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Synergism in a HDEHP/TOPO Liquid−Liquid Extraction System: An Intrinsic Ligands Property? O. Pecheur,† S. Dourdain,*,‡ D. Guillaumont,† J. Rey,† P. Guilbaud,† L. Berthon,† M.C. Charbonnel,† S. Pellet-Rostaing,‡ and F. Testard§ †

Nuclear Energy Division, RadioChemistry & Processes Department, CEA, 30207 Bagnols-sur-Cèze, France ICSM, Institut de Chimie Séparative de Marcoule UMR 5257, 30207 Marcoule, Bagnols-sur-Cèze, France § LIONS, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif sur Yvette Cedex, France ‡

S Supporting Information *

ABSTRACT: Among the proposed mechanisms to predict and understand synergism in solvent extraction, the possibility of a preorganization of the mixture of extractant molecules has never been considered. Whether involving synergistic aggregation as for solubilization enhancement with reverse micelles or favored molecular interaction between the extractant molecules, evaluation of this hypothesis requires characterization of the aggregates formed by the extractant molecules at different scales. We investigate here the HDEHP/ TOPO couple of extractant with methods ranging from vibrational spectroscopy and ESI-MS spectrometry to vapor pressure osmometry and neutron and X-ray scattering to cover both molecular and supramolecular scales. These experimental methods are subjected to DFT calculations and molecular dynamics calculations, allowing a rationalization of the results through the different scales. Performed in the absence of any cation, this original study allows a decorrelation of the mechanisms at the origin of synergy: it appears that no clear preorganization of the extractants can explain the synergy and therefore that the synergistic aggregation observed in the presence of cations is rather due to the chelation mechanisms than to intrinsic properties of the extractant molecules.

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INTRODUCTION Chemical systems are widely used in synergy for different fields of applications as catalysis, fluid transportation, pharmaceutical industry, or separation science.1−9 In solvent extraction, synergistic systems are used to separate precious metal or rare-earth elements from other solutes. They rely on mixtures of extractant molecules diluted in aliphatic solvents, which are able to extract more efficiently than pure systems, a target ion from an aqueous phase.10−12 The term of synergy was first used by Blake et al. in the 1950s when he observed that combining a neutral organophosphorous molecule with di-2-ethyl-hexyl phosphoric acid (HDEHP) showed a greater extraction power than expected for the recovery of uranium from mineral acid media.13 While applied in many processes, elucidating the synergistic mechanisms in solvent extraction remains a major challenge. To date, synergism was mainly attributed to the formation of mixed metallic complexes with the participation of all extractant molecules. For classical acidic extractant systems, the most common mechanisms propose the addition of neutral donor ligand to the acidic extractant.1−3,12,14,15 However, in the © XXXX American Chemical Society

literature most of the studies rely on experimental determination of the mixed species stoichiometry from the analysis of the metal ion distribution ratio for various experimental factors. This approach results from macroscopic data of extraction and does not take into account molecular interactions between the ligands and the cations or the extractants self-assembling properties. In solvent extraction studies, molecular associations between extractants are often considered as responsible for antagonistic effects, implying that less chelating sites are available. However, such associations are seldom fully characterized, while they could also induce a specific organization which could also favor metal ions extraction.16−18 Few groups have performed a detailed characterization of the organic phase organization from the atomic scale to the supramolecular scale of the self-assembled aggregates.19−23 To our knowledge, the role of a preorganization of the solution Received: December 1, 2015 Revised: January 18, 2016

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DOI: 10.1021/acs.jpcb.5b11693 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

proven to be very efficient to increase the distribution ratio of uranium when extracted from phosphate rocks. The maximum of uranium extraction at a TOPO:HDEHP molar ratio of 1:4 has been exploited industrially.6 Our study relies on a complete description of the molecular organization and supramolecular properties of the synergistic HDEHP/TOPO system without ions in the organic phase using a combination of experimental and theoretical approaches. At the molecular scale, interactions between extractant molecules have been investigated though infrared spectroscopy and electrospray ionization mass spectrometry (ESI-MS). The supramolecular properties of the organic solution have been characterized through vapor pressure osmometry (VPO) and small angle X-ray and neutron scattering (SAXS and SANS). Here, the aim was to determine if the system presents particular aggregation properties at the synergistic ratio before contact with an aqueous phase, especially by estimating the critical aggregation concentrations of the various ligand mixtures. The experimental approaches have been coupled with density functional theory (DFT) calculations to support the interpretation of infrared spectra and molecular dynamics (MD) simulation in order to give an atomistic description of the solution, as it was first proposed by Ferru et al.36 The combination of these experimental methods and simulations offers, for the first time, a complete atomistic and molecular description of an extractant reverse micellar system prior to its contact with an ion-rich aqueous phase. We foresee that knowledge on the preorganization of the organic phase in the absence of metallic complexes is expected to bring new insights into the origins of synergism.

prior to any contact with the target ions has never been evaluated to explain the synergistic mechanisms. In a recent study, the structural properties of an organic phase containing HDEHP/TOPO synergistic mixtures were investigated in the presence of metal ions. Taking benefit of the general knowledge of the supramolecular structure of extractant phases,24−26 the organic phase was treated as a water in oil microemulsion, able to solubilize water and solutes. It was shown that the synergistic extraction of uranium by HDEHP/ TOPO could be related to a synergistic aggregation of the two extractants.27 A maximum of extraction was found to be concomitant with a minimization of the micellization energy. The results suggested for the first time that the aggregation properties of extractant molecules can play a major role in the synergistic mechanisms. However, it was not possible to discriminate if the origin of the synergistic extraction is due to the favored micellization properties of extractants or a consequence of a higher uranium extraction. It appears indeed essential to determine if a nucleation effect by uranyl cations as reported with acids on different extractions systems28 induces the lower micellization energies or not. The question of a preorganization of the organic phase due to specific molecular or supramolecular interactions between the extractants that would be at the origin of synergism is therefore crucial. This possibility assumes the presence of preformed mixed aggregates in the solution with a geometry well suited to the metal ion or a minimization of the free energy of the system for a given composition of mixed extractants. To further investigate this hypothesis, a detailed description of the supramolecular properties of the organic solution and its molecular organization must be given before any contact with the aqueous solution. Without chelation mechanisms induced by the cations, the basic phenomena structuring the organic phase of solvent extraction are analogous to those encountered in surfactant systems, where the surfactant molecules self-assemble thanks to pure amphiphilic properties to form reverse micelles that may solubilize water and solutes in their polar core.29 In the classical field of reverse micelles, it has been proved that a mixture of surfactants can enhance the performance over the individual components. One of the best known effects is the enhanced water solubilization in reverse micelles by using a mixture of surfactants. These synergistic effects have been observed and applied for different applications like drug solubilization, enzymatic activities, synthesis, etc.30−33 A significant synergism is also observed with nonionic surfactant mixtures where surfactants of similar structure are used.34 The origin of such synergism has been attributed to the interplay between the interfacial curvature and the interdroplets interaction or to the formation of an optimal hydrophilic−lipophilic balance (HLB) value or both.32 In direct systems (surfactants in water solution), synergistic effects have been rationalized by considering the composition dependence of the free energy per aggregated surfactant.35 The main contribution results from entropic effects related to the surfactant head groups rather than specific interactions between surfactants. As such approach was not fully rationalized for reversed micellar systems, the origin of synergism in such phases is still under debate. In this study, we aim at evaluating if synergy of extraction could originate from a pure self-assembled preorganization of the organic phase. We investigate the synergistic combination of the HDEHP acidic extractant with the trioctylphosphine (TOPO) neutral organophosphorous extractant that has been

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RESULTS AND DISCUSSION A. Molecular Interactions. Infrared spectroscopy can reflect changes in the local environment around the functional groups of HDEHP and TOPO and is a good probe of the molecular interactions between the extractants. FT-IR has been previously applied to the HDEHP−TOPO mixture in chloroform solution to evidence the formation of adducts between the two molecules.7 In the present study, the FT-IR spectra of HDEHP, TOPO, and HDEHP−TOPO mixture have been recorded in dodecane and simulated from DFT. Experimental and calculated spectra are shown in Figure 1. The DFT calculations were performed on TOPO, on the dimeric species HDEHP2, and on a mixed species HDEHP2·TOPO where TOPO creates a hydrogen bond with HDEHP (as depicted on Figure 1). In HDEHP2 both HDEHP are engaged in two hydrogen bonds, while in HDEHP2·TOPO one HDEHP is engaged in only one hydrogen bond and one PO group is free. On the experimental spectra of HDEHP in dodecane, two strong absorption bands occur in the 1190−1300 and 960− 1080 cm−1 regions (bands A and B in Figure 1). Band A is assigned the phosphoryl PO stretching vibration, and B is assigned to P−O−C and P−OH stretching vibrations. The spectra of TOPO exhibit low-intensity absorption bands in the 1140−1200 cm−1 region corresponding to the PO stretching vibrations. For the HDEHP−TOPO mixture, three new bands or shoulders are found in the ranges 1260−1300, 1100−1150, and 950−1010 cm−1 (denoted, respectively, C, D, and E). According to the calculated spectra, band C is assigned to a phosphoryl PO stretching of an unbound phosphoryl group. Band D is attributed to TOPO PO stretching vibration resulting from the creation of a hydrogen bond between B


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When diluted in an organic solvent, HDEHP is known to form small aggregates in the organic phase. Dimeric species are considered to be the predominant species, but trimeric species may also exist in the organic solution.37 Electrospray ionization mass spectrometry, ESI-MS, based on a soft ionization mode, is a powerful technique for producing ions in vacuo from large and complex species in solution.38,39 Previous studies have shown that dimerization or oligomerization equilibria can be investigated by ESI-MS.40−43In a review41 Ceraulo et al. reported that the structural organization of the surfactant in the gas phase depends on the nature and composition of the starting solution (called the mirror effect). Organic solutions have been analyzed in the presence of both extractants in order to determine the interactions between the neutral extractant TOPO and HDEHP (Figure 2). Table 1 reports the identified species by comparison with a simulated isotopic pattern. Table 1. List of the Main Species Identified on the ESI-MS Spectruma m/z 345.2 645.5 667.4 967.7

Figure 1. Experimental and calculated infrared spectra. Experimental FT-IR spectra (bottom) were measured for HDEHP (0.3 M), TOPO (0.3 M), and HDEHP (0.3 M) + TOPO (0.3 M) in n-dodecane (ndodecane signal has been subtracted). Calculated spectra (top) were simulated from DFT (B3LYP) by broadening the calculated transitions as a sum of Lorentzian functions with bandwidths at half height of 10 cm−1. A−E identify remarkable peaks that are commented in the body of the text.

1289.9 1311.9 1634.1

HDEHP species [HA·Na]+ [(HA)2· H]+ [(HA)2· Na]+ [(HA)3· H]+ [(HA)4· H]+ [(HA)4· Na]+ [(HA)5· Na]+

m/z

TOPO species

m/z

HDEHP−TOPO species

387.4 773.8

[T·H]+ [T2·H]+

731.6 1031.8

[HA·T·Na]+ [(HA)2·T·H]+

795.7

[T2·Na]+

1117.9

[T2·HA·Na]+

1376.1

[(HA)3·T·Na]+

1440.2

[(HA)2·T2·Na]+

1504.3

[HA·T3·Na]+

1698.3

[(HA)4·T·Na]+

1762.4 2036.5 2100.6

[(HA)3·T2·Na]+ [A·(HA)4·T·Ca]+ [A·(HA)3·T2· Ca]+

a

HA and T stand, respectively, for HDEHP and TOPO. Some sodium or calcium adducts are formed during the desolvation/ionization process in low acidic solution.44

HDEHP and TOPO. Band E corresponds to P−O−C and P− OH stretching vibrations of the HDEHP molecule which is engaged in only one hydrogen bond. To summarize, IR spectroscopy shows that in the presence of TOPO the hydrogen bond network between HDEHP is partly broken in favor of hydrogen bond formation with the TOPO PO group.

Several mixed species with HDEHP and TOPO are detected, involving up to six ligands and up to three TOPO. This species suggests the formation of adducts in the gas phase including the

Figure 2. ESI-MS spectrum of HDEHP/TOPO organic phase, [HDEHP] = 0.4 M, [TOPO] = 0.1 M in n-dodecane after 1/1000th dilution in methanol (ISCID voltage 50 eV). The smaller inset shows a zoom of the less intense peaks for the 950−2200 m/z range. C


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SAXS represents the scattering from the core of the aggregates, richer in electrons. Looking at the evolution of the absolute intensities at low angles, it appears that SANS spectra show a linear increase of intensity with the TOPO ratio, while the SAXS spectra show an increase until 20% of TOPO followed by a decrease. While slight and close to the error bar of the technique, this peculiar feature at the synergistic ratio was reproduced and confirmed on several series of samples and in various diluents (see Supporting Information, Figure SI-4). To interpret this behavior, the classical equation of scattering is expressed when q tends to zero

two ligands. These mixed ions could be isolated before fragmentation by collision with N2 gas, indicating that they are stable. Some of the species could be formed in the gas phase during the ionization process. Fragmentation spectra were recorded for some mixed species ([(HA)3·T·Na]+ (m/z = 1376.1); [(HA)2·T2·Na]+ (m/z = 1440.2); [HA·T·Na]+ (m/z = 731.6)); all show consecutive losses of HDEHP ligand (Figure SI-1 in ESI). The presence of Na+ and Ca2+ should not be taken into account in the interpretation. This is a common artifact observed for low acidic samples of the ESI-MS method, which is very sensitive to any ions present in solution even at a very small amount, which may come from glassware, impurity in the solvent, etc. These results support the existence of multiple aggregates including HDEHP and TOPO molecules (Figures SI-1 and SI-2 in ESI). Therefore, infrared spectroscopy and mass spectrometry indicate the presence of molecular interactions between HDEHP and TOPO in the organic phase, resulting in the formation of mixed molecular species. B. Self-Assembling Properties. As VPO measurements and molecular dynamics calculations can only be perfomed in n-heptane, aggregates of HDEHP and TOPO have been characterized in n-heptane instead of n-dodecane in order to obtain comparable measurements of the self-assembling properties. It was shown in a previous study that the chain length of the diluent does not influence the synergistic ratio of uranium extraction of this system.45 Absolute SANS and SAXS intensities are depicted in Figure 3. Both methods show flat

ISAXS(q → 0) =

NA(C − CAC) . (VcoreΔρcore + Vag Δρag )2 Nag (1)

where NA is Avogadro’s number, C is the total concentration of ligands, CAC is the critical aggregation concentration, Nag is the aggregation number equal to the average number of ligands in each aggregate, Vcore and Vag are the core and aggregate volumes, and Δρcore = (ρshell − ρcore) and Δρag = (ρshell − ρsolvant) are the scattering length density contrasts. At first approximation, the increase of SAXS intensity at TOPO 20% could be attributed to a decrease of CAC or Nag or to a smaller Vcore, which would be in favor of a specific preaggregation state at the synergistic ratio. However, it may also be attributed to an intrinsic evolution of Δρcore or to a combined evolution of all the parameters. Since this feature is reproducible and as it might be important to take into account in the interpretations, it is essential to determine precisely and separately the different terms of eq 1. To this end, CAC and aggregation numbers were determined experimentally by vapor pressure osmometry, and a combined fit of SANS and SAXS patterns was performed to complete the estimations of the parameters. CAC determination in noncontacted organic phases is more complex than in organic phases previously contacted with water and solutes.48,49 Indeed, the most common and applied method for extractant molecules is based on the interfacial (oil/ aqueous) surface tension measurements. As it would induce water extraction during the measurement, this method cannot be used here for noncontacted solutions. An organic/air interface can also not be used as the extractant molecules are not amphiphilic enough to migrate and saturate the interface before forming aggregates in the bulk organic phase. Experiments have therefore been performed with vapor pressure osmometry (VPO) and small angle neutron scattering (SANS) to determine the critical aggregation concentration. In the case of ideal solutions (i.e., diluted solutions), the VPO signal is expected to be proportional to the molality of the solution (dashed line in Figures 4 a−c). When the concentration of the solution increases, interactions between the species make the solution no longer ideal. VPO signals show therefore a deviation from ideality at a molality that can be interpreted at first approximation as the critical aggregation concentration.46 Figure 4 presents CAC measurements from VPO and indicates that CAC are about 0.3 mol/kg (i.e., ∼0.2 mol/L) for TOPO ratios 0%, 20%, and 50%. Similar series of organic phases were measured with small angle neutron scattering. For diluted solutions, the initial scattered intensity (extrapolated for q = 0) being proportional to the volume fraction of scattering objects is linearly related to

Figure 3. SANS and SAXS diagrams of noncontacted HDEHP/TOPO heptane phases ([extractants] = 0.5 mol/L: light gray disks stand for 0% TOPO, gray for 20% TOPO, and black disk for 50% TOPO). Lines stand for the calculated spectra. Bottom inset illustrates the reverse aggregates formed by the extractant molecules in the organic phase.

spectra at low angles, followed by a strong decrease of intensity, occurring at ca. 0.2 Å−1 for SANS and 0.3−0.4 Å−1 for SAXS. This first qualitative observation is in favor of the presence of globular aggregates in the organic phases. The scattering signal is less intense for SAXS than for SANS, with more than one decade of difference in absolute units. As neutrons and X-rays are, respectively, sensitive to protons and electrons density contrasts, it is coherent with a description of the aggregates with a spherical core−shell model, which is often considered in solvent extraction.46,47 SANS is the signature of the scattering from the hydrogenated aggregates in deuterated solvent, while D


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Figure 4. CAC determination by VPO (top insets) and SANS (bottom insets).

As VPO signals are related to the molality S of the species formed in solution, the aggregation number can be derived by dividing the total molality of molecules introduced (B) by S36

the molarity of extractant molecules in the organic phase. For concentrations over the CAC, as the extractant molecules start to form aggregates a break in linearity occurs due to an increase of the scattered intensity for objects of bigger volume. In this case, the CAC also corresponds to the abscissa of the break in linearity (Figure 4 d−f). For TOPO ratios 0% and 50%, no clear linear deviation allowed a determination of CAC. This may happen when interactions between species, which result in a decrease in the scattered intensity, intervene in the same range of concentrations than the monomer to micellar transition. For 20% TOPO ratio a value of 0.2 mol·L−1 comparable to the one obtained by VPO was obtained. This concentration is significantly higher than monomers concentration extracted from dimerization constants experimentally determined for diluted HDEHP in toluene or alkane ([monomers] < 0.002−0.07 mol/L).50,51 In this particular system, where HDEHP is known to form mainly dimers, the CAC can be interpreted as the concentration from which extractant molecules start to form aggregates bigger than monomers and dimers and can therefore not be compared to the concentrations calculated from the dimerization constants. From these two data sets, CAC values close to 0.2 mol/L whatever the TOPO ratio are obtained. Contrary to what was shown for the same organic phases contacted to aqueous phase containing uranium,27 no apparent minimization of CAC appears when no cations are extracted. According to this result, enhanced synergistic extraction properties of HDEHP/ TOPO mixture can therefore not be related to a synergism in the micellization properties of pure/native extractants. Aggregation numbers ⟨Nag⟩, averaged on all aggregates species including the monomers, have been estimated from VPO, SAXS, and SANS fitting and compared to results obtained from molecular dynamics calculations.

Nag =

B S

where S is obtained from the VPO signal and a calibration constant determined on a solute that forms nearly ideal mixtures. Trioctylamine was chosen as the reference system (Figure SI-3 in ESI). Aggregation numbers of 2.1−2.3 were obtained without significant inflection at the synergistic ratio. To confirm these results and to complete the description of the aggregates (internal structure of the aggregates and distribution of aggregation numbers) simultaneous fitting of the SANS and SAXS spectra for all TOPO ratios have been done following the methodology described in the Experimental and Theoretical Section. The structural quantities adjusted for the fit are the average aggregation number ⟨Nag⟩, and the chain length of the diluent penetrating in the shell of aggregates, x. They are reported in Table 2 for all TOPO ratios tested, together with the deduced averaged radii of the aggregates and of the cores and the scattering length densities constants. The spectra simultaneously calculated for SAXS and SANS are presented as straight lines in Figure 3. They are in good agreement with the experimental data, which validates the core−shell model used with a modulated penetration of the diluent in the aggregates. SAXS fits are however not perfect in the intermediate q range. They have been considered as satisfying for such a constrained model with only two fitting parameters. Discrepancies observed on the calculated SAXS spectra can be attributed to the behavior of the solvent signal in E


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the ligands as well as the structure of supramolecular aggregates with derived parameters as accurate as the statistical distribution of the aggregation numbers. Simulations were therefore conducted to confirm the nature of the interactions between the ligands and the supramolecular parameters obtained experimentally and described in this paper. Experimental densities and water concentrations were measured for six HDEHP/TOPO molar ratios. MD simulations boxes were built with the compositions given in the SI, with molecules randomly distributed. Applying a radial distribution function on the MD simulation boxes’ snapshots, the distribution of the extractant engaged in n-mers has been established (Table 3) and was used to calculate the average aggregation numbers for each TOPO ratio.

Table 2. Calculated and Adjusted Parameters Used for the SAXS and SAXS Calculated Spectra at [HDEHP + TOPO] = 0.5 M in Heptane TOPO ratio

x

⟨Nag⟩

⟨Rag⟩ (nm)

⟨Rcore⟩ (nm)

0% 20% 50%

1.6 1.7 1.9

2.7 2.8 2.7

0.83 0.86 0.87

0.35 0.33 0.28

this range. As shown in Figure SI-5, signals of organic solvents as heptane present a shoulder before the well-known solvent peak that can be attributed to interchains interactions. In this q range the solvent structure is expected to be modified by the presence of the extractant molecules alkyl chains. These kinds of interactions are not taken into account in our model. As shown in Table 2, average aggregation numbers of 2.7− 2.8 are obtained with no significant inflection at the TOPO ratio of 20% if we consider the error bar estimated to 0.2. Averaged core and aggregate radii do not vary significantly with the TOPO ratio. As previously noticed, the evolution of SAXS intensity at low q with TOPO ratio was singular at the synergistic ratio. It appears that the rationalized calculation of SAXS curves shows the same increase of intensity for TOPO 20% without being related to the specific behavior of the CAC or Nag. Taking into account the main parameters influencing intensity at low q as proposed in eq 1 and their quantitative estimation in Table 2, it is possible to interpret the phenomenon as a simple effect of scattering length densities and volume of cores and aggregates. The two terms of the sum VcoreΔρcore + VagΔρag show, respectively, an increase and a decrease with the TOPO ratio that compensates at 0% and 50%, leading to maximum values for intermediate ratios. The evolutions of these terms are detailed in the ESI. Three main conclusions can be issued from the fitting of the combined SANS and SAXS patterns constrained by the composition of the sample. First, there is no significant variation of the radius (∼3 Å for the polar radius and ∼8 Å for the aggregates) at the synergistic ratio (20% of TOPO). Second, the experimental singular excess of SAXS intensity at 20% is retrieved by the calculation and is not related to a decrease of the CAC or clear inflection of aggregation number. Third, the continuous increase of SANS intensities at low q with the TOPO ratio is due to both an increase of the aggregate volume and the scattering length density contrast. These results confirm that the extraction synergistic behavior of HDHEP/ TOPO in the presence of uranium is not driven by a premicellization effect of the extractant molecules. The rather simple model used to calculate SANS and SAXS patterns appears consistent but shows some limitations to describe the exact structure of the organic solutions. The main problems are due to the very small sizes of the aggregates, which lead to difficult discrimination of the scattering signals from aggregates and solvent, especially on SAXS fitting. The model of core−shell reverse aggregates becomes moreover debatable when aggregation numbers as small as 2−4 are considered and when the polar entities expected to be gathered into the polar cores concern one or less molecules per aggregate. To go beyond these limitations, we turn toward molecular dynamics, which can give a complete description of the solution at different scales.52 C. Molecular Dynamics Simulations. Molecular dynamics simulations can describe the molecular interactions between

Table 3. Aggregates’ Repartition in Simulation Boxes (in % of aggregates) for Various Ratios of TOPO and Derived Average Aggregation Numbers % TOPO

0

20

50

monomers dimers trimers tetramers pentamers hexamers ⟨Nag⟩

31.3 39.6 25 4.2 0 0 2.02

33.3 33.3 27.1 4.2 2.1 0 2.08

36 34 22 6 2 0 2.04

Results show that a mixture of monomers and aggregates of various sizes and compositions coexists in solution. Main species are illustrated in Figure 5, showing the nature of interactions between the ligands. In agreement with the results obtained from IR analysis, HDEHP molecules interact with themselves and with TOPO through the formation of a hydrogen bond between the acidic function of HDEHP and TOPO oxygen atom. In some of the aggregates a water molecule can also participate with the hydrogen bond network. TOPO dimers interact through dipole−dipole interactions. Table 3 gives also the composition of the aggregates. When only HDEHP is present in the heptane solution (TOPO 0%), the maximum aggregation number is 4. As reported in the literature, HDEHP dimers and trimers are the predominant species.25,53 In the presence of TOPO, mixed aggregates are formed in solution with up to 2 TOPO per aggregate. The maximum aggregation number reaches 6, which is consistent with the tendency obtained by ESI-MS, reinforcing also the major conclusion: the favored interaction between the two extractants molecules induces a large variety of mixed species in the organic solution with an aggregation number ranging from 2 to 6. In addition, when the TOPO ratio increases, the number of mixed HDEHP−TOPO species raises gradually while the number of HDEHP aggregates with no TOPO decreases. This effect appears clearly in Figure 6, where yellow to orange masses represent pure HDEHP species and the green to blue ones mixed aggregates. This representation confirms also that there is no peculiar molecular organization at a synergic ratio of 20%. Beyond the scope of understanding the synergism in mixed extractant system, this study demonstrates that MD can be useful to describe stable mesoscale heterogeneities of extractant in an F


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Figure 5. MD simulation snapshots of HDEHP, TOPO, or HDEHP−TOPO aggregates: orange, HDEHP; blue, TOPO. Some of them contain a water molecule (white and red).

Figure 6. Snapshots of boxes composed of various TOPO ratios in heptane. Yellow to orange spots indicate pure HDEHP aggregates having increasing Nag from 1 to 3. Green to cyan colors are used for mixed HDEHP/TOPO aggregates, and blue spots show pure TOPO aggregates.

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CONCLUSIONS For the first time a complete description at different scales of a synergistic extractant solution is obtained prior to its contact with water. Combined IR, DFT, ESI-MS, and molecular dynamics simulations have shown the existence of significant molecular interactions between the two extractants HDEHP and TOPO. When the neutral TOPO extractant is introduced in an organic solution containing the acidic HDEHP extractant, HDEHP-to-HDEHP hydrogen bonds are broken while hydrogen bonds between TOPO and HDEHP are majorly formed. This interaction is evidenced by a strong structural reorganization of the solution with the formation of mixed HDEHP− TOPO aggregates. According to molecular dynamic simulations, the maximum aggregation number is six, but a mixture of monomers and aggregates of various sizes and compositions coexists in solution. The number of mixed aggregates gradually increases with the increase of TOPO:HDEHP molar ratio with no specific organization at a synergistic 1:4 molar ratio. The average aggregation number, determined from VPO, SAXS, SANS, and molecular dynamics, is close to 2, and the CAC is found to be close to 0.2 M, without any significant variation with the TOPO molar ratio. It is found that HDEHP/ TOPO mixture does not aggregate synergistically prior to contact with an aqueous phase containing cations. This indicates that the synergistic aggregation observed after contact with aqueous phase is a consequence of the maximized uranium

organic binary system without extra added water, a domain which is poorly studied.19,54,55 The results obtained experimentally can be compared and confirmed with those obtained from molecular dynamic simulations. VPO, SANS/SAXS, and MD give mean aggregation numbers in the same order of magnitude, as summarized in Table 4. Table 4. Determination of Aggregation Numbers by VPO, SANS, and SAXS Fitting and Molecular Dynamics % TOPO

⟨Nag⟩ VPO

⟨Nag⟩ SAXS/SANS fitting

⟨Nag⟩ molecular dynamics

0% 20% 50%

2.1 ± 0.2 2.3 ± 0.2 2.2 ± 0.2

2.7 ± 0.3 2.8 ± 0.3 2.7 ± 0.3

2.1 ± 0.2 2.1 ± 0.2 2.0 ± 0.2

Aggregation numbers derived from MD simulations are in the same range as those obtained from VPO and SANS/SAXS analysis. Mean aggregation numbers obtained from SAXS/ SANS data may appear slightly higher despite the uncertainties. The fit of the data with a pure spherical model may be slightly constraining and may explain a slight overestimation of the aggregation numbers. However, looking at the three methods, the most important results remains that the order of magnitude of mean aggregation number is of 2−3 molecules with no clear deviation at the synergistic ratio. G


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min−1, nebulizer gas 5 psi, temperature 250 °C, ion spray voltage 4000 V. Spectra were acquired over a mass range of m/z 40−3000. The QTOF was calibrated daily using an Agilent (G2421A) ES-TOF tuning solution. Fragmentation mass spectra were obtained from collision-induced dissociation with nitrogen. A syringe infusion pump (Cole Palmer) delivered samples at 90 μL·h−1 to the electrospray source. Organic phases were diluted 1/10 000th in methanol before analysis. VPO Data Measurement and Analysis. VPO measurements of the organic phases were performed at 25 °C using a Knauer K-7000 vapor pressure osmometer. The calibration of the osmometer was carried out using trioctylamine and octanoic acid, respectively, as monomeric and dimeric standards. The results provided a linear relationship between the measured signal (mV) and the mole fraction of the solute. The resulting slope values were used to calculate the sum of the concentrations of all species (S). The uncertainties of the VPO measurement are estimated to be ±10%. The studies were performed with n-heptane as a diluent. MD Simulations. MD simulations were performed using the AMBER 10 software with the parm99 force field57,58 and taking explicitly into account polarization effects which are essential for a good representation of the dispersion forces and polarization interactions.59 Atomic partial charges on HDEHP, TOPO, and n-heptane were calculated with the restricted electrostatic potential procedure.60,61 Water molecules were described using the POL3 model.62 The MD simulation boxes compositions were based on the corresponding experimental organic phases analyses, for which all compound concentrations have been determined. MD simulation boxes were built starting with random positions of the solute and solvent molecules throughout space (simulated boxes characteristics are given in Table 4) and using periodic boundary conditions. Equations of motion were numerically integrated using a 1 fs time step. A 15 Å truncation cutoff was used, and long-range interactions were calculated using the particle-mesh Ewald method.10 Boxes were equilibrated for at least 500 ps in the NPT (constant number of atoms, constant pressure and temperature) ensemble. For each simulated solution, the equilibration achievement was verified by comparing calculated with experimental densities at the end of the equilibration. MD simulation runs were subsequently collected for 5 ns. Trajectories were then analyzed using VMD software.11 Small Angle X-ray Scattering. SAXS experiments were carried out on a bench built by Xenocs and using Mo radiation (λ = 0.71 Å). A large online scanner detector (MAR Research 345) located at 750 mm from the sample stage was used to record the scattered beam. Thanks to off-center detection, a large scattering vector q range was covered (2 × 10−1 nm−1 < q < 30 nm−1). Collimation was applied using a 12:∞ multilayer Xenocs mirror (for Mo radiation) coupled to two sets of Forvis scatterless slits providing a 0.8 mm × 0.8 mm X-ray beam at the sample position. The experimental resolution is Δq/q = 0.05. The detector count is normalized to differential cross section per volume (in cm−1) with either a 2.36 mm thick high-density polyethylene sample (from Goodfellow) (Imax = 4.2 cm−1) or 3 mm water for which the level of scattering at low q is known (1.64 × 10−2 cm−1). Data preanalysis was performed using FIT2D software, taking into account the electronic background of the detector (the flat field response being homogeneous), transmission measurements (using a photodiode that can be inserted upstream the sample), and empty cell subtraction. The

extraction at the synergistic TOPO ratio as a result of an aggregates nucleation effect, as it was recently described by Dejugnat et al.28 In their study, higher aggregation numbers, smaller CAC and therefore favored aggregation states were evidenced for higher acidic concentrations in the aqueous phases. This feature is also observed in this study where aggregation numbers as low as 2−3 are obtained, compared to values of 5−6 in the presence of acid and metallic cations. We can conclude that the synergistic extraction properties of a HDEHP/TOPO mixture cannot be related to a synergism in the micellization properties of pure extractants. The nature of the ions to be extracted with the curvature effect due to water solubilization properties has to be taken into account to go further in the understanding of synergism in extractant solutions. Beyond this single aspect, this article is very promising to go toward the description of a reverse micellar system without water solubilization, which is still poorly studied in the literature.

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EXPERIMENTAL AND THEORETICAL SECTION Materials and Methods. HDEHP (purity > 97%) and trin-octylphosphine oxide (TOPO) (purity > 98.5%) from SigmaAldrich were used as received. n-Heptane, n-dodecane, and nheptane D16 from Carlo Erba and Eurisotop were used without prior purification. Sample Preparation. For all of the samples solutions of HDEHP and TOPO were prepared by weighting the appropriate compounds in n-heptane and n-dodecane to attain the final chosen concentration. For HDEHP/TOPO mixture a total (i.e., [HDEHP] + [TOPO]) concentration is prepared for different weight fraction of TOPO X = mTOPO/(mTOPO + mHDEHP) going from 0% to 20% to 50%. The compositions of the different samples used for the different analytical tools are summarized in Table 5. Table 5. Samples Compositions Used for the Different Techniques (see ESI for more details) techniques IR

ESI-MSa SANS, SAXS, VPO a

[HDEHP] [TOPO] 0.3 0 0.3 0.4

0 0.3 0.3 0.1

[HDEHP + TOPO]

X = mTOPO/mTOPO + mHDEHP

0.3 0.3 0.6 0.5 0.01−0.5 M

0 1 0.5 0%, 20%, and 50%

Dilution 1/1000 in methanol.

Infrared Spectroscopy and DFT Calculations. FTIR spectra were recorded using a PerkinElmer 100 FT-IR spectrometer equipped with the universal attenuated total reflectance (ATR) sampling accessory. Infrared spectra were simulated from DFT calculations using the Gaussian 09 software package.56 Frequencies and intensities were obtained from B3LYP/6-311G(d,p) calculations in the presence of a continuum solvent model (IEFPCM) corresponding to n-dodecane. Electrospray Ionization Mass Spectrometry. Mass spectrometry measurements were recorded in positive ionization mode using a Bruker micrOTOF-QIII equipped with an electrospray ionization source and a time-of-flight analyzer. The experimental conditions were as follows: drying gas (N2), 5 L· H


Article


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scattering intensities were thus expressed versus the magnitude of the scattering vector. Small Angle Neutron Scattering. SANS experiments were performed at the French neutron facility Laboratoire Léon Brillouin/Orphée (LLB/Orphée) on PACE and PAXY spectrometers. Two configurations were used on the PAXE spectrometer with one sample to detector distance of 1145 mm, two values of wavelength λ equal to 7.8 and 4 Å (Δλ/λ ≈ 10%), and a standard two-diaphragm collimation geometry (7 mm/20 mm, collimation distance of 2500 mm). By shifting the position of the detector relative to the neutron beam center we can increase both the accessible qmax and the overlap between the two configurations which permit one to cover a total q range from 0.01 to 0.6 Å−1. All measurements were done under atmospheric pressure and room temperature. Measurements were performed in quartz Hellma cells with an optical path of 2 mm. Pasinet software (ref Pasinet, http://pasinet-mat.software. informer.com) was used for data reduction, standard corrections (for sample volume, neutron beam transmission, empty cell signal subtraction, inelastic scattering, and detector efficiency), and normalization procedure (from the incident neutron beam). The methodology employed for the simultaneous fit of the SANS and SAXS data is detailed in the ESI.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b11693. Sample compositions, ESI-MS, vapor pressure osmometry, SANS data treatment, and molecular dynamics (PDF)

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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 This work was supported by the program NEEDS Resources. We also acknowledge Jacques Jestin and Bruno Corso for their help in measuring and T. Zemb and P. Bauduin for fruitful discussions.

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REFERENCES

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