micellar units but the superaggregates which they constitute interact. Just before the critical point, this second peak becomes very broad, remarkably similar in shape to the acidic LOC PDDF (green curve in Figure 6b). In the Glatter paper,36 this broadening of the second peak was attributed to strong interactions between cylindrical aggregates, preluding and perhaps initiating phase transition after the critical point.

= 70 Å. Bell-shaped PDDF curves are typical of globular micelles and the asymmetry in the shape shows a transition from globular to short rodlike aggregates, where the maximum extension, rmax, corresponds to the maximum length of these rods (70 Å for the neutral system without Ce(III)).83 Increasing Ce(III) concentration in the neutral system increases rmax to 100 Å (blue curve in Figure 6a) and then 300 Å at the LOC (green curve in Figure 6a), generating PDDFs that are typical for wormlike micelles, showing a pronounced peak in the low-r regime and an extended tail in the high-r regime.83 The results obtained from fits to the I(Q) data with a cylindrical form factor (Supporting Information, Figure S11a) provide cylinder lengths (L, Table 2) that agree with the PDDF response. The increased height of the PDDF curve for the LOC is from increasing contrast arising from high Ce(III) and water concentrations (Figure 3). The PDDF curves for the acidic system show that, without Ce(III), the micelles are already assembled into long chains of up to 140 Å (red curve in Figure 6b). Adding Ce(III) increases the length of these chains to ca. 290 and 600 Å at the LOC (blue and green curves in Figure 6b, respectively). These values are corroborated by the cylinder lengths (L, Table 2) obtained by fits to the I(Q) data with a cylinder form factor (Supporting Information, Figure S11b). For the LOC system, a second, broad and diffuse peakreminiscent of growth in two dimensions82with an inflection point of ca. 45 Å becomes prominent in the PDDF. Glatter et al. observed the development of a similar broad second peak in the PDDF of a nonionic surfactant system near its critical point, analogous to the LOC.36 The nonionic surfactant, in a similar manner to the solvent extraction system in the present study, formed small globular interacting micelle units that self-assembled into cylinders, yielding PDDF curves showing a single sharp peak

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DISCUSSION SUMMARY AND CONCLUDING REMARKS The findings in this study on the effect of acid on the extractive properties of the Ce(III)−malonamide system and the relation to micro- and macromolecular organic phase structures are summarized in Figure 7. Initially, in the neutral system, small, poorly hydrated micelles predominate. Upon Ce(III) extraction, these micelles swell and incorporate more water (step 1N, Figure 7). Hydration of the organic phase and subsequent reorganization of the supramolecular structure into larger micelles disfavors Ce(III) extraction. In contrast, the acidic system initially contains large hydrated micelles that incorporate associated HNO3 molecules. Ce(III) enters into these swollen, hydrated cavities, displacing acid (step 1A, Figure 7). This process does not require extracting more water into the organic phase and the impact on the micellar structure is minimal. Prehydration and preorganization of micellar assemblies by acid favors 5995

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Ce(III) extraction, which is reflected in the relative DCe values of the acidic and neutral systems. Although acid is apparently not directly involved in the extraction mechanism outlined in eq 1, its presence impacts the Ce(III) coordination chemistry, namely, nitrate coordination, which is essentially monodentate in the acidic system and bidentate in the neutral one. The difference in nitrate coordination was attributed to the presence of water and/or nitric acid that would stabilize the metal complex in the micellar environment either by satisfying the inner-coordination sphere via direct metal coordination or by interacting with the nitrate in the outer-coordination sphere. In the neutral system, incorporation of Ce(III) into the micellar structure and the resulting increased attraction between cores caused the growth of micelle chains (step 2N, Figure 7), which were identified and monitored using SAXS data interpreted with GIFT. In the acidic system, the solution contained sizable chains even without Ce(III). These chains grew as [Ce(III)] increased toward the LOC (step 2A, Figure 7) and an interaction peak, attributed to attraction between micellar chains, was observed at the LOC point. The growth of micelle chains was associated with third-phase formation, and the presence of acid clearly promoted their growth. This was reflected in the solvent extraction study that showed that thirdphase formation happens more readily (lower LOC) in the acidic system (step 3A, Figure 7) than for the neutral one (step 3N, Figure 7). This multipronged approach of conducting a simultaneous investigation into both the coordination and supramolecular structures of the Ce(III)−malonamide solvent extraction system afforded new insights into how extracted solutes affect organic phase assemblies. In particular, the results have shown that the structure of macromolecular architectures formed as a result of micellar coagulation is more ordered than previously thought and that extraction of solutes stimulates one-dimensional growth of chains that increase in length and begin to interact in a second dimension as the LOC is approached. This study serves as a foundation stone to a new approach for looking at the structure and properties of solvent extraction systems and bridges the gap between soft matter chemistry and industrial separations science, giving new insights into how acid affects coordination chemistry within reverse micelles and the macromolecular architectures that prelude phase transition.

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Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences, under contract No DE-AC0206CH11357. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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ASSOCIATED CONTENT

S Supporting Information *

EXAFS and FT-IR spectra as well as SAXS data. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Drs. Laurence Berthon and Manuel Miguirditchian from the CEA, who generously provided the DMDBTDMA, and our Argonne colleague Dr. Renato Chiarizia and Laura D’Amicoa student from ParisTech, Chimie Paris, France for assistance during various stages of this research. This work and the use of the Advanced Photon Source are supported by the U.S. Department of Energy, Office of Science, Office of 5996

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