DOI: 10.1002/chem.201403859

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& Solvent Extraction |Very Important Paper |

Complexation-Induced Supramolecular Assembly Drives Metal-Ion Extraction Ross J. Ellis,*[a] Yannick Meridiano,[b] Julie Muller,[b] Laurence Berthon,*[b] Philippe Guilbaud,*[b] Nicole Zorz,[b] Mark R. Antonio,[a] Thomas Demars,[a] and Thomas Zemb[c]

Abstract: Combining experiment with theory reveals the role of self-assembly and complexation in metal-ion transfer through the water–oil interface. The coordinating metal salt Eu(NO3)3 was extracted from water into oil by a lipophilic neutral amphiphile. Molecular dynamics simulations were coupled to experimental spectroscopic and X-ray scattering techniques to investigate how local coordination interactions between the metal ion and ligands in the organic phase combine with long-range interactions to produce spontaneous changes in the solvent microstructure. Extraction of the Eu3 + –3(NO3 ) ion pairs involves incorporation of the “hard” metal complex into the core of “soft” aggregates. This seeds the formation of reverse micelles that draw the

Introduction Amphiphiles allow hydrophilic and lipophilic environments to coexist within solution by assembling into nanoscale structures that stabilize the mutually exclusive domains. Such systems are understood as “soft matter”, where weak forces beyond the first neighbor control the solution microstructure. In the case of solvent-rich ternary or quaternary solutions with low water content, physical properties can be derived from knowledge of the phase diagram, microstructure, and surfactant film properties.[1] In contrast, metal-ion coordination complexes are understood from concepts rooted in organometallic chemistry, considering metal–ligand complexes as “hard” science, where strong and directional dative interactions bind electron donor [a] Dr. R. J. Ellis, Dr. M. R. Antonio, Dr. T. Demars Chemical Sciences & Engineering Division Argonne National Laboratory, Argonne, IL, 60439 (USA) E-mail: [email protected] [b] Dr. Y. Meridiano, Dr. J. Muller, Dr. L. Berthon, Dr. P. Guilbaud, Dr. N. Zorz Nuclear Energy Division, Radiochemistry and Processes Department, CEA 30207 Bagnols–sur–Ceze (France) E-mail: [email protected] [email protected] [c] Prof. T. Zemb Institut de Chimie Sparative de Marcoule, UMR5257 CEA/CNRS/UM2/ENSCM 30207 Bagnols–sur–Cze (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201403859. Chem. Eur. J. 2014, 20, 12796 – 12807

water and “free” amphiphile into nanoscale hydrophilic domains. The reverse micelles interact through attractive van der Waals interactions and coalesce into rod-shaped polynuclear EuIII-containing aggregates with metal centers bridged by nitrate. These preorganized hydrophilic domains, containing high densities of O-donor ligands and anions, provide improved EuIII solvation environments that help drive interfacial transfer, as is reflected by the increasing EuIII partitioning ratios (oil/aqueous) despite the organic phase approaching saturation. For the first time, this multiscale approach links metal-ion coordination with nanoscale structure to reveal the free-energy balance that drives the phase transfer of neutral metal salts.

ligands to metal cations.[2] Yet, despite these scientific distinctions, many important natural and technological systems combine coordination metal-ion complexes within aggregating soft-matter systems. Examples include the synthesis of nanoparticles[3] and mesomaterials[4] as well as the transfer of metal ions through lipid bilayers that underpins biology.[5] Of central relevance to all of these technologies is the amphiphile-facilitated interfacial transfer of metal ions between hydrophilic and hydrophobic regions/phases—a process that is analogous to solvent extraction. Solvent extraction is the amphiphile-facilitated transfer of a coordinating metal ion from an aqueous phase, through the water–oil interface, and into a water-poor microemulsion.[1, 6] This process, effectively segregating metal ions on a tie line in a ternary phase diagram,[7] is ubiquitous in industrial metal refining.[8–10] Separation is achieved by mixing a hydrocarbon oil containing an amphiphilic “extractant” with an aqueous phase bearing dissolved metal salts. The extractant binds selectively to the target metal ion or salt and draws it into the oil, thus removing it from the aqueous phase: the driving force is the step of free energy between water-rich and water-poor coexisting phases.[11] The result is a solution of hard metal complexes in soft amphiphile-in-oil aggregates, present either as discrete free aggregates or loose continuous networks.[12] Apart from being of interest to the field of metal-ion separations, solvent extraction systems provide a general opportunity to understand how metal ions impact the hierarchical structure

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Full Paper of hydrophobic soft-matter systems and how this influences transfer between phases. Before the recognition of cooperative processes due to extractant self-assembly linked to the free energy of extraction,[11] the properties of solvent-extraction systems have traditionally been understood from concepts rooted in coordination chemistry,[13] where the structural focus is on the dative interactions that connect the extractant amphiphile to the hard metal center.[14] More recently, however, the soft-matter-like behavior of the extractant amphiphiles—such as tri-n-butyl phosphate (TBP),[15] and N,N,N’,N’-tetraoctyl-3-oxapentanediamide (TODGA),[16]—has also been shown to underpin important properties. In these studies, in the dilute part of the phase diagram, extractants are understood to be aggregated into the form of reverse micelles that interact with increasing attractive potential as polar solutes are extracted. This interaction between aggregates was shown to underpin a hidden liquid–gas type transition common to all van der Waals fluids known as “third phase formation”, where the organic phase spontaneously splits into two domains at high solute concentrations.[17] Solvent extraction systems have, therefore, both hard- and soft-matter character where structures and interactions between nearest neighbors and long-range interactions help define the emergent extractive properties. Understanding how metal ions are solvated in the hierarchical solutions formed in amphiphile–oil microemulsions is an important step in defining the complex energetics of solvent extraction separation systems. This could improve the efficacy of metal separations in an array of processes that are of societal importance, including the efficient reprocessing of nuclear waste and the refining of rare-earth elements. In a broader sense, there is a dearth of knowledge regarding the hierarchical nature of supramolecular mesostructures that involve the solvation of metal ions within reverse micellar structures in general. These systems, which hinge on both short-range coordination bonds and long-range interactions, are of central relevance, not just in established technologies, such as solvent extraction or detergents, but also in a range of emerging applications, such as controlled synthesis of nanoparticles[18] and drug delivery by adsorption via controlled nanometric rafts in the rapidly growing area of nanomedicine.[19] Herein we explore the effect of metal-ion concentration of the hierarchical structure of an amphiphile–oil solvent by combining experimental measurements with molecular dynamics (MD) simulations. In this way, we utilize tools that we have recently developed combining X-ray scattering techniques with small-scale MD simulations to define the structure of reversemicelle-type aggregates formed by extractant molecules.[20] We have increased the scale of the simulations and combined it with a comprehensive experimental study to resolve a detailed system-wide structural image of metal-complex coordination and self-assembly in a solvent extraction organic phase and observed how this image evolves in response to metal-ion extraction. The solvent extraction system chosen for investigation was the malonamide extractant amphiphile DMDOHEMA (N,N’-dimethyl-N,N’-dioctylhexylethoxymalonamide, Figure 1) in n-hepChem. Eur. J. 2014, 20, 12796 – 12807

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tane, which has current technological relevance to nuclear-fuel reprocessing where it is used to extract actinide and lanthanide nitrate salts from aqueous Figure 1. The structure of amsolutions.[21] The behavior of phiphilic malonamide extracDMDOHEMA in hydrocarbon sol- tant N,N’-dimethyl-N,N’-diovents is well defined from solvent ctylhexylethoxymalonamide extraction, as well as structural and (DMDOHEMA). physical studies,[22] making it a good candidate for this investigation combining organometallic, supramolecular, and colloidal chemistry. We are particularly interested in how the presence of the metal ion in this water-in-oil microemulsion system perturbs the hierarchical phase structure, and how this in turn influences the distribution of metal ions between the coexisting oil and water phases.

Results and Discussion Solvent extraction The interfacial transfer behavior of Eu(NO3)3 between the aqueous phase and oil (DMDOHEMA–heptane) was investigated by mixing the biphasic system with increasing concentrations of Eu up to the point at which the organic phase collapsed into extractant-rich and extractant-poor regions via a liquid–gas transition driven by van der Waals interactions (a phenomenon known as third phase formation).[23] Figure 2 shows how EuIII distribution between oil and water phases (quantified by the value DEu = [Eu]org/[Eu]aq) changes as a function of Eu concentration. An initial increase in DEu is observed in the low-tomedium Eu concentration range, followed by a slight decrease at very high concentrations.

Figure 2. Variation of DEu = [Eu]org/[Eu]aq as a function of aqueous-phase EuIII concentration ([Eu]aq, M) corresponding to sample 1 to 7 reported in Table 5. Dashed line is a guide to show the general trend.

Such behavior is difficult to explain by referring to equilibria models involving the transfer of metal ions into the organic phase as well defined, discrete stoichiometric malonamide– metal “complexes”. However, Chiarizia and co-workers have observed similar nonlinear phenomena in ion-extraction studies. In this case, the increasing distribution ratios with metal concentration were attributed to the formation of polynuclear

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Full Paper metal complexes in the organic phase, which competes with saturation at high concentrations as the free-extractant concentration diminishes.[24] This kind of buffered mechanism, involving the competition between polynuclear metal complex formation and saturation, could explain the domed profile of the distribution data in Figure 2. Deriving quantitative extraction equilibria models to fit data from such complex systems is difficult and beyond the scope of this structure-focused study, but a more detailed discussion of the solvent extraction data is given in the Supporting Information, SI5. EuIII coordination structures: hard-matter perspective The focus of this study was to understand how the structural roots and interactions drive the nonclassical extractive behaviors at increasing EuIII concentrations as indicated by DEu in Figure 2. Towards this end, we conducted a multiscale investigation into the structural hierarchies incorporating both the hard inner-coordination sphere EuIII complex and the soft aggregated solvent. Extraction of EuIII into the organic phase is classically understood in colloidal chemistry through the multiple-equilibrium model,[25] whereas in supramolecular and coordination chemistry, primary consideration is given to the coordination of a metal ion by the electron-donor ligands available as first neighbors in solvation layers (in this case DMDOHEMA, water and nitrate).[14] The coordination structure is one of the terms defining the extractive properties so that evolutions in the coordination environment, while also considering long-range interactions, could explain the nonclassical extraction behavior, that is, deviation from values as expected from known and fixed equilibrium constants. To investigate the coordinating (dative) interactions between the solubilizing ligands and the EuIII in the organic solvent, FTIR spectroscopy is considered first. In Figure 3 we compare the FTIR spectra from the 0.5 m DMDOHEMA solution before (dotted line) and after (dashed and solid lines) EuIII extraction. Of particular interest are the peaks that correspond to the polar malonamide head groups; namely the carbonyl C=O stretch (1600–1700 cm 1) and the amide C N (1400– 1500 cm 1) stretch. These peaks change notably after extrac-

Figure 3. FTIR spectra from solutions of increasing organic-phase EuIII concentrations produced by solvent extraction of EuIII from 3 m LiNO3 into 0.5 m DMDOHEMA in n-heptane: EuIII 0 mm (g), EuIII 60 mm (b), EuIII 230 mm (c). Chem. Eur. J. 2014, 20, 12796 – 12807

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tion of EuIII, especially the carbonyl C=O stretch, showing the coordination of the malonamide amphiphile to the EuIII center through the oxygen donors (this is consistent with previous studies on malonamide–LnIII extraction systems).[26] The other spectral change observed upon EuIII extraction is the manifestation of peaks at about 739, 816, 1030, 1300, and 1480 cm 1 corresponding to the bending/stretching vibrations of coordinated nitrate,[27] showing that an associated europium nitrate complex, coordinated by extractant amphiphiles, pertains in the solvent. Water coordination to EuIII can be studied further in solution by using time-resolved laser-induced fluorescence spectroscopy (TRLIFS). This was conducted for 0.5 m DMDOHEMA solutions formed from extracting EuIII from a 3 m LiNO3 aqueous phase (results published in our recent study).[22d] These results showed that approximately 0.5 molecules of water are coordinated directly to each extracted EuIII center so that approximately 3=4 of the water that is co-extracted with the EuIII (see the Supporting Information, SI4) is in the outer-coordination sphere. These results show that all electron-donor ligands in the organic phase (extractant, nitrate, and water) are involved in the nearest neighbor EuIII coordination shell. To investigate further the immediate coordination environment around EuIII as a function of mole ratio between extractant and europium in the organic phase, EXAFS measurements were taken from both the aqueous (Figure 4 a) and organic (Figure 4 b) phases. The spectra for each series are virtually superimposable, suggesting that little or no change in immediate coordination environment occurs neither in the aqueous nor organic phase as the concentration of europium is increased. The Fourier transforms (FTs) of the EXAFS data give a physical portrayal in real space of the scattering paths, which correspond to the structure of coordinating ligands around the EuIII center. As the EuIII concentration increases, the intensity of the main peaks—corresponding to the coordinating donor oxygen atoms—decrease, which is attributed to the vitiating effects of self-absorption for concentrated solutions probed by fluorescence EXAFS.[28] The positions of the main peaks for the organic and aqueous series do not change significantly with EuIII concentration, and even the minor features remain unaffected, showing that the immediate environment around the EuIII center remains similar whatever point in the phase diagram is considered. This conclusion is supported by UV/Vis spectra from DMDOHEMA solutions with increasing concentrations of NdIII (see the Supporting Information, SI6), suggesting that the trend in partitioning ratio DEu is not driven by the evolution of dative interactions with the first neighbors. A more quantitative understanding of the coordination environment around EuIII can be drawn from the EXAFS data by conducting stepwise fits of increasing complexity. This was conducted only on the low-concentration samples (10 and 2.5 mm Eu in aqueous and organic solutions, respectively) because the coordination numbers (CN) are perturbed by the self-absorption effects at high metal concentration. First, unconstrained curve fitting with one-shell O was conducted to account for the principle scattering peak in the FT-EXAFS data (Figure 4), giving good correspondence between the model

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Full Paper terer after the inner-sphere O.[22d, 30] As expected, two-shell (O and N) modeling of the data gives statistically better fits than the one-shell O model (see the Supporting Information, SI2). The metrics to these fits are presented in Table 2 for the low EuIII concentration organic phase. The fitting gave a nitrate N CN close to 3, showing that the majority of the nitrate counterions are in direct coordination with the EuIII ion. Assuming that the total Eu O distance is reflective of the nitrate O component, the difference in distance between the N and O shells can give an indication of nitrate denticity. For purely monodentate nitrate, a difference between O and N of 1.24  is expected, whereas for bidentate nitrate a distance of 0.44  is expected based on simple geometry. The experiFigure 4. k3c(k) EXAFS and corresponding FT data from a) aqueous (ca. 3 m LiNO3,) and b) organic (0.5 m DMDOHEmental distance difference of MA in n-heptane) solutions of increasing EuIII concentration generated in the solvent extraction experiment (Figure 2). 0.52  suggests a mixed mode of coordination in the organic solvent, in alignment with our previous experimental[22d] and and experiment (see the Supporting Information, SI2). Table 1 compares the metrical parameters from the one-shell O fit to theoretical[31] studies. III the low Eu concentration EXAFS data for the organic and aqueous phases. The aqueous phase result is in good agreeTable 2. Summarized metrics from the two-shell O,N fit to the k3c(k) ment with the calculated and experimentally determined EuIII [29] EXAFS data for europium species in low concentration organic phase CN in water reported in a recent study by Duvail et al. The (sample 3). average Eu O distance for the organic phase (2.434 ) is slightly longer than the aqueous phase Eu O (2.429 ) and the DE [eV] CN r [] s2 [2] III CN of Eu is decreased by 1 in the organic solvent. org (O) 10.4(9) 2.440(4) 0.0134(7) 2.2(3) org(N)

2.6(5)

2.96(8)

0.007(2)

2.2(3)

Table 1. Summarized metrics from the simple one-shell O fit to the k3c(k) EXAFS data for europium species in organic and aqueous phases.

aq(O) org(O)

CN

r []

s2 [2]

DE [eV]

9.6(9) 8.9(9)

2.429(3) 2.434(4)

0.0091(4) 0.011(6)

1.5(3) 2.0(3)

This suggests that the aqueous phase offers a better coordination environment for EuIII than the organic phase, with more numerous and stronger dative interactions that may account, in part, for the low (< 1) partitioning ratios (Figure 2). Nitrate anions must be co-extracted into the organic phase to account for the + 3 charge on the EuIII cation, and the FTIR results suggest that these are coordinated to the metal center. The next step was therefore to fit a second shell to the EXAFS data collected from the organic-phase samples to account for the nitrate N as this is expected to be the next-strongest scatChem. Eur. J. 2014, 20, 12796 – 12807

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To supplement the experimental approach and to provide a visual platform from which to understand the structure of the EuIII complex, MD simulations were conducted. The systems were simulated by using the concentrations (translated to number of molecules per simulated volume) of Eu3 + , NO3 , n-heptane, H2O, and DMDOHEMA found experimentally in the organic solutions after extraction of EuIII. To investigate the effect of EuIII concentration on the system structure, the simulation was performed at low (0.015 m) and high (0.126 m) EuIII concentrations. A typical cluster at low EuIII concentration is shown in Figure 5 a, where nitrate, water, and malonamide all coordinate, as observed in the experimental studies. Table 3 summarizes the average EuIII coordination environment in relation to the other hydrophilic solutes calculated from radial distribution functions (RDFs) centered on the EuIII center for both the dilute and concentrated EuIII-containing systems.

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Full Paper was explored relative to the representative model by making systematic changes to coordination mode, including changing nitrate and malonamide denticity and water CN. The representative model was the only configuration that gave a good fit to the EXAFS data with realistic metrics, supporting the conclusion that the MD simulation, at least at low Eu concentration, is a close representation of the experiment. At low concentrations of EuIII, Table 3 shows that an average of three molecules of DMDOHEMA coordinate to the EuIII center; two through the bidentate mode and one through the monodentate mode. This agrees with our solvent extraction results that also predict three molecules of DMDOHEMA per EuIII at trace levels of metal ion (see the Supporting Information, SI8). This, therefore, supports the validity of our simulation methodology in predicting speciation under these conditions. To balance the positively charged EuIII center, 2.8 molecules of nitrate are coordinated to the inner-sphere with an additional 0.2 molecules dissociated 5  away from the EuIII ion. This small amount of dissociation is also consistent with the Eu N CN calculated from the two-shell model of the EXAFS data, Figure 5. Illustrative examples of EuIII complexes isolated from MD simulawhich was 2.6. In agreement with the EXAFS modeling, the tions (snap-shots) of a) dilute and b) concentrated systems corresponding to the composition of samples 3 and 7, respectively. EuIII center shown by simulation suggests nitrate of mixed denticity at low EuIII conlight-blue sphere, coordinated by nitrate O (blue spheres), malonamide O centration, with an average of 0.7 molecules in the bidentate (green spheres) and water O (red spheres). Other atoms shown as colored mode and 2.1 in the monodentate mode. The CN of water was sticks (red = O, blue = N, grey = C). Average MD cluster model fit to the 0.8, which is within error of the result from the experimental 10 mm c) EXAFS data and d) corresponding FT (solid lines for experimental data, dotted lines for model fit). TRLIFS result (see above) and shows that the majority of the water that is co-extracted into the organic phase with EuIII at a ratio of 2 (H2O):1 (Eu) (see the SupportIII Table 3. Average Eu coordination environment and distances in relation to the other ing Information, SI5) is not directly involved in metal hydrophilic solutes calculated from RDFs centered on the EuIII ion for both the dilute coordination. and concentrated EuIII-containing systems. Ccent is the central malonamide carbon The total number of atoms coordinated in the atom. The bidentate and monodentate coordination modes of DMDOHEMA and niinner-sphere is not changed significantly in the high trate attachment to Eu are indicated. EuIII concentration simulation (CN = 9.2) with respect Molecule Atom Assignment Low conc (sample 3) High conc (sample 7) to the low-concentration case (CN = 8.9). These r CN r CN changes in inner-sphere coordination are relatively O coordinating 2.46 5 2.43 2.7 minor and would not, therefore, be expected to sigbidentate 4 2 4 1 Ccent DMDOHEMA nificantly affect the experimental EXAFS data. The Ccent monodentate 4.6 1 4.6 0.8 major differences occur in the outer-coordination O outer sphere 4.3 0.5 4.4 1.5 O coordinating 2.4 3.4 2.41 5.1 sphere, where the most prominent structural change N monodentate 2.85 0.7 2.84 1.6 is indicated by 1) the presence of a significant peak nitrate N bidentate 3.57 2.1 3.58 1.9 attributable to a Eu···Eu correlation in the RDF (beN dissociated 5 0.2 – – tween 5 and 6.4 , see the Supporting Information, O coordinating 2.43 0.8 2.43 1.1 water O outer sphere 4.7 0.2 4.6 0.6 SI7b) and 2) the increase of bound nitrate above the O outer sphere – – 6.1 1.6 number three (which is required to form neutral III Eu bridged – – 5 1 Eu mononuclear Eu(NO3)3 complexes). This is a result of the formation of binuclear clusters of EuIII bridged by nitrate ions (accounting for the increase in nitrate coordination above three due to the sharing of a 1 charge), as A model cluster was constructed to reflect as closely as posshown in Figure 5 b. Such a distant correlation is impossible to sible the average speciation determined from the RDFs (see resolve in the solution phase EXAFS data obtained at RT. The the Supporting Information, SI7) of the low-concentration MD high EuIII concentration also results in a decrease in the aversimulation, and this was used to fit the experimental EXAFS data from the 10 mm solution. The model was constructed age number of malonamide molecules and in a small increase with a EuIII center coordinated by two bidentate malonamides, of water molecules bound directly to each EuIII center (from 3 one monodentate malonamide, two bidentate nitrates, one to 1.8, and 0.8 to 1.1, respectively). Another notable feature in monodentate nitrate, and one water molecule, giving a good the high concentration simulation is the presence of more fit to the data with realistic optimized metrics (Figure 5 c, d and outer-sphere interactions (+ 1 DMDOHEMA in average) where the Supporting Information, SI2). The conformational space water bridges between the hard europium nitrate complex Chem. Eur. J. 2014, 20, 12796 – 12807

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Full Paper and the soft aggregating solvent (Figure 5 b), as witnessed in our recent study.[32] The MD simulations suggest, therefore, that increasing EuIII concentration drives structural changes on the supramolecular level, extending beyond the outer-coordination sphere of the metal ion, involving in particular the formation of polynuclear nitrate-bridged EuIII clusters. Because EXAFS and UV-visible spectroscopy probe the organic phase EuIII complexes from a metal-centric perspective and so are of limited use in diagnosing the polynuclear EuIII assemblies proposed by the MD simulation at high metal concentration, we have undertaken ESI-MS experiments. Electrospray is a soft ionization method allowing the transfer pre-existing ions from solution into the gas phase so that speciation information in solution can be obtained. This technique has been used successfully to characterize metal–ligand complexes in solution involving lanthanides and heavy metals based on the molecular mass of the associated complex.[33] Figure 6 portrays ESI-MS data for dilute (0.048 m) and concentrated (0.159 m) EuIII organic phases, showing peaks that correspond to several Eu–malonamide species that involve two to seven malonamide molecules for each metal center. The influence of the cone voltage was explored and the intensity of ions, such as [EuLx]3 + , where x  5, strongly decrease as the cone voltage increases, showing that these species are not very stable and are probably formed during the ionization process as already observed for Nd extraction.[22e] The data show that the malonamide:EuIII complex stoichiometries in the organic phase depend on the europium concentration.

um concentrations, the loss of one or two nitrates and water molecules leaves the metal insufficiently coordinated, resulting in rearrangement to produce L3Eu(NO3)2 + or attachment of a further malonamide to produce L4Eu3 + as observed for complexes of lanthanide nitrates with trialkylphosphine oxide.[34] For high europium concentrations, the loss of bridging nitrates in bimetallic species followed by rearrangements of the metal coordination sphere leads to monometallic species (such as L2Eu(NO3)2 + ) as observed in the mass spectra. Thus, although not conclusive in providing structural confirmation for the polynuclear complexes witnessed in the theoretical study, the ESI-MS results provide stoichiometric evidence that is consistent with what would be expected from the transfer of the mononuclear/binuclear species from the solution to the gas phase.

Supramolecular organization: soft-matter perspective We now explore the structure of self-assembling aggregates of supramolecular and colloidal chemistry, sometimes referred to as soft matter. Defining the structure of water-poor microemulsions—such as in the subject malonamide/n-heptane solvent extraction system—can be challenging, particularly when the aggregates are small in size, dynamic, and contain exotic constituents, such as coordinating metal ions. It is, therefore, required to complement scattering studies with a thermodynamic constraint,[35] in this work osmotic compressibility as derived from the solvent activity. Vapor pressure osmometry (VPO) gives an estimate for the average total aggregation number that can then be used to corroborate analysis of X-ray scattering data. VPO measurements were performed on the organic phase with increasing concentrations of extracted EuIII. Figure 7 shows the average total DMDOHEMA aggregation number (n) resulting from the VPO experiments (see the Supporting Information, SI9 for full data), as a function of EuIII concentration in the organic phase. Here, we must stress that the average number per water-in-oil aggregate determined is averaged over the whole sample. In classical reverse-micelle studies, the average aggregation number is defined as the average number of aggregate in the micellar pseudo-phase (N), that is,

Figure 6. Positive ESI-MS data from solutions of 48 mm and 159 mm organicphase EuIII concentrations produced by solvent extraction of EuIII from approximately 3 m LiNO3 into 0.5 m DMDOHEMA in n-heptane (L stands for DMDOHEMA).

Three main EuIII species are observed in the mass spectrum with a 5:1; 4:1, and 3:1 malonamide:EuIII stoichiometry. Under “dilute” conditions, the 2:1 species is negligible, whereas under concentrated conditions the relative intensity of the 3:1 and 2:1 malonamide:EuIII species is significantly increased. These observations are consistent with the cluster models proposed by the MD simulations. In the gas phase, the water molecule in the metal coordination sphere can be easily released and metal–nitrate interactions are weakened.[33e, 34] For low europiChem. Eur. J. 2014, 20, 12796 – 12807

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Figure 7. VPO-determined average aggregation number (n; including the fraction present as monomers) for solutions of increasing EuIII concentrations produced by solvent extraction of EuIII from approximately 3 m LiNO3 into 0.5 m DMDOHEMA in n-heptane.

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Full Paper not taking into account monomers.[25] The low numbers obtained can differ significantly. A steady increase is observed in n from just under 2 when considered averaged over the whole sample at low concentrations to about 7 at very high concentrations of EuIII. As the average total aggregation number, n, accounts for both the aggregated and monomeric DMDOHEMA molecules. At low EuIII concentrations, n is largely reflective of the high proportion of monomers relative to aggregates that are known to persist under such conditions. For example, we have previously shown that a 0.5 m DMDOHEMA n-heptane solution without metal ion is composed of 0.21 m of monomer and 0.072 m of tetramers.[22a] The introduction of EuIII at low concentrations seeds the formation of aggregates of the type shown in Figure 5 a, where three malonamides interact with the EuIII center, so that the proportion of monomers steadily decreases resulting in an increase in total average aggregation number, n. At very high EuIII concentrations, the VPO data gives n < 7, which is higher than what would be expected from mononuclear assemblies of the type shown in Figure 5 a and is more consistent with polynuclear aggregates of the type shown in Figure 5 b. Thus, the VPO data support the conclusion drawn from the distribution ratio, ESI-MS and MD simulation results that suggest the supramolecular evolution of aggregates containing more than one EuIII center with a high number of malonamide amphiphiles in participation: this is typical of sphere-to-rod transitions in reverse aggregates, as has been observed in the case of aluminum extraction.[36] To move beyond physical/thermodynamic platforms and toward a structural understanding of the nano/mesoscale assemblies, scattering experiments including small/wide-angle Xray scattering (SAXS/WAXS) are necessary. Due to the small size of the aggregates and the sensitivity to conditions, electron microscopy techniques cannot be employed to take direct images in real space, so we used SAXS to probe the supramolecular aggregate structures in reciprocal space. The electronic density contrast of the nanoscale polar cores of the reverse-micelle-type aggregates scatter X-rays against the background apolar n-heptane solvent and surrounding aliphatic corona. The scattering pattern holds information on the shape, size, and arrangement of these polar scattering cores and, by employing various data-interpretation techniques, their supramolecular structure can be elucidated. As explained in numerous previous publications,[15, 16, 23, 37] these data-interpretation techniques must take into account both the intra-aggregate factor, that is, the scattering produced by the core structure of individual assemblies, as well as inter-aggregate scattering produced by adjacent cores through the solvent medium, known as the “structure factor”. Normalized, background-subtracted SAXS data collected from organic phases generated from the extraction of increasing concentrations of EuIII are shown in Figure 8 a. The shape of the curves are typical for particle scattering—as would be expected for reverse micelles—and are consistent with previous studies of malonamide–alkane systems at similar concentrations.[23, 37a, 38] These data were interpreted first with the Baxter model for hard sticky spheres in the same way as in the numerous previous publications[15, 16, 23, 37, , 38c, d] (a detailed exChem. Eur. J. 2014, 20, 12796 – 12807

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Figure 8. a) SAXS data collected from 0.5 m DMDOHEMA in n-heptane organic phases generated in the solvent extraction experiments (Figure 2) containing up to 125.9 mm EuIII (dashed line shows gradient of 1 on the log–log scale). b) PDDF functions calculated from the corresponding SAXS data using GIFT. Dotted line shows approximate position of second inflection point calculated by using the second differential, corresponding to aggregate polar core diameters.

planation and discussion of the application of the Baxter model to the subject system can be found in the Supporting Information, SI4) and a selection of the modeling parameters are presented in Table 4. The data show that the solution becomes more structured as EuIII is extracted, manifest in a significant decrease in “free” malonamide (monomer concentration) as the amphiphiles are drawn toward the EuIII centers. Aggregates also become larger as more EuIII is extracted, with the average polar core radius increasing by 30 % from 5.8 to 7.6  and the average aggregation number of reverse micelles (N, exclusive of monomers) also increasing from 4 to 6.5. The SAXS-determined N is therefore lower or equal to the VPO-determined solvent-averaged aggregation number n. N converges with n at high EuIII concentrations as the proportion of monomers becomes less significant. The SAXS analysis therefore confirms the VPO result that suggests, at high EuIII concentrations, the aggregation number is approximately 7. This means that the concentration of reverse micelles present in the solvent phase in the system is 0.07 m (calculated simply by dividing the total concentration of extractant by the aggregation number), which is less than the highest concentration of EuIII in the solvent extraction experiment (0.125 m, Figure 2). Therefore, either the reverse-micellar aggregates must contain more than one EuIII ion per reverse aggregate or they are connected wormlike oligomers. Although the analysis at the resolution of SAXS/WAXS does not give insight into the interactions between EuIII centers, it is very likely that it involves bridging nitrate of the kind predicted in the MD simula-

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Full Paper tions (Figure 5 b) as the repulsion between cationic EuIII centers must be overcome on the molecular level. In addition to determining metrical parameters for the reverse-micellar aggregates, the Baxter model also approximates the sum of the long-range interactions between them as a short-range step function characterized by one parameter only: the function depth or “stickiness”.[39] This level of approximation has allowed the prediction of the position of instability lines in the phase diagram when micelles are spherical, but not when they are present as wormlike or interconnected structures.[17, 23] These interactions define the inter-aggregate scattering (structure factor), which is produced by the relative arrangement of polar micelle cores. For any “reverse” aggregate in solvent, the dominant interactions that produce this arrangement are: 1) the hard-sphere interaction that is an infinite repulsion describing the volumeexclusion effect; 2) an attractive interaction between micelles caused by electrostatic fluctuations in the cores, and 3) the steric repulsion between aggregates due to the protruding hydrocarbon chains. The Baxter model works at a level of approximation for which these three interaction modes can be accounted for as a step function and analytic expressions are available to calculate the resulting structure factor. Table 4 shows the energy of the attractive interaction, U/kBT, as a function of europium concentration. As more EuIII is extracted, the attractive potential at contact increases until reaching a value of approximately 2kBT per aggregate at which point phase splitting occurs, agreeing with previous studies.[17, 40] Thus, the sticky-sphere approximation predicts the reverse micelles to randomly assemble as they attract. However, the model is limited in that it does not predict the higher-ordered structures of resulting architectures that can evolve to increasing curvature as the concentration of ionic entities (acid or metal ions) approaches the point of phase splitting, as observed in some cloud point studies of microemulsions.[41] Taking into account bicontinuous aggregates requires modelling of form and structure factors in one unique step and generating free energy as well as microstructures in the same calculation including thermalization with a reservoir, that is, simulations in a grand canonical ensemble. This refinement has

been made only once, to our knowledge, for water-poor microemulsions[42] and is only qualitatively different in results when a broad peak generated by a connected network of polar cores is observed in the scattering pattern. This broad peak is not apparent in the scattering patterns shown here and we relied on the more approximate picture of individual noncoalescing globular identified aggregates. In this case, a qualitative description of the morphology of these larger architectures may be indicated by power law behavior in the low-q region of the SAXS data (Figure 8 a). In this region, at low EuIII concentrations, the SAXS waves tend toward a plateau (i.e., q0 power law behavior), which is typical for globular particles with no extended structure. As the concentration of EuIII increases toward the critical limit (the point of phase splitting), the SAXS waves in the low-q region tend toward a straight line of gradient 1 on the log–log scale, or a q 1 power law behavior, suggesting the development of elongated one-dimensional rod-shaped reverse micelles.[43] To investigate the extended structures in more detail, and to corroborate the Baxter modelling, the SAXS data were interpreted independently by using the generalized indirect Fourier transform (GIFT) method.[41, 44] GIFT is a model-free method for noninteracting particles, but needs injection of an assumed structure factor, since only the form factor is Fourier transformed, if the system is interacting. It has been demonstrated that in the case of elongated cylindrical connected or branched structures, using the PY-structure factor formula for hard spheres to model volume exclusion effects still holds (extensive literature exists on the development, applications, and underpinning physical/mathematical theories of the GIFT method, and a summarized discussion of this, along with a justification for its applicability to the current system, is given in the Supporting Information, SI4).[45] By approximating the repulsive interactions governing the structure factor function S(q), the GIFT method can then resolve the form factor, which corresponds to the total scattering produced by both the independent reverse micellar units and the higher-ordered architectures that develop as a result of the mesoscopic assembly. Taking the Fourier transform of the resolved form factor generates a pair distance distribution function (PDDF) that serves as a real-space description of the average shape and size of the scattering architectures. The PDDFs shown in Figure 8 b are, therefore, probability distribution functions representing scattering distances between Table 4. Column 2: Concentrations of EuIII in the 0.5 m DMDOHEMA in n-heptane orpoints within all of the aggregated polar assemblies, ganic phases generated in the solvent extraction experiments (Figure 2). Columns 3– with r being the distance and p(r) the relative 6: summary of the parameters used in the interpretation of the SAXS data in Figure 8 by using Baxter’s model for hard sticky spheres. Column 7: polar core radii (Rp) calcunumber of particular distances that occur. lated from the PDDFs generated by using GIFT. At low concentrations of EuIII the PDDF curves are bell-shaped, as expected from globular aggregates of N [mono][a] [M] Rp [] U/kBT[b] Rp [] (GIFT) Sample [EuIII]org [mM] little extended structures, in agreement with the q0 1 0 4 0.21 5.8 0.51 5.2 SAXS-wave behavior described and the relatively low 2 2.5 4 0.21 5.8 0.61 5.5 attractive interaction potential reported from the 3 15 4 0.19 5.9 1.23 5.5 4 30.7 4.5 0.13 6.2 1.50 6.0 Baxter modelling in Table 4. As the concentration of 5 64.5 5 0.08 6.9 1.76 7.0 EuIII increases, a tail grows in the high-r region, turn6 103.4 5.8 0.02 7.2 1.97 7.0 ing the PDDF into a ski-slope-shaped function. Such 7 125.9 6.5 0.01 7.6 2.04 7.3 functions are typical of cylindrical or rod-shaped mi[a] Monomer concentration.[b] kB is Boltzman’s constant and T is temperature celles[46] and are in agreement with the q 1 power

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Full Paper law behavior in Figure 8 a. The maximum extent (or long axis diameter) of these aggregates can be read from the maximum extent of the PDDF functions. This shows a steady growth from 15  at low EuIII concentrations (globular micelles) to 100  at high concentrations (cylindrical micelles). The PDDFs can also be used to estimate the short-axis diameter of the polar core, which is physically comparable (when divided by two) to the polar core radius in the Baxter model.[47] This is taken from the position of the second inflection point (determined from the second derivative of the function) and is indicated approximately by the dotted line in Figure 8 b. The migration of the second inflection point to higher r shows, again, a swelling of the polar core as more EuIII is extracted. The final column in Table 4 shows the short axis radii calculated from the PDDFs, which are very close to the values used in the Baxter model, thus corroborating both analytical techniques. In the MD simulation, the aggregating behavior was investigated by taking snapshots of the entire system at low (Figure 9 a) and high (Figure 9 b) EuIII concentrations. At low concentration, the EuIII has a limited influence on the distribution of DMDOHEMA amphiphile, much of which is located in poorly-defined aggregates or monomers (as also suggested by the VPO and SAXS data). This is also true for water that often pertains elsewhere within the aggregating amphiphile solvent, away from the EuIII centers. At higher concentrations, the DMDOHEMA and water are drawn around the EuIII clusters, with significantly less independent extractant monomer distributed in the available space. This is consistent with the Baxter model interpretation of the SAXS data that shows EuIII to trigger the formation of more defined aggregates and hence decrease monomer concentration in equilibrium with the aggregates. As water and amphiphile are drawn around the clusters,

they interact with the EuIII complex through outer-sphere H bonds that often involve water bridging between coordinated nitrate and aggregating amphiphiles, as shown in Figure 5 b. This has the effect of swelling the polar reverse-micelle core that is also suggested by the PDDFs from the experimental SAXS data (Figure 8 b). At constant curvature constraint, swelling is always linked to sphere-to-rod transition in all reverse micelles studied yet.[48] The aggregate-in-oil system described here is no exception to this general behavior. In addition to drawing the malonamide and water molecules, the EuIII clusters also interact with each other to produce assemblies of aggregates, as shown in Figure 9 b’. This increasing associative interaction between clusters is consistent with the increase in attractive potential (U/kBT) from the Baxter modelling of SAXS data (Table 4) and shows the beginnings of a collapse of metal-containing aggregates into a separate domain that would manifest in phase splitting. When in close proximity, the aggregates interact with each other through outer-sphere H bonds that involve water. Figure 10 shows an example of a binuclear EuIII-containing cluster interacting with a mononuclear cluster through water molecules that bridge between coordinated nitrate and malonamide ligands. The Hbond-tethered clusters might be an intermediate for forming the polynuclear EuIII species that are bridged by nitrate. These could lead to the more elongated structures that are suggested by the PDDFs (Figure 8 b) that extend up to 10 nm in length. The mediation of the approach of the micelle cores by H-bonding water would circumvent the hard-sphere repulsion of the cores and allow for the reconfiguration of nitrate ligand in the inner-sphere to form stable, nitrate-bridged, polynuclear EuIII assemblies.

Figure 10. Snapshot of an illustrative example of two EuIII-containing aggregates interacting through H-bonding water molecules in the outer sphere.

Figure 9. Snapshots of the entire MD simulation boxes at a) low and b) high EuIII concentrations corresponding to the composition of samples 3 and 7, respectively. Figure a’ and b’ are from the same snapshot but without the nheptane solvent and “free” DMDOHEMA molecules. The n-heptane solvent is shown in orange, DMDOHEMA in green, nitrate in red/blue, water in red/ white, and EuIII as large blue spheres. Chem. Eur. J. 2014, 20, 12796 – 12807

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Combining the MD simulation with the experimental SAXS investigation suggests a supramolecular structural evolution driven by EuIII extraction that could account for the nonclassical extractive behaviors. As the concentration of EuIII increases, the solution becomes more structured as free malonamide amphiphile and water are drawn to the aggregates. In this way, EuIII extraction leads to deviation from ideal behavior (observed empirically from the VPO result). The aggregates interact and cluster together to form nanoscale hydrophilic domains that contain high concentrations of structured O-donor ligands and

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Full Paper anions. These self-assembled polar domains are chemically and physically different from the poorly structured 0.5 m DMDOHEMA/n-heptane solvent that pertains at low EuIII concentration, and possibly favors extraction. The structural information obtained in this study can be used to estimate an entropic term which corresponds to the inclusion of a EuIII cation in a DMDOHEMA aggregate by using statistical thermodynamics.[49] This entropic term depends only on partition functions of the cation in the aggregates, that is, the possible number of bonds that can be formed inside the aggregate. At low EuIII concentration, the cation is bonded, on average, to three DMDOHEMA with two bidentate and one monodentate ligand, which corresponds to a 1.1 kT entropic term. At a higher EuIII concentration, each of the two EuIII is bonded (in average), to two bidentate, two monodentate, and two outer-spheres DMDOHEMA ligands, which corresponds to a more favorable 4.2 kT entropic term. Therefore, the increase of the aggregate size—due to the increase of DMDOHEMA, water, europium, and nitrate participating in the aggregate at the higher cation concentrations—favors additional EuIII phase transfer. This entropic “win” is due to the number of equivalent configurations that are possible between the EuIII center and the coordinating ligands available locally in the aggregate. The result is a system that is driving towards more favorable extraction of the order of 10 kJ mol 1, without ANY variation of the complexation. It is very important to note that this order of magnitude of 10 kJ mol 1 is the optimal formulation in practical design of extraction plants based on liquid–liquid extraction.[50]

3) MD simulations are consistent with the above experimental observations and suggest that the repulsions between cationic Eu centers are mediated by bridging nitrate. The preorganized hydrophilic domains, formed through long-range interactions between aggregating reverse-micelle units, contain high concentrations of O-donor ligands and anions that provide improved EuIII solvation environments and drive the observed extractive properties. Thus, predictive modeling of equilibria must take into account, not only complexation equilibria between the metal and nearest neighbor ligands, but also the long-range interaction terms. This study gives structural insight into the intricate relationship between the hard metal-ion complexation interactions and the soft long-range interactions between aggregates. This fundamental research moves toward a predictive understanding of interfacial metal-ion transfer that is not only central to extractive separations, but also to a plethora of applications, involving metal-ion transport in soft-matter environments, such as nanosynthesis and life-mimicking systems.

Experimental Section Chemicals DMDOHEMA was supplied by Pharmasynthse (Lisses, France) with purity higher than 99 %. n-Heptane (99 %) was supplied by Sigma– Aldrich (France). All other reagents were of analytical grade and used as received.

Liquid–liquid extraction

Conclusion

Solvent-extraction experiments and analysis of solutes (metal, acid,

water) were performed by using the standard methods reported in Increasing concentrations of europium nitrate transferred from our previous publications.[22d, 30a, 32, 51] Details are given in the Supan aqueous phase into a water-poor amphiphile-in-oil system porting Information, SI1. (DMDOHEMA in n-heptane) resulted in a nonclassical distribuTo investigate the effect of increasing Eu concentration on the Eu tion behavior. This is the result of a combination of coordinapartitioning ratio DEu and solution structure, biphasic extraction extion and colloidal chemistry, where structural evolutions within periments were performed to produce aqueous and organic soluthe organic phase, driven primarily by long-range interactions, tions with water, LiNO3, Eu, and DMDOHEMA concentrations or acchange the properties of the solvent. Extraction occurs initially tivities according to Table 5. All organic solutions 1–7 were anaby direct coordination of the EuIII ion by the O-donor ligands lyzed with SAXS and EXAFS, and solutions 3 and 7 were simulated available in the organic phase (nitrate, water, and DMDOHEby MD calculations. Most of the structural analysis was, therefore, performed on exactly the same solutions that correspond to the MA). This seeds the formation of reverse micelles that draws trend in DEu. the “free” water and amphiphile into nanoscale hyIII drophilic domains. At increasing Eu concentrations, the reverse-micelle clusters attract each other to Table 5. Solute concentrations and activities (a) in aqueous (aq) and organic (org) form larger polynuclear EuIII aggregates with metal samples generated in biphasic extraction experiments for investigating trend in DEu centers perhaps bridged by nitrate. This is supported and solution structure as a function of Eu concentration. The lithium concentration in by: the organic phase was below detection limits.

1) Metal-dependent solvent extraction data that is characteristic of systems in which the polymerization and saturation effects buffer each other to result in the domed profile of Figure 2. 2) Supramolecular SAXS and VPO investigations show that there is more than one Eu per aggregate so that Eu centers must be in close proximity. Chem. Eur. J. 2014, 20, 12796 – 12807

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Sample

[LiNO3]aq [mol L 1]

[Eu]aq [mol L 1]

aH2Oaq

[Diamide]org [mol L 1]

[H2O]org [mol L 1]

[Eu]org [mol L 1]

DEu

1 2 3 4 5 6 7

2.93 2.93 2.83 2.76 2.67 2.37 2.34

0 0.0066 0.037 0.0711 0.1391 0.2981 0.2855

0.8701 0.8694 0.8718 0.8721 0.8700 0.8701 0.8732

0.493 0.49 0.488 0.487 0.487 0.485 0.483

0.062 0.063 0.07 0.076 0.16 0.221 0.246

0 0.0025 0.0148 0.0307 0.0645 0.1034 0.1259

0 0.38 0.4 0.43 0.46 0.35 0.44

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Full Paper UV-visible absorption spectrometry UV-visible measurements were performed on a Varian Cary 50 Scan spectrometer between 500 and 900 nm in 0.2 or 1.0 cm quartz cuvettes.

IR spectrophotometry Infrared measurements were performed with a Bruker Equinox 55 FT-IR spectrometer equipped with a diamond attenuated total reflectance cell (ATR). All spectra were collected between 650 and 4000 cm 1 during eight scans and a resolution of 4 cm 1.

EXAFS data collection and analysis EXAFS data collection and treatment using the shell-fitting procedure with EXAFSPAK was the same as previously described.[22d] Data treatment with a model cluster representing the average species in the low Eu concentration MD simulation was carried out with the Athena software.[52] The edge energy E0 is determined by finding the maximum of the first derivate of the m0(E) spectrum. Normalized data were obtained after subtracting a linear pre-edge background and a three-term quadratic function for the atomic absorption background and normalized by the Lengeler–Eisenberger procedure.[53] Pseudo-radial distribution functions (PRDF) were obtained by Fourier transformation in k3c(k) with the ARTEMIS software[54] between 2 and 10.8 A 1. Phases and amplitudes were calculated with the FEFF8.03 code based on the MD calculations. Details regarding EXAFS data collection and fitting is expanded on in the Supporting Information, SI2.

ESI-MS The mass spectrometric measurements were recorded in the positive ionization mode by using a Bruker Esquire-LC quadrupole ion trap equipped with an electrospray interface. Experimental conditions were positive-ion mode, drying gas (N2): 5 L min 1, nebulizer gas 5 psi, 250 8C, ion-spray voltage of 4000 V, cap-exit offset of 60 V, skimmer 1 of 20–90 V, skimmer 2 of constant 10 V, trapdrive 90. Spectra were acquired over a mass range of m/z = 45–2200. A syringe infusion pump (Cole Palmer) delivered the sample at 90 mL h 1 to the electrospray source. The organic phases were diluted 1:10 in ethanol and 1:100 in acetonitrile/water before analysis.

VPO VPO measurement and data analysis were performed in a similar manner as described previously.[22a] The details are given in the Supporting Information, SI3.

SAXS measurements SAXS data collection and treatment was the same as previously described in our recent publication,[22d] and is repeated in SI4. The monomer concentration [mono], the aggregation number of reverse micelles N and the aggregate concentrations calculated from the Baxter modelling of each SAXS data set was checked against the average total aggregation number n obtained by the VPO measurement.

MD simulations MD simulations were performed with the AMBER 10 software and the parm99 force field[55] taking explicitly into account polarization Chem. Eur. J. 2014, 20, 12796 – 12807

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effects. The inclusion of the explicit polarization term in the force field is essential for a good representation of: 1) the interactions around a + 3-charged cation, such as europium(III), and 2) the dispersion forces and polarization interactions that are essential for ions located near dielectric jumps, such as those appearing in reverse micelles, as stated by Kunz et al.[56] Water molecules were described by using the POL3 model.[57] Eu3 + and NO3 parameters were determined in previous studies to reproduce the experimental structural properties in solution.[31, 58] Atomic partial charges on DMDOHEMA and n-heptane were calculated with the restricted electrostatic potential (RESP) procedure.[59] Periodic boundary conditions were applied to the simulation boxes and long-range interactions were calculated by using the particle-mesh Ewald method.[60] Equations of motion were numerically integrated by using a 1 fs time step. The MD simulation box compositions were defined based on the corresponding experimental organic phase analyses, for which all compound concentrations have been determined. The resulting molar fractions are the sole experimental data introduced in the MD calculations. MD simulation boxes were then built starting with random positions for the ions, water, and the extractant molecules, and equilibrated during at least 500 ps in the NPT ensemble. Reproduction MD simulation runs were subsequently collected for 5 ns.

Acknowledgements We thank Dr. Sçnke Seifert for assistance at the APS (Sector 12). This work is supported by the U.S. Department of Energy, Office of Basic Energy Science, Division of Chemical Sciences, Biosciences and Geosciences, under contract No DE-AC02– 06CH11357 (for the parts performed at Argonne National Laboratory). The work at the CEA is supported by the Direction de l’Innovation et du Soutien Nuclaire/aval du cycle futur/SEPOU. Keywords: coordination chemistry · self-assembly · soft matter · solvent extraction [1] H. F. Eicke, H. Christen, J. Colloid Interface Sci. 1974, 48, 281 – 290. [2] G. A. Lawrance, Introduction to Coordination Chemistry Inorganic Chemistry: A Textbook Series, Wiley, New York, 2010, p. 304 pp. [3] M.-P. Pileni, Nat. Mater. 2003, 2, 145 – 150. [4] a) W. Fan, M. A. Snyder, S. Kumar, P.-S. Lee, W. C. Yoo, A. V. McCormick, R. Lee Penn, A. Stein, M. Tsapatsis, Nat. Mater. 2008, 7, 984 – 991; b) M. Choi, H. S. Cho, R. Srivastava, C. Venkatesan, D.-H. Choi, R. Ryoo, Nat. Mater. 2006, 5, 718 – 723. [5] J. A. Lemire, J. J. Harrison, R. J. Turner, Nat. Rev. Microbiol. 2013, 11, 371 – 384. [6] H. F. Eicke, H. Christen, Helv. Chim. Acta 1978, 61, 2258 – 2263. [7] C. Bauer, P. Bauduin, J. F. Dufreche, T. Zemb, O. Diat, Eur. Phys. J.: Spec. Top. 2012, 213, 225 – 241. [8] F. L. Bernardis, R. A. Grant, D. C. Sherrington, React. Funct. Polym. 2005, 65, 205 – 217. [9] R. G. Bautista in Separation chemistry, Vol. 21 (Eds.: K. A. Gschneidner, L. Eyring), North-Holland, Amsterdam, 1995, pp. 1 – 27. [10] F. Habashi, Handbook of Extractive Metallurgy, Wiley-VCH, Weinheim, 1997, p. 2500. [11] T. Zemb, M. Duvail, J.-F. Dufreche, Isr. J. Chem. 2013, 53, 108 – 112. [12] B. Abcassis, F. Testard, T. Zemb, L. Berthon, C. Madic, Langmuir 2003, 19, 6638 – 6644. [13] A. M. Wilson, P. J. Bailey, P. A. Tasker, J. R. Turkington, R. A. Grant, J. B. Love, Chem. Soc. Rev. 2014, 43, 123 – 134. [14] P. A. Tasker, P. G. Plieger, L. C. West in Metal complexes for hydrometallurgy and extraction, Vol. 9 (Ed. M. D. Ward), Elsevier, New York 2004, pp. 759 – 808.

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