Giuseppe Cavallaro

Giuseppe Lazzara
email: giuseppe.lazzara@unipa.it

Frédéric Pignon

Leonardo Chiappisi

Erwan Paineau
email: erwan-nicolas.paineau@universite-paris-saclay.fr.

Effect of Polymer Length on the Adsorption onto Aluminogermanate Imogolite Nanotubes

This study evidences the adsorption of model non-ionic polymers onto aluminogermanate imogolite nanotubes, an attractive porous nanofiller with potential molecular loading and release applications. We resolve the underlying mechanisms between nanotubes and polyethylene glycols with different molecular weight by means of nano-isothermal titration calorimetry. The analysis of the results provides a direct thermodynamic characterization, allowing to propose a detailed description of the energetics involved in the formation of polymer/imogolite complexes. The affinity towards the nanotube surface is enthalpy-driven and strongly depends on the polymer chain length, which significantly affects the polymer configuration and the flow properties of the resulting complexes, probed by small-angle neutron scattering and rheology, respectively. These findings open new avenues for the rational design of these hybrid mixtures for advanced applications.

INTRODUCTION

Imogolite nanotube (INT) is an aluminosilicate clay mineral, naturally occurring and widespread in well-drained volcanic ash soils. [START_REF] Dahlgren | Mineralogical Characteristics of Volcanic Ash Soils[END_REF] Unlike halloysite, another tubular clay mineral, [START_REF] Joussein | Halloysite Clay Minerals-a Review[END_REF] INTs can be readily synthesized through hydrothermal methods. [START_REF] Farmer | Synthesis of Imogolite -Tubular Aluminum Silicate Polymer[END_REF][START_REF] Paineau | Imogolite Nanotubes: A Flexible Nanoplatform with Multipurpose Applications[END_REF] The structure of these nanotubes is rather unique. The external walls consist of gibbsite-like sheet Al(OH) 3 while the cavity interface is formed by isolated (SiO 3 )OH tetrahedron units connected upright to the octahedral vacancies by covalent bonding. [START_REF] Cradwick | a Hydrated Aluminum Silicate of Tubular Structure[END_REF] Silicon can be replaced by germanium allowing to increase the diameter of the inner cavity from 1.5 to 3 nm for single-walled INTs. [START_REF] Mukherjee | Highly Ordered, Single-Walled Mixed-Oxide Nanotubes Assemble from Amorphous Nanoparticles[END_REF][START_REF] Thill | How the Diameter and Structure of (OH)(3)Al2O3SixGe1-XOH Imogolite Nanotubes Are Controlled by an Adhesion versus Curvature Cornpetition[END_REF] Furthermore, modifications of the synthesis conditions offer a convenient way for designing innovative INTs with well-defined morphologies (single or double-walled structures), [START_REF] Thill | Physico-Chemical Control over the Single-or Double-Wall Structure of Aluminogermanate Imogolite-like Nanotubes[END_REF] modular interfaces (e.g. hydrophilic or hydrophobic cavities) [START_REF] Kang | Direct Synthesis of Single-Walled Aminoaluminosilicate Nanotubes with Enhanced Molecular Adsorption Selectivity[END_REF][START_REF] Amara | Tunable-Diameter, Metal Oxide Nanotubes for Trapping of Organic Molecules[END_REF][START_REF] Picot | Dual Internal Functionalization of Imogolite Nanotubes as Evidenced by Optical Properties of Nile Red[END_REF] and high colloidal stability in aqueous media. [START_REF] Lisuzzo | Stability of Halloysite, Imogolite, and Boron Nitride Nanotubes in Solvent Media[END_REF][START_REF] Paineau | Colloidal Stability of Imogolite Nanotube Dispersions: A Phase Diagram Study[END_REF][START_REF] Paineau | Role of Initial Precursors on the Liquid-Crystalline Phase Behavior of Synthetic Aluminogermanate Imogolite Nanotubes[END_REF] All these properties have led in recent years to a renewed interest in synthetic imogolite-type nanotubes as promising nano-reactors notably for molecular confinement [START_REF] Kang | Solution-Processed Ultrathin Aluminosilicate Nanotube-Poly(Vinyl Alcohol) Composite Membranes with Partial Alignment of Nanotubes[END_REF][START_REF] Fernandez-Martinez | Curvature-Induced Hydrophobicity at Imogolitewater Interfaces[END_REF][START_REF] Nasi | Hybrid Organic-Inorganic Nanotubes Effectively Adsorb Some Organic Pollutants in Aqueous Phase[END_REF][START_REF] Monet | Solid Wetting-Layers in Inorganic Nano-Reactors: The Water in Imogolite Nanotube Case[END_REF] or photocatalytic applications. [START_REF] Poli | The Potential of Imogolite Nanotubes as (Co-)Photocatalysts: A Linear-Scaling Density Functional Theory Study[END_REF][START_REF] Elliott | Chemically Selective Alternatives to Photoferroelectrics for Polarization-Enhanced Photocatalysis: The Untapped Potential of Hybrid Inorganic Nanotubes[END_REF][START_REF] Olson | Utilizing Imogolite Nanotubes as a Tunable Catalytic Material for the Selective Isomerization of Glucose to Fructose[END_REF][START_REF] Poli | The Role of Cation-Vacancies for the Electronic and Optical Properties of Aluminosilicate Imogolite Nanotubes: A Non-Local, Linear-Response TDDFT Study[END_REF][START_REF] Li | Metal Chalcogenides Based Heterojunctions and Novel Nanostructures for Photocatalytic Hydrogen Evolution[END_REF] INT offers similar characteristics to those of carbon nanotubes (CNTs) in terms of rigidity, sizes and aspect ratios while retaining transparency and self-organization in highly dilute conditions. [START_REF] Paineau | A Liquid-Crystalline Hexagonal Columnar Phase in Highly-Dilute Suspensions of Imogolite Nanotubes[END_REF][START_REF] Paineau | Influence of the Al/Ge Ratio on the Structure and Self-Organization of Anisometric Imogolite Nanotubes[END_REF] Although they display larger band gaps than CNTs, [START_REF] Poli | Termination Effects in Aluminosilicate and Aluminogermanate Imogolite Nanotubes: A Density Functional Theory Study[END_REF] they are cheaper to produce with monodisperse diameters without further sorting techniques. Consequently, imogolite nanotubes represent an interesting alternative of the carbon nanotubes in numerous applications including the fabrication of polymer/filler nanocomposites with multifunctional characteristics. Within this, imogolite was successfully employed with various polymers as a reinforced filler for functional nanocomposites with enhanced mechanical, optical, or electrical properties, including self-healing capability. [START_REF] Park | Design and Characterization of Hybrid Hydrogels Composed of Imogolite Fibrous Nanotubular Clay and Hyaluronic Acid[END_REF][START_REF] Shikinaka | Design of Stimuli-Responsive Materials Consisting of the Rigid Cylindrical Inorganic Polymer "Imogolite[END_REF][START_REF] Ryu | Structural Response of Imogolite-Poly(Acrylic Acid) Hydrogel under Deformation[END_REF][START_REF] Su | Properties of Single-Walled Aluminosilicate Nanotube/Poly (Vinyl Alcohol) Aqueous Dispersions[END_REF][START_REF] Li | Effects of Aluminogermanate Imogolite Nanotube Orientation on Mass Transport across Polyamide Nanocomposite Membranes[END_REF][START_REF] Lee | Inorganic Nanotube Mesophases Enable Strong Self-Healing Fibers[END_REF][START_REF] Li | Structure and Properties of Hybrid Film Fabricated by Spin-Assisted Layerby-Layer Assembly of Sacran and Imogolite Nanotubes[END_REF][START_REF] Mukai | Preparation of an (Inorganic/Organic) Hybrid Hydrogel from a Peptide Oligomer and a Tubular Aluminosilicate Nanofiber[END_REF] An interesting advantage of INTs is the high density of hydroxyl groups on the outer surface (~ 18 OH/nm²). It should promote different possibilities of polymer adsorption, [START_REF] Bonini | Adsorption of Amino Acids and Glutamic Acid-Based Surfactants on Imogolite Clays[END_REF] or change in conformation as shown recently by nuclear magnetic resonance. [START_REF] Kang | Single-Walled Aluminosilicate Nanotube/Poly(Vinyl Alcohol) Nanocomposite Membranes[END_REF][START_REF] Lange | Partial Transformation of Imogolite by Decylphosphonic Acid Yields an Interface Active Composite Material[END_REF] To date, however, the underlying mechanisms remain largely unexplored.

One of the most used techniques to probe the polymer-nanoclay interaction mechanism is isothermal titration calorimetry (ITC), which allows the experimentalist to determine the thermodynamic parameters for the adsorption phenomenon through direct measurements. It allowed clarifying the interactions in terms of hydrophobic/electrostatic forces and polymer degree of freedom through the evaluation of the entropic contribution. [START_REF] Bertolino | Biopolymer-Targeted Adsorption onto Halloysite Nanotubes in Aqueous Media[END_REF][START_REF] Archer | Isothermal Titration Calorimetry: Practical Approaches and Current Applications in Soft Matter[END_REF] Moreover, the determination of the adsorption isotherm and the maximum amount of polymer adsorbed per mass of nanoclay in comparison with the polymer gyration radius and geometric features of the nanoparticle, provided an indirect representation on the polymer conformation at the interface. [START_REF] Cavallaro | Aqueous Phase/Nanoparticles Interface: Hydroxypropyl Cellulose Adsorption and Desorption Triggered by Temperature and Inorganic Salts[END_REF] It should be noted that isothermal calorimetry is the only correct approach to determine the enthalpy change of a reaction if the stoichiometry may change with temperature as, in the latter case, the well-known van't Hoff approach cannot be used. [START_REF] Holtzer | Use of the van't Hoff Relation in Determination of the Enthalpy of Micelle Formation[END_REF] Here, we investigated the adsorption of non-ionic polymers with variable molecular weight on the outer surface of anisometric double-walled germanium-based imogolite (Ge-DWINTs) with outer diameter of 4.5 nm and average length around 100 nm (Figure 1). [START_REF] Paineau | Role of Initial Precursors on the Liquid-Crystalline Phase Behavior of Synthetic Aluminogermanate Imogolite Nanotubes[END_REF] Polyethylene glycols (PEGs) with variable molecular weight are selected as model hydrosoluble non-ionic polymers that are frequently added with clay minerals. [START_REF] Cavallaro | Films of Halloysite Nanotubes Sandwiched between Two Layers of Biopolymer: From the Morphology to the Dielectric, Thermal, Transparency, and Wettability Properties[END_REF][START_REF] Chiu | Self-Assembly Behavior of Polymer-Assisted Clays[END_REF][START_REF] El Rifaii | Destabilization of the Nematic Phase of Clay Nanosheet Suspensions by Polymer Adsorption[END_REF] We proposed the first detailed description on the energetics involved in the formation of polymer/imogolite complexes by combining ITC measurements with small-angle neutron scattering (SANS) and rheological experiments. Determination of thermodynamic parameters allowed predicting the fraction of polymer adsorbed onto the nanotubes surface. The adsorption mechanism is dependent of the PEG chain length, modifying the conformation and the flow properties of the resulting mixtures. Oxygen and hydrogen atoms are represented in red and white, respectively.

EXPERIMENTAL SECTION

Materials. Polyethylene glycols (PEGs) with various average molecular weight (M w = 200, 400, 2k, 3k and 20k g.mol -1 ), were purchased from Sigma Aldrich. Polymer solutions have been prepared by weight (±0.01 mg) and keeping under magnetic stirring overnight. Details on the synthesis and characterizations of Ge-DWINT clay nanotubes have been reported elsewhere. [START_REF] Amara | Single-Step Formation of Micron Long (OH)3Al2O3Ge(OH) Imogolite-like Nanotubes[END_REF] After dialysis, as-obtained Ge-DWINTs form stable colloidal dispersions (C = 10 g.L -1 ), where the nanotubes are individually dispersed. [START_REF] Paineau | A Liquid-Crystalline Hexagonal Columnar Phase in Highly-Dilute Suspensions of Imogolite Nanotubes[END_REF] Mixtures of Ge-DWINTs with PEGs were prepared by adding the appropriate amount of polymer at different polymer/imogolite mass ratio (R P/INTs ) while keeping constant the imogolite concentration.

Isothermal Titration Calorimetry (ITC).

ITC experiments were performed by using the ultrasensitive nano-ITC200 calorimeter (MicroCal). An amount of approximately 40 µL of the water/polymer mixture was injected into the thermally equilibrated ITC cell (200 µL) containing the water/imogolite dispersion (concentration of 4.47 mg.L -1 ). Each addition step was 0.49 µL. Volumes have been calibrated by NaCl dilution experiment. [START_REF] Tellinghuisen | Calibration in Isothermal Titration Calorimetry: Heat and Cell Volume from Heat of Dilution of NaCl (Aq)[END_REF] The concentration of the polymer solutions was ca. 5 g.L -1 . The cell was thermally equilibrated at 25.000 ± 0.005 °C. The calorimeter sensitivity is at least 2 nanoW. The raw data were corrected for the instrument time constant and an appropriate baseline was subtracted.

Small-Angle Neutron Scattering (SANS). SANS measurements were carried out at Institut

Laue-Langevin (ILL), Grenoble (France), on the instrument D11. [START_REF] Lieutenant | A New Design for the Standard Pinhole Small-Angle Neutron Scattering Instrument D11[END_REF] The experiments were conducted at two different configurations with sample-to-detector (and collimation in parenthesis) distances of 2 m (5 m), 8 m (8 m), using a wavelength λ of 6 Å and Full Width at Half Maximum (FWHM) of 10%. The scattering vector q was calculated as , θ

being the scattering angle. The two-dimensional patterns were corrected for the detector efficiency using the scattering of a 1 mm H 2 O sample and for the dark current signal. The contribution from the H 2 O/D 2 O solvent was subtracted and finally, the patterns were radially averaged, the scattering being isotropic. Data reduction was performed with BERSANS, [START_REF] Keiderling | The New 'BerSANS-PC'Software for Reduction and Treatment of Small Angle Neutron Scattering Data[END_REF] while SASfit 0.94.11 software was used for the analysis of SANS curves. [START_REF] Breßler | A Tool for Small-Angle Scattering Data Analysis Using a Library of Analytical Expressions[END_REF] The experiments were performed diluting the stack Ge-DWINTs dispersion with D 2 O and the final solvent was As evidenced from Table 1, the polymer molecular weight affects both the thermodynamic parameters and the maximum mass of PEG adsorbed onto Ge-DWINTs. The equilibrium constant displays a monotonic evolution according to the PEG chain length and it is related to different affinity. As shown for clay nanosheets, a limited affinity for the clay surface was observed for PEG with low molecular weight (1000-4000 g.mol -1 ) whereas higher values resulted in an increase of the adsorbed mass of polymer in conjunction with a change of polymer conformation. [START_REF] Burchill | Smectite-Polymer Interactions in Aqueous Systems[END_REF][START_REF] Luckham | The Colloidal and Rheological Properties of Bentonite Suspensions[END_REF] The negative values of standard free energy indicate that PEG adsorption onto Ge-DWINTs surface is energetically favored. Both and decrease with PEG molecular weight, which indicates an enthalpy driven adsorption process for all the mixtures.

Given that the enthalpy of adsorption per mole of polymer is always negative, PEG/imogolite interactions are the dominant forces in the adsorption process although the enthalpic contribution is much more relevant for polymers with a longer chain. As concerns , we determined positive values for all the PEGs used in this work (except for PEG20k). In general, the change in entropy is related to the following contrasting effects: (i) a loss of configuration freedom of the polymer in the adsorbed state, which induces a decrease of entropy ( < 0) or (ii) a water release from the polymer ( > 0). The positive entropy values highlight that de-hydration from the adsorbed PEG represents the dominant entropic factor during the adsorption. On the other hand, < 0 was observed for the highest molecular weight suggesting that the dehydration phenomenon is less relevant for this mixture.

The maximum amount of PEG molecules adsorbed onto the imogolite surface decreases sharply with the polymer molecular weight (Z values in Table 1) and therefore the most hydrated structure is expected for PEG20k compared to the other polymers. Based on the thermodynamic parameters (Table 1), we can deduce the fraction of polymer adsorbed onto imogolite surfaces at there is more place for small polymers (expressed in total mass), but less tendency to fill that space as evidenced by the equilibrium constants.

The dependence of the affinity toward the Ge-DWINTs surface from the PEG molecular weight was further investigated by SANS for PEG2k and PEG20k. It should be noted that the scattering from dilute Ge-DWINTs dispersions (1 g.L -1 ) in the same solvent has a negligible flat scattering intensity in the investigated scattering vector regime (Figure S1). As a general feature (Figure 4), the addition of Ge-DWINTs to PEG solutions does not influence the shape of the curves. All scattering curves were analysed with a model of a Gaussian coil [START_REF] Debye | Molecular-Weight Determination by Light Scattering[END_REF] :

with I 0 and R g being the forward scattering intensity and the radius of gyration of the chain, respectively. The values are given in Table 2. The overlap concentration was calculated from the values of the radii of gyration, as:

Confirming that the experiment were performed in the dilute regime, the interactions between the polymer chains can be estimated between the ratio of the determined forward scattering and the predicted value, 53 calculated as:

In eq. 6, is the polymer concentration in volume fraction, d p its density (a value of 1.199 g cm -3 was used) [START_REF] De Lisi | Thermodynamic Behavior of Non-Ionic Tri-Block Copolymers in Water at Three Temperatures[END_REF] , and the scattering contrast. A ratio of 0.5 and 0.1 for PEG2k and PEG20k, respectively, between the experimental and the calculated forward scattering intensity indicates a significant repulsive interaction between the polymer chains, in agreement with previous studies (Table 2). 53 The analysis of the SANS data shows that the conformation of the polymer is not, or only minimally, affected by the presence of the clay nanotube, as deduced from constant value of radius of gyration obtained. In contrast, an increase in scattering intensity is observed upon addition of Ge-DWINTS to the dilute PEG solution. The origin of this increase has to be found in the attractive interaction between the nanoclay and the polymer. In fact, both the adsorption of PEG on the Ge-DWINTs and a reduced repulsion between the PEG chains in solution would result in an increase of the forward scattering intensity. The effect is more pronounced for the longer PEG chains for which the radius of gyration increases from 2.5 to 3 nm with decreasing R P/INTs (Table 2). This slight stretching of PEG20k chains can be ascribed to a crowding phenomenon due to the larger amount of polymer adsorbed onto Ge-DWINTs predicted by the ITC experiments, ca 0.96 compared to 0.87 for R P/INTs = 4 and 18, respectively. The stretching of the polymer chain is an indication of a relatively compact organization at the solid/liquid interface. This is confirmed by the grafting density of ca. 1.6 chains nm -2 , estimated from Z values reported in Table 1 and by using the experimental specific volume of Ge-DWINTs (0.3949 cm 3 g -1 ) and the characteristic sizes of the nanotubes. [START_REF] Paineau | Role of Initial Precursors on the Liquid-Crystalline Phase Behavior of Synthetic Aluminogermanate Imogolite Nanotubes[END_REF] It should be noted that the fits of the scattering curves for PEG2k provided a radius of gyration of 1.5 nm with and without Ge-DWINTs in agreement with the thermodynamic prediction of negligible interactions in this case.

Therefore, we should expect different rheological and colloidal behaviors for these systems. In particular, depletion processes can play a role for PEG2k/Ge-DWINTs and steric colloidal stabilization for PEG20k/Ge-DWINTs (as a consequence of the large amount of polymer adsorbed onto Ge-DWINTs surfaces).

We investigated the flow properties of PEGs/Ge-DWINTs complexes (PEG2k and PEG20k) prepared at a fixed R P/INTs = 10 while varying the concentration of imogolite nanotubes (C INTs ). In this case, the fraction of polymer adsorbed onto Ge-DWINTs is the same whatever the value of C INTs but varies by a factor of 2 between the two series (Figure 3). This should allow us to probe how the difference in amount of PEG adsorbed onto the nanotube surface impact the viscosity of the suspensions. The flow curves (viscosity  vs. shear stress ) exhibit a shear thinning behavior whatever the molecular weight or the nanotube concentration (Figure 5a,b).

With increasing shear stress, hydrodynamic interactions tend to overcome electrostatic interaction between PEGs/Ge-DWINTs complexes that end up aligning in the flow. To go further, we reproduced flow curves by adjusting a viscosity model based on the Quemada's approach: [START_REF] Baravian | Rheological Determination of Interaction Potential Energy for Aqueous Clay Suspensions[END_REF][START_REF] Paineau | Aqueous Suspensions of Natural Swelling Clay Minerals. 2. Rheological Characterization[END_REF] where represents the viscosity for infinite shear stress and  c is a critical shear stress. Its value is close to  when hydrodynamics effects are comparable to Brownian and interaction energies.  is positive when a viscosity plateau at 0-shear is measured while it is negative when a yield stress occurs in the dispersion such as:

. By adjusting eq. 7a using the least-square method, fair agreement was obtained between experimental and calculated flow curves for both PEG2k and 20k/Ge-DWINTs complexes (Figure 5a,b). The evolution of yield stress with Ge-DWINTs concentration depending the PEG molecular weight is represented in Figure 5c. The yield stress remains the same for low Ge-DWINTs concentrations but starts to increase when C INTs is above 3 g.L -1 . For the longest PEG chain, we observed a sharp increase of the yield stress right from the first concentrations investigated until reaching a constant value around 0.15 Pa. This difference is directly related to the larger amount of PEG adsorbed on the imogolite surface for PEG20k and the modification of the polymer conformation. This is particularly noticeable when looking at the evolution of infinite shear viscosity (Figure 5d). An almost linear increase is observed with varying the imogolite concentration but the values are always lower for PEG2k compared to PEG20k. Bridging interactions between the nanotubes seems unlikely since the average distance is at least higher than 60 nm for the most concentrated suspension (~ 9 g.L -1 ) that is ~14 times the average diameter of the nanotubes. [START_REF] Paineau | A Liquid-Crystalline Hexagonal Columnar Phase in Highly-Dilute Suspensions of Imogolite Nanotubes[END_REF][START_REF] Paineau | Influence of the Al/Ge Ratio on the Structure and Self-Organization of Anisometric Imogolite Nanotubes[END_REF] PEG/INTs mixtures rather look like a "decorated" nanotube with more or less amount of polymer as sketched in Figure 4. For PEG20k, steric interactions between the polymer and the nanotubes may form PEG/Ge-DWINTs complexes that are larger than the nanotubes themselves, which interfere more significantly thus raising the viscosity when shearing.

CONCLUSIONS

In summary, we investigated the adsorption mechanism of non-ionic polymers onto geo-inspired inorganic nanotubes in aqueous dispersions. Modeling nano-ITC measurements allowed us to quantify the standard variation of enthalpy, entropy and free energy during the adsorption of PEGs. Their affinity with the nanotube surface is dominated by favorable enthalpic contribution.

We also evidenced that the adsorption mechanism depends on the length of the polymer molecular weight. Within the investigated polymer/Ge-DWINTs range, we estimated that more than 95% of polymer is adsorbed with the longest chain (PEG20k) against 40% for a 10-fold

The authors declare no competing financial interest.

ACKNOWLEDGMENT

The present work has benefited from Imagerie-Gif core facility supported by l'Agence Nationale

Figure 1 .

 1 Figure 1. Representative TEM image of aluminogermanate double-walled imogolite

H 2 O

 2 /D 2 O with a mass ratio of 1/10. The density (± 1×10 -6 g cm -3 ) of the Ge-DWINTs dispersion was determined by using a DSA 5000 M (Anton Paar) at 25 °C (±0.001°C). The specific volume of the INTs was calculated as: being d the experimental density for Ge-DWINTs dispersion at concentration w (expressed as g of Ge-DWINTs/kg of solvent) and d 0 is the water density.Rheology. Rheological measurements were performed on a controlled-stress rheometer (Discovery HR-3, TA Instruments) using a cone-plate geometry (60 mm in diameter, 1° in angle) and a solvent trap cover to prevent sample evaporation. Flow curves of PEGs/Ge-DWINTs (R P/INTs = 10) were measured under a controlled shear rate from 1 to 2200 s -1 .RESULTS AND DISCUSSIONThermodynamic characterization of PEG/Ge-DWINTs dispersions. The thermodynamics of the polymer adsorption onto aqueous dispersions of Ge-DWINTs was investigated by ITC through the stepwise injection method. The effects of dilution of the polymer and imogolite were measured and subtracted from the heats of titration to obtain the thermal effects of the polymer/imogolite interaction at each step. Based on the ITC data, we determined the cumulative variation of enthalpy (ΔH ic ) at variable polymer/imogolite mass ratio (R P/INTs ) for PEGs with different molecular weight (Figure2). Details for the calculation of ΔH ic are presented in Supporting Information.

Figure 2 .

 2 Figure 2. Isothermal titration data of Ge-DWINTs with PEGs of different molecular weight.

  variable R P/INTs . Based on the fitting parameters of ITC curves (see details in Supporting Information), we calculated the fraction of adsorbed polymer onto imogolite as a function of the R P/INTs for PEG2k/INTs and PEG20k/INTs complexes (Figure 3).

Figure 3 .

 3 Figure 3. Calculated fraction of polymer adsorbed onto Ge-DWINTs at variable R P/INTs for Ge-

Figure 4 .

 4 Figure 4. Neutron Scattering Intensity function after background (BKG) subtraction and

Figure 5 .

 5 Figure 5. Flow curves for PEGs/imogolite mixtures prepared at different C INTs and PEGs molecular weight: (a) M w = 2000 g.mol -1 and (b) M w = 20000 g.mol -1 . Continuous lines

  de la Recherche (ANR-11-EQPX-0029/Morphoscope; ANR-10-INBS-04/FranceBioImaging; ANR-11-IDEX-0003-02/Saclay Plant Sciences). SANS data (https://doi.ill.fr/10.5291/ILL-DATA.9-12-557) using D11 at Institut Laue Langevin took place with the approved proposal 9-12-557. The Partnership for Soft Condensed Matter (PSCM) is acknowledged for provided the laboratory infrastructure for sample preparation and pre-characterization.

Table 1

 1 collects the thermodynamic parameters and the stoichiometry values for the adsorption of PEGs with variable molecular weights onto imogolite.

Table 1 .

 1 Investigated polyethylene glycol (PEG) with various molecular weight and the corresponding adsorption parameters deduced from ITC 141

	142	measurements performed at 298 K.
			Polymer	Molecular weight (g.mol -1 )	K ads (dm 3 mol -1 )	Z a (mmol P g INTs	-1 )	∆H 0 (kJ mol -1 ) ads	∆G 0 (kJ mol -1 ) ads	∆S 0 (kJ mol -1 K -1 ) ads
			PEG200	200		(9.5 ± 1.7) • 10 4	37 ± 2		-2.15 ± 0.17 -28.8 ± 0.5 0.088 ± 0.007
			PEG400	400		(1.6 ± 0.4) • 10 6	30.6 ± 0.7		-2.18 ± 0.16 -35.4 ± 0.7 0.115 ± 0.004
			PEG2k	2000		(3.3 ± 1.7) • 10 6	2.5 ± 0.2		-9.65 ± 1.24 -37.2 ± 1.6	0.09 ± 0.01
			PEG3k	3000		(8.0 ± 3.0) • 10 6	4.8 ± 0.2		-9.3 ± 0.7 -39.4 ± 1.2 0.101 ± 0.010
			PEG20k	20000		(1.4 ± 0.3) • 10 8	1.00 ± 0.02	-55.5 ± 1.6 -46.5 ± 0.7 -0.030 ± 0.001
	143	a	Moles	of	polymer	chain	per		gram	of	imogolite	nanotubes

Table 2 . Forward scattering intensity I 0 and radius of gyration R g deduced from the modelling of SANS curves with a Gaussian coil model.

 2

	System	R P/INTs	(cm -1 )	R g (nm)	(cm -1 )
	PEG2k	-	0.08 ± 0.01 1.56 ± 0.08	0.16	0.51
	PEG2k/INTs	22	0.10 ± 0.02 1.54 ± 0.05	0.15	0.62
	PEG20k	-	0.18 ± 0.02 2.49 ± 0.06	1.61	0.11
	PEG20/INTs	18 4	0.17 ± 0.02 2.61 ± 0.06 0.05 ± 0.01 3.02 ± 0.08	1.22 0.30	0.14 0.18

lower polymer molecular weight. The role played by the molecular weight on the adsorption of PEG drastically impacts the high shear flow properties of the resulting complexes due to different interaction and conformation of the polymer coil at the interface with the nanotubes.

These findings will open a fresh look on fundamental understanding of polymers with these modeled nanotubes but will also aid in an improved rational design of hybrid polymer/imogolite mixtures during the elaboration of nanocomposites.

ASSOCIATED CONTENT Supporting Information. Details on the ITC data analysis and additional SANS data of Ge-DWINTs dispersion. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*E-mail: giuseppe.lazzara@unipa.it. Phone: +39 09123897962 *E-mail: erwan-nicolas.paineau@universite-paris-saclay.fr. Phone: +33 (0) 169 156 051.

Funding Sources

Laboratoire

Notes


Sayed-Ali Moussaoui

Anne Lélias

Bertrand Braibant

Daniel Meyer

Damien Bourgeois
email: damien.bourgeois@umontpellier.fr

Solvent extraction of palladium(II) using diamides: A performing molecular system established through a detailed study of extraction kinetics

Keywords: Solvent extraction, Kinetics, Palladium, Diamides, Synergy

Palladium isolation and refining through solvent extraction suffer from limited stability of commercial reagents or limited availability of robust reagents in nitrate media. Several performing extraction solvents based on diamide reagents have been proposed, but their preparation requires synthetic chemistry skills. In the present work, we propose the use of readily accessible N,N,N',N'-tetrahexylmalonamide (THMA). This reagent revealed superior distribution ratios and selectivity in comparison with previously used diamides for palladium(II) extraction from nitrate media in the frame of palladium recovery from waste. Furthermore, the benefit of excess dihexylamine (DHA) employed during the preparation of THMA was demonstrated through a gain in kinetics performance. Detailed analysis enabled to determine the best performing formulation of extraction solvent, which can be readily prepared from commercially available chemicals without any complex synthesis procedure nor purification step.

Introduction

Recycling of platinum Group Metals (PGM) has continuously been the object of extensive R&D efforts due to their elevated cost and importance in many industrial sectors. [START_REF] Fröhlich | Valuable Metals-Recovery Processes, Current Trends, and Recycling Strategies[END_REF][START_REF] Jasiński | The life cycle impact for platinum group metals and lithium to 2070 via surplus cost potential[END_REF][START_REF] Ilyas | Hydrometallurgical recycling of palladium and platinum from exhausted diesel oxidation catalysts[END_REF] The supply of PGM is limited by their very low natural abundance and the availability of natural ores in only a few countries (South Africa, Russia). [START_REF] Sverdrup | A system dynamics model for platinum group metal supply, market price, depletion of extractable amounts, ore grade, recycling and stocks-inuse[END_REF][START_REF] Cowley | Matthey Pgm Market Report[END_REF][6] The recent sanitary crisis has further revealed the weaknesses of the supply chain in most developed countries. Public policies push towards increasing the recovery of PGM from secondary resources to ensure the economy's resilience. [START_REF] Mdr | Advances in sustainable approaches to recover metals from e-waste-A review[END_REF][START_REF] Chauhan | Novel technologies and conventional processes for recovery of metals from waste electrical and electronic equipment: Challenges & opportunities -A review[END_REF][START_REF] Gu | Recovering materials from waste mobile phones: Recent technological developments[END_REF][START_REF] Işıldar | Electronic waste as a secondary source of critical metals: Management and recovery technologies[END_REF] Various complementary solutions have been proposed, based either on pyrometallurgical or hydrometallurgical processing. [START_REF] Ilyas | Hydrometallurgical recycling of palladium and platinum from exhausted diesel oxidation catalysts[END_REF][START_REF] Padamata | Recovery of Noble Metals from Spent Catalysts: A Review[END_REF][START_REF] Ashiq | Hydrometallurgical Recovery of Metals From Ewaste[END_REF] Nevertheless, whatever the technology employed, final isolation and purification of PGM rely generally on a solvent extraction stage. [START_REF] Narita | Recent Research in Solvent Extraction of Platinum Group Metals[END_REF][START_REF] Kaya | Recovery of metals and nonmetals from electronic waste by physical and chemical recycling processes[END_REF] The existing industrial extracting molecules, mostly dialkyl sulfides and fatty amines, could not be successfully applied to hydrometallurgical processes dedicated to the recovery of PGM from waste as their selectivity scope or their stability is insufficient. [START_REF] Poirot | Palladium Extraction by a Malonamide Derivative (DMDOHEMA) from Nitrate Media: Extraction Behavior and Third Phase Characterization[END_REF] As a consequence, several organic molecules have been developed after molecular design and synthesis route selection. [START_REF] Wang | Extractant structures and their performance for palladium extraction and separation from chloride media: A review[END_REF] The chemical synthesis of tailor-made molecules is not limited to solvent extraction: They can be employed in solvent impregnated [START_REF] Lee | Separation of platinum, palladium and rhodium from aqueous solutions using ion exchange resin: A review[END_REF][START_REF] Zhang | Development of palladium separation process from nitric acid medium utilizing a new hybrid multi-nitrogen adsorbent[END_REF] or chelating resins, [START_REF] Wu | Novel polyazamacrocyclic receptor impregnated macroporous polymeric resins for highly efficient capture of palladium from nitric acid media[END_REF] as ion exchange is also a technique of interest for the recovery of PGM in dilute solutions.

Amide compounds play a central role, as the amide moiety is very stable in the highly acidic and oxidizing media employed. [START_REF] Manchanda | Amides and diamides as promising extractants in the back end of the nuclear fuel cycle: An overview[END_REF] Monoamides are not efficient enough for PGM extraction. Thus, sulfide containing monoamides and various diamides, [START_REF] Costa | Liquid-Liquid Extraction of Platinum from Chloride Media by N,N′-Dimethyl-N,N′-Dicyclohexyltetradecylmalonamide[END_REF][START_REF] Costa | N , N ′tetrasubstituted succinamides as new molecules for liquid-liquid extraction of Pt(IV) from chloride media[END_REF] including aza-or thio-bearing diamides [START_REF] Narita | Extraction Properties of Palladium(II) in HCl Solution with Sulfide-Containing Monoamide Compounds[END_REF][START_REF] Matsumiya | Separation of palladium by solvent extraction with methylamino-bis-N,N-dioctylacetamide and direct electrodeposition from loaded organic phase[END_REF] have been synthesized and successfully used in solvent extraction of PGM from chloride media. Malonamides proved to lead to very stable Pd(II) complexes, and to efficient extraction from nitrate media. [START_REF] Poirot | Palladium Extraction by a Malonamide Derivative (DMDOHEMA) from Nitrate Media: Extraction Behavior and Third Phase Characterization[END_REF][START_REF] Poirot | Metal Recognition Driven by Weak Interactions: A Case Study in Solvent Extraction[END_REF] Based on these results, we recently proposed a simple process to recover palladium from electronic waste using diamides previously developed in the frame of nuclear fuel reprocessing. [START_REF] Manchanda | Amides and diamides as promising extractants in the back end of the nuclear fuel cycle: An overview[END_REF][START_REF] Mastretta | Palladium Isolation and Purification from Nitrate Media: Efficient Process Based on Malonamides[END_REF][START_REF] Bourgeois | A simple process for the recovery of palladium from wastes of printed circuit boards[END_REF] The approach relies on the reformulation of the extraction solvent using existing available molecules. Although the approach seems promising, it relies on a molecule that has to be specifically prepared and which is only available in large quantities through custom synthesis companies, and not through the regular chemical suppliers. To the best of our knowledge, none of the above-presented new molecules has passed the proof of concept stage at the laboratory level due to the complexity of obtaining them on a large scale for further process development. Finally, it is essential to mention that in Europe, the marketing of a new molecule is hampered by REACH regulation, so that efficient solutions based on existing registered molecules are still needed. During our previous work, the fundamental studies performed along with the process development studies revealed that the initial design of the chosen diamide, N,N'-dimethyl-N,N'dibutylteradecylmalonamide (DBMA, Figure 1), was not optimized for selective Pd(II) extraction. [START_REF] Mastretta | Palladium Isolation and Purification from Nitrate Media: Efficient Process Based on Malonamides[END_REF][START_REF] Dul | Perfluoroalkyl-vs alkyl substituted malonamides: Supramolecular effects and consequences for extraction of metals[END_REF][START_REF] Dul | Synthesis of fluorinated malonamides and use in L/L extraction of f-elements[END_REF][START_REF] Braibant | Impact of the Long-Range Electronic Effect of a Fluorous Ponytail on Metal Coordination during Solvent Extraction[END_REF] DBMA was selected as this molecule is the former lanthanide/actinide separation molecule, so that it was readily available from French CEA. However, its molecular design has been performed targeting high distribution ratios for lanthanides(III) cations, [START_REF] Mowafy | Extraction Behaviors of Trivalent Lanthanides from Nitrate Medium by Selected Substituted Malonamides[END_REF][START_REF] Mcnamara | Extraction of Europium(III) ion with tetrahexylmalonamides[END_REF][START_REF] Sasaki | Extraction of Actinides(III), (IV), (V), (VI), and Lanthanides(III) by structurally tailored diamides[END_REF] and its ability to also extract palladium(II) was instead a side-effect. [START_REF] Serrano-Purroy | Recovery of minor actinides from HLLW using the DIAMEX process[END_REF][START_REF] Modolo | DIAMEX Counter-Current Extraction Process for Recovery of Trivalent Actinides from Simulated High Active Concentrate[END_REF] Thus, the possibility to envision a more simple diamide based solvent dedicated to selective palladium(II) extraction rapidly appeared, using N,N,N',N'-tetrahexylmalonamide (THMA, Figure 1), previously described for uranium(VI) extraction. [START_REF] Bao | Extraction of uranium(VI) with N,N,N′,N′-tetrabutylmalonamide and N,N,N′,N′-tetrahexylmalonamide[END_REF][START_REF] Lumetta | Synthesis and characterization of monoand bis-(tetraalkylmalonamide)uranium(VI) complexes[END_REF] Although the newly proposed diamide is not commercially available, it can be prepared in situ and employed without any purification in the extraction process. Thus, as starting reagents are available and easy to handle, it is possible to prepare the extraction solvent without specific operation in chemical synthesis. Furthermore, the study of the influence of the stoichiometry of reagents employed revealed that a slight excess of amine is beneficial to the performance of the extraction process. A detailed study of the thermodynamic and kinetic features enabled to evidence a synergy phenomenon between amine and diamide for Pd(II) extraction. Finally, the performance of the THMA could be extended to the extraction of Pt(IV) from chloride media. and platinum(IV) chloride (99.9% purity) were purchased from Strem Chemicals. Iron(III) nitrate nonahydrate (Fe(NO 3 ) 3 .9H 2 O, ≥98% Fe), neodymium(III) nitrate hexahydrate (Nd(NO 3 ) 3 .6H 2 O, 99.9% purity), diethyl malonate (99% purity) and dihexylamine (97% purity) were purchased from Sigma-Aldrich and used without further purification. Toluene, Solvesso TM 150, diethyl ether, concentrated nitric acid 69.5% (w/w), concentrated hydrochloric acid 37% (w/w) and anhydrous magnesium sulfate were purchased from Carlo Erba reagents.

Synthesis and characterization of N,N,N',N'-tetrahexylmalonamide (THMA)

A mixture of diethylmalonate (8.09 g, 0.05 mol) and dihexylamine (20.1 g, 0.105 mol, 2.1 equiv.) was heated under reflux (ca. 180°C) for 7 h. Every 30 min, an argon flow was injected during 1 min to evacuate the produced ethanol through a distillation apparatus (see SI, Figure S1). The reaction mixture was then cooled to room temperature. The resulting product could be used directly in solvent extraction experiments. For characterization and detailed study, purification was performed as follows: The resulting mixture was dissolved in 100 mL diethyl ether and then washed with an aqueous 1M HCl solution to remove unreacted dihexylamine (4 x 30 mL), with an aqueous saturated solution of NaHCO 3 (1 x 30 mL) and finally with brine (1 x 30 mL). The recovered organic layer was dried over anhydrous MgSO 4 and concentrated in vacuo. N,N,N',N'-tetrahexylmalonamide (THMA, Figure 1) was obtained as a viscous pale yellow liquid (21.5 g, 98% yield). [START_REF] Fröhlich | Valuable Metals-Recovery Processes, Current Trends, and Recycling Strategies[END_REF] H NMR (400 MHz, CDCl 3 )  (ppm): 3.45 (s, 2H, COCH 2 CO) 3.34 (dd, J = 15.4, 7.8 Hz, 4H, N-CH 2 ), 3.32 (dd, J = 15.4, 7.7 Hz, 4H, N-CH 2 ), 1.57-1.55 (m, 8H, N-CH 2 -CH 2 ), 1.32-1.29 (m, 24H, CH 2, ) 0.92 (t, J = 6.7 Hz, 6H, CH 3 ), 0.88 (t, J = 6.6 Hz, 6H, CH,); [START_REF] Narita | Recent Research in Solvent Extraction of Platinum Group Metals[END_REF] C NMR (100.6 MHz, CDCl 3 ) δ (ppm): 166.6, 48.4, 46.0, 40.8, 31.5, 31.5, 28.9, 27.5, 26.6, 26.5, 22.5, 13.9, 13.9, IR (neat)  (cm -1 ): 2927, 2903, 2850, 1634 (C=O), 1456, 1420, 1376.

Solvent extraction

Aqueous phases containing the desired metallic cations were prepared from the corresponding metallic salts (Pd(NO 3 ) 2 .xH 2 O, PdCl 2 .2H 2 O, Fe(NO 3 ) 3 .9H 2 O, Nd(NO 3 ) 3 .6H 2 O, PtCl 4 ) dissolved in an aqueous HNO 3 or HCl solution of desired concentration as described in the text. Organic phases were prepared by diluting THMA, DMBA and/or DHA in the appropriate diluent (toluene, Solvesso TM 150) to reach the concentrations given in the text.

The organic phases were pre-equilibrated by bringing them into contact for 30 min with a five-fold volume of an aqueous solution of HNO 3 or HCl of molarity identical to that of the aqueous metal solution employed for the extraction step.

Extraction experiments were carried out in 2 mL Eppendorf tubes with equal volumes of the aqueous and organic phases (A/O =1). The two phases were shaken with a thermostated orbital mixer (Eppendorf Thermomixer® C) for 1 h to 24 h at 20°C ± 2°C. Unless otherwise stated in the text, the stirring speed was set at 2000 rpm. Tubes were then centrifuged to ensure good phase separation.

For each extraction performed, aliquots of the aqueous phases (100 µL), before and after extraction, were directly diluted with a 2% HNO 3 /HCl aqueous solution (90/10). After extraction, aliquots of the loaded organic phases (500µL) were back-extracted during 1 h with 1 mL of a 0.1M aqueous thiourea solution. Resulting phases were separated and 250 µL of the final aqueous phase were taken and diluted with a 2% HNO 3 /HCl aqueous solution (90/10).

Evidence from preliminary investigations showed that all extracted metals were totally stripped from loaded organic phases during the process, and that no metal precipitation occurred prior to analysis. Dilution factors have been chosen so that a minimum 10-fold dilution is operated to avoid matrix effects during analysis.

Analytical determination of metal content

Metals concentration in each solution was determined using inductively coupled plasma atomic emission spectroscopy (ICP/AES, SPECTRO ARCOS ICP Spectrometer, AMETEK Materials Analysis). The selected spectral lines to assay metals were free from interference. The given concentrations were calculated as the average of three replicates at different wavelengths for each metal. Relative standard deviations have been determined and lie between 1 and 3%. Quantification limits (LoQ) in each analyzed phase were determined for each metal from the dilution factor applied and the background equivalent concentration calculated by the spectrometer for each optical line. Confidence intervals were selected taking into account the error of the spectrometer and the precision of the equipment used for dilution.

Altogether, the relative errors were found to be between 5% and 10%. All results showed a total mass balance in each metal in the range of ±5% of the initial load in the aqueous phase.

Expression of results

Assuming that the volume change of both phases is neglected during extraction, the metal distribution ratio ( ) and the metal extraction percentage ( ) were determined using equations ( 1) and (2) below:

(1) (2)
Where , and denote, respectively, the metal concentration in the initial aqueous phase, the metal concentration in the aqueous phase and the organic phase after extraction, and V aq and V org denote, respectively, the volumes of aqueous and organic phases.

The separation factors were calculated using the following equation ( 3):

(

Where and denote the distribution ratios of the metal and the metal respectively.

Interfacial tension measurements

All interfacial tension measurements were carried out using a Krüss spinning drop tensiometer (SDT) equipped with a high resolution USB3 camera. The tensiometer is combined with the ADVANCE software. This software performs image analysis applying either the approximation functions of Vonnegut, or making use of an evaluation algorithm based on a Young-Laplace fit. For the current measurements, Young-Laplace-fit equation was used for interfacial tension determination. The densities of all samples were determined using DSA 5000 densimeter at 23°C. All the organic phases were pre-equilibrated by contacting them for 30 min with a five-fold volume of 3M aqueous nitric acid concentration. The capillary was filled with the heavy phase (3M HNO 3 ). A drop of the sample (organic phase)

was injected with a syringe against the inner wall of the horizontally held capillary, so that the drop remained stuck approximately in the center of the capillary. The capillary was then placed on the filled closure cap so that it was sealed. Calibration of the tensiometer were performed before each measurement to obtain the image scale. Interfacial tension measurements were performed at a controlled temperature of 23°C at a high rotation rate (>5000 rpm). The reported values were recorded after the drops reached equilibrium, and no further decrease in interfacial tension values was observed.

Results and discussion

Extraction of Pd with THMA and DBMA

In addition to their capacity to remove minor actinides, diamides have shown an affinity to extract Pd(II) from nitric acid media. Accordingly, the selective extraction of Pd(II) from nitrate media was reported with DBMA. DBMA remains complicated to prepare (three synthesis steps, and purification on silica gel column), making a competitive process dependent on excellent recycling of extraction solvent. Furthermore, for efficient extraction of Pd(II), it was necessary to increase the concentration of the extractant to 0.6 mol/L, i.e. about 300 g/L, which makes the initial investment in extraction solvent substantial. On the contrary, THMA is a N,N,N',N'-tetrasubstituted malonamide that is much simpler to prepare. as previously reported. When using THMA, the distribution ratios of both Fe(III) and Nd(III) also increased with concentration, but remained very low (below 0.1) so that S Pd/Fe and S Pd/Nd were still much higher than those obtained using the same DBMA concentration (Table 1). Note: * When concentration in the final aqueous phase is below the detection limit, only a lower D value can be determined. Similarly, when concentration in the final organic phase is below the detection limit, only a higher D value can be determined.

Altogether, this analysis rapidly emphasized the very interesting features of THMA over DBMA in the selective extraction of Pd(II). Regarding the Pd(II) selective isolation process we recently proposed, using THMA instead of DBMA should lead to a more efficient process, based on lower stage numbers in extraction and scrubbing sequences, and using a lower amount of extracting molecule. However, we were surprised to notice a sudden drop in performance when lowering the THMA concentration to 0.2M or below. Particularly low Pd(II) distribution ratios were sometimes obtained, depending on the batches of THMA used.

Careful examination of the NMR spectra of the corresponding batches was conducted and revealed residual traces of dihexylamine (DHA) in the solvent. Also, equilibrium was not reached after 1 h extraction: The extraction with THMA is incomplete after 1 h shaking (see SI, Figure S2), whereas with DBMA we found during our previous studies that extraction equilibrium was reached after 10 min only, working in toluene and using the same aqueous layer ([HNO 3 ] = 3M). This drawback led us to investigate in detail the impact of THMA purity on extraction performances, and especially on the kinetics of Pd(II) extraction.

Impact of added DHA on Pd(II) extraction with THMA

Synthesis of THMA involves DHA as a starting material. High molecular weight amines and quaternary ammonium salts have been used among nitrogen-containing extractants for the extraction of PGM from chloride media. [START_REF] Swain | Separation of platinum and palladium from chloride solution by solvent extraction using Alamine 300[END_REF][START_REF] Lee | Liquid-liquid extraction/separation of platinum(IV) and rhodium(III) from acidic chloride solutions using tri-iso-octylamine[END_REF][START_REF] Lee | Extraction and separation of Pt(IV)/Rh(III) from acidic chloride solutions using Aliquat 336[END_REF][START_REF] Cieszynska | Extraction and separation of palladium(II), platinum(IV), gold(III) and rhodium(III) using piperidine-based extractants[END_REF][START_REF] Belova | Extraction of platinum-group metals from chloride solutions by salts of quaternary ammonium bases and binary extractants[END_REF][START_REF] Nguyen | Separation of platinum(IV) and palladium(II) from concentrated hydrochloric acid solutions by mixtures of amines with neutral extractants[END_REF][START_REF] Raju | Separation of platinum and rhodium from chloride solutions containing aluminum, magnesium and iron using solvent extraction and precipitation methods[END_REF][START_REF] Inoue | Solvent extraction of palladium (II) with trioctylmethylammonium chloride[END_REF][START_REF] Jaree | Separation of concentrated platinum(IV) and rhodium(III) in acidic chloride solution via liquid-liquid extraction using tri-octylamine[END_REF] In addition to their extraction properties, their role as a catalyst in the extraction of Pd(II) from aqueous hydrochloric acid solutions has been documented in previous studies. [START_REF] Szymanowski | The surface activity of triisooctylamine and the mechanism of palladium (II) extraction by dihexyl sulphide in the presence of triisooctylamine[END_REF][START_REF] Sj | Phase transfer catalysis in palladium extraction by 1-(2pyridylazo)-2-naphtol (PAN)[END_REF][START_REF] Rong | The catalytic effect of quaternary ammonium salt On the Extraction of Palladium(II) With 2-Hydroxy-5-Nonyi-Benzophenone Oxime(LIX 65N) In Heptane-Chloroform Solvents[END_REF][START_REF] Szymanowski | Catalytic effects in solvent extraction[END_REF] Incorporation of an amine moiety into an amide based extractant proved superior performances for Pd(II) from nitrate media.

However, Pd(II) extraction with amine-based functions from nitrate media has been rarely reported in the literature. [START_REF] Mezhov | Study of Extraction of Palladium from Nitric Acid Solutions with Nitrogen-Containing Compounds, as Applied to Recovery of Fission Palladium from Spent Nuclear Fuel of Nuclear Power Plants: 1[END_REF][START_REF] Mikulaj | Simultaneous extraction of palladium and technetium from nitrate solutions with tri-n-octylamine[END_REF][START_REF] Zhang | Extraction Chemistry of Palladium(II). Mechanism of Antagonistic Synergistic Extraction of Palladium By A 4-Aroyl Derivative of 1-phenyl-3-Methyl-Pyrazolone-5-one and Trialkylamine of High Molecular Weight[END_REF] Thus, the impact of controlled addition of DHA to pure THMA on the extraction of Pd(II) from nitric acid media was investigated. Firstly, a well-purified batch of THMA was prepared and fully characterized (see SI, Figures S3 andS4, and Table S1). Working with this batch confirmed i) the excellent performance regarding Pd(II) extraction, and ii) the slower kinetics of Pd(II) extraction at low THMA concentrations, compared to previous results with DBMA. Then, the concentration of THMA was set at 0.2M, and DHA was added in quantity varying from 0 to 100 mol% respective to THMA. For a contact period of 24 h, excellent Pd(II) distribution ratios could be obtained even at this relatively low THMA concentration (D Pd = 28 with 0.2M THMA in toluene after 24 h). DHA addition only had marginal effect, since the D could reach only 32-35 (see Figure 3). Finally, using only DHA 0.2M in toluene, very low palladium distribution ratio was found (D Pd = 0.1 at equilibrium, after 24 h) which confirms that secondary amines are not good Pd(II) extracting molecules from nitrate media.

In order to investigate in detail the kinetics effects, extraction conditions were modified in order to disfavor mass transfer through lowering of the interfacial area between aqueous and organic phases. Although it is difficult to determine with precision the interfacial area, it is clear that it depends on the shearing rate, thus on stirring rate. A visual study of the impact of the stirring rate on the mixing of both phases revealed that dispersion of organic phase into aqueous phase occurs at stirring rate between 800 and 1000 rpm (see SI, Figure S5). At higher speed, emulsification of the biphasic system is well visible, and it can be assumed that the concentration in metal is homogeneous in each phase. Mass transfer is, nevertheless, probably still limited by the reduced interfacial area, and thermodynamic equilibrium is reached after longer shaking time. This was well evidenced through the study of Pd(II) extraction yield after 1 min (see SI, Figure S5). Therefore, the stirring speed was set at 1200 rpm. In these conditions, the palladium distribution ratio (D Pd ) drops to 3 (75% extraction percentage) using pure THMA 0.2M in toluene after 1 h. When DHA was added to 0.2M THMA in toluene, the Pd(II) distribution ratio determined after 1 h extraction increased significantly (Figure 3). It reached a maximum at 14 (93% extraction yield) when 50 mol% DHA were added to THMA (thus, the extraction solvent is a 0.2M THMA / 0.1M DHA solution). The increase in the palladium distribution ratio was somehow still limited, and remained below the distribution ratio obtained in the same conditions after 24 h (Figure 3).

Then, above 50 mol% of added DHA, there is no more benefit of DHA addition. Moreover, a further increase in the molar percentage of added DHA led to a decrease in the extraction efficiency of Pd(II) (see SI, Figure S6). For instance, when using THMA with 90 mol% added DHA, D Pd dropped to 2 (69% extraction yield).

The marked increase of Pd(II) extraction at short durations could not be related to a change in interfacial surface, as no effect of DHA on interfacial tension was observed (see SI, Figure S7). Neither did viscosity change significantly up to 50 mol% added DHA. We therefore assume that the addition of DHA does not induce any extra interfacial activity, and that the observed results may instead be linked to chemical limits of the extraction kinetics.

An increase in viscosity of the extraction solvent was noticed when the added DHA quantity exceeded 80 mol%. This increase in viscosity can be invoked here to explain the lower extraction yield at short durations as the increase in viscosity leads to reduced diffusion of molecules and ions. High molecular weight amines have been reported to extract nitric acid, [START_REF] Smith | The acid-binding properties of long-chain aliphatic amines[END_REF] and the increase in viscosity probably originates in the aggregation of DHA-nitrate species formed upon extraction of nitric acid by DHA. [START_REF] Tsujino | Effect of Temperature on the Extraction of Nitric Acid, Uranium and Fission Products with Tri-n -Octyl Amine[END_REF][START_REF] Lloyd | Extraction of Hexavalent Uranium by Trilaurylamine Nitrate[END_REF] Visually, mixing of both phases was still efficient. A dedicated experimental set-up with a controlled interfacial area would be required to investigate in detail the effect of added DHA on the Pd(II) transfer kinetics. However, the experimental approach we propose is simple for preliminary investigations and we limited the following studies to 50 mol% added DHA. (4)

Pd extraction kinetics with

The integration of equation (4) gives equation ( 5):

(

) 5
Where k is the rate constant, t is the time, is the concentration of Pd in the aqueous phase at time t, is the concentration of Pd in the aqueous phase at equilibrium (see SI, Table S2) and is the initial concentration of Pd in the aqueous phase. In Figure 4 The extraction kinetics of Pd(II) is enhanced in the presence of DHA, with a regular increase in the observed rate constants (Figure 4). The rate constant increased from 0.028 min -1 for the extraction with pure THMA to 0.116 min -1 with 50 mol% added DHA, corresponding to an extraction half-time of 25 min and 6 min respectively. Thus, without DHA, Pd(II) extraction is slow, but occurs. The addition of 10 mol% DHA leads to doubling of the rate constant (half-time divided by almost 2). The increase is then proportional to the quantity of added DHA (see SI, Figure S8). Interestingly, the observed rate constant (k obs ) can be decomposed into two contributions, k THMA (obtained when no DHA is added, k THMA = 0.028 min -1 ) and k DHA (equation ( 6)).

(6)

The contribution k DHA is then proportional to the DHA quantity (see SI, Figure S8), as

given by equation (7), where [DHA] is the concentration in DHA in mol.L -1 : (7) As thermodynamic outcome of the Pd(II) extraction is almost the same in each case (Figure 3), this result demonstrates that DHA has principally a kinetic effect. Analysis of the observed rate constant suggests that two mechanisms occur simultaneously, i) a direct extraction with THMA, and ii) an indirect one mediated by DHA, and of first order respective to DHA. As stated above, a more detailed study employing adequate experimental tools would be necessary to reach a complete mechanistic understanding. These results are, nevertheless, sufficient to justify i) that slow kinetics are only observed at low THMA concentration, and ii) that DHA is involved in the formation of intermediate molecular species responsible for Pd(II) extraction.

The effect of the aqueous nitric acid concentration was studied, and results showed that the kinetics enhancement is strongly dependent on the aqueous nitric acid concentration:

When using a 5M aqueous HNO 3 solution, no more effect of added DHA was observed (Figure 5). Palladium distribution ratio obtained after 1 h extraction remained constant (~2)

for the entire range of added DHA. After 24 h of extraction, palladium distribution were found to lie around 10 (see SI, Figures S9 andS10). The lack of kinetic effect when using a higher concentration of aqueous HNO 3 is possibly due to the fact that under these conditions, in the organic phase, DHA (pK a = 11.0 in ethanol-water mixture) is fully protonated by HNO 3 . Thus, using a 5M aqueous HNO 3 solution, only the extraction promoted by THMA may be observed.

Extraction solvent formulation: optimization of added DHA quantity

These results suggest that addition of 50 mol% DHA is the optimum for improvement of extraction kinetics. However, when the THMA quantity is fixed, the addition of extra DHA leads to a higher total amount of extractant employed. Thus, it could be argued that better performance results also from the use of a higher amount of extractant. We therefore examined the impact of controlled DHA addition to THMA when total concentration in extracting molecules is fixed, ie. when [THMA] + [DHA] = 0.2M. Thus, the addition of DHA is here accompanied by a lowering of the THMA concentration. The total mass of used extractant also decreases, as the DHA is lighter than the THMA. As it is preferable to present results using molar quantities than weight, we did not perform the study using a constant weight in extractant. The Pd(II) distribution ratios obtained were plotted against the molar ratio of DHA (x DHA ) in the extraction solvent (Figure 6): extraction efficiency decreases, as the THMA content in the organic phase decreases. Thus, it can be concluded that the benefit of DHA addition is optimum at 10 mol%. Further addition of DHA does increase the kinetics, but the increase is insufficient to counterbalance loss of efficiency due to lower amount of THMA. Extraction half-time is lowered from 13 min to 10 min upon doubling of the DHA quantity from 10 mol% to 20 mol%. Thus, unless kinetics are the most important parameter, we think that it is not worth adding more than 10 mol% DHA. It is worth mentioning here again that these results have been gathered in conditions voluntarily chosen in order to reduce mass transfer during the liquid-liquid extraction process.

The simple set-up we used, and the low quantities of reagents employed enabled a rapid screening of several different conditions. Once optimum conditions are chosen, i) they can be employed for further development in regular conditions, ie. with adequate phase mixing, and ii) the potential kinetics limits have been identified, as well as key parameters to study. The way experimental results are presented is similar to classical studies which evidence synergistic phenomena. In our case, it is clear that mass transfer is limiting, at least on the left side of Figure 6, and that thermodynamic equilibrium is not reached. The results should not be taken as evidence of a synergy. The non-linear behaviour of the extraction results obtained with THMA/DHA mixtures results from an enhancement of the extraction kinetics, counterbalanced by thermodynamic constraints due to poor Pd(II) extraction with pure DHA.

The rise in extraction yield when x DHA ≤ 0.1 (x THMA ≥ 0.9), where D Pd with THMA/DHA mixtures are higher than D Pd with THMA, have been proven to result from enhancement of kinetics by DHA. Then, for the range of 0.1 ≤ x DHA ≤ 0.4-0.6, a decrease in D Pd was observed, yet they were still higher than D Pd with THMA at a 0.2M concentration, thus probably higher than D Pd at corresponding THMA concentration. The thermodynamical constraints are still partially overcome by the kinetic acceleration. Finally, when x DHA > 0.6, the system is entirely governed by thermodynamic limitations. This approach demonstrates that studies of synergism under thermodynamic conditions require proof that there is no kinetic limitation.

Kinetics studies are often overlooked, although our results demonstrate that they could be at the origin of an apparent synergy.

Extraction of Pd with crude THMA

As DHA proved to have a beneficial effect on the extraction of Pd(II) with THMA from nitric acid media, we investigated the possibility to employ unpurified THMA to prepare the extraction solvent. When DHA is employed in excess (more than 2.0 equiv.) respective to diethyl malonate in the synthesis of THMA, the crude reaction mixture only contains residual DHA in addition to THMA. Thus, a batch of crude (unpurified) THMA was evaluated for Pd(II) extraction. This THMA batch was prepared using excess DHA (10 mol%), with the same synthetic procedure, except that all purification stages (dilution in diethyl ether, HCl washing, drying over magnesium sulfate and distillation of diethyl ether) were not performed:

The synthesis was run solvent free by heating at 180°C, and the crude mixture was employed directly, after cooling, through dilution of the viscous yellow liquid obtained in toluene.

Determination of the molar ratio of residual DHA was performed through integration of corresponding signal areas in These experiments have been carried out with standard shaking (2000 rpm), and proper emulsification of aqueous and organic phases. No issue was encountered with crude THMA compared to purified THMA during phase disengagement, as the protonated amine does not significantly modify interfacial tension. The results suggest that equilibrium is reached after 1 h using crude THMA. Thus, the kinetics limitations associated with THMA are successfully overcome by using crude THMA. The extraction performance of the crude THMA was better compared with the purified one (DHA free). Indeed, the palladium distribution ratio increased from 28 when using purified THMA to 34 when using crude THMA.

Extraction of Pd(II) and Pt(IV) from chloride media

THMA has proven to be a performing extractant for Pd(II) from nitric acid media. The investigation was further extended to assess the efficiency of THMA in the extraction of Pd(II) from chloride media. From an industrial application perspective, toluene was replaced by Solvesso TM 150 as diluent. These experiments were carried out with crude THMA using the batch and the procedure described above. THMA concentration was set at 0.2M. THMA

showed an efficient extraction of Pd(II) ( Altogether, these preliminary results on platinum group metals (PGM) extraction from chloride media demonstrate the potential interest of THMA for the extraction and purification of these metals. Significant distribution ratios could be obtained in classical conditions, ie.

after 1 h, with crude THMA. As a comparison, the extraction of PGM with alkyl sulfides from chloride media require the addition of a fatty amine to obtain acceptable kinetics. It could be worth envisioning the study of the optimum DHA quantity before adapting the technology to PGM separation from chloride media. [START_REF] Malik | A Novel Solvent Extraction Route for the Mutual Separation of Platinum, Palladium, and Rhodium in Hydrochloric Acid Media[END_REF] It is finally interesting to take into consideration the cost of reagents: DHA is the expensive reagent employed for THMA synthesis (Table 4): Solvesso TM 150), as there is no organic solvent employed for the synthesis. This approach is particularly interesting as both reagents, DEM and THA, are registered chemicals, easy to handle, and not flammable. Consequently, there is no regulation issue with the use of THMA as long as it is prepared in situ. Extraction performances were extended to Pd(II) and Pt(IV) extraction from chloride media. Potential applications of the proposed extraction solvent in platinum group metals (PGM) refining is underway. Also, deep understanding of the impact of the molecular topology on extraction outcome will be continued.

The kinetics study was performed via simple experimental models. By deliberately lowering the stirring speed, we demonstrated that relevant data can be acquired. A deeper understanding of the kinetics phenomena involved, and the determination of precise absolute mass transfer rate constants require more complex devices (single drop technique, rotating membrane, cell with controlled interface) that cannot be employed for intensive screening.

Our approach enabled to precise the importance of different parameters (DHA concentration, THMA concentration, aqueous HNO 3 and Pd(II) concentrations…) so that dimensioning of adapted kinetics studies will be straightforward. More generally, the proposed method is worth considering when kinetics limitations are suspected. Reaction:

Set-up: (a)

Figure 1 :

 1 Figure 1: Chemical structures of diamides employed in the study

Furthermore, it revealed

  to be an excellent candidate for the selective extraction of Pd(II): The Pd(II) extraction performances of THMA have been assessed in comparison with DBMA, in conditions where DBMA presents good Pd(II) distribution ratios and good selectivity regarding Fe(III) and Nd(III) (Table1). THMA exhibited overall much better extraction of Pd(II) than DBMA across the whole range of extractant concentrations. Palladium distribution ratios D Pd with THMA were between 10 and 100 times higher than that with DBMA, and still using only 0.3M THMA, the concentration of residual Pd(II) in the final aqueous layer was below the detection limit of the ICP-OES analysis (D Pd > 70). The quantitative extraction of Pd(II) with THMA can induce a significant reduction in extractant charge in the organic phase since both diamides are structural isomers, having the same number of carbon atoms(27), and thus the same molecular weight (438.7 g.mol -1 ).It was previously reported that the selective extraction of Pd(II) over Nd(III) and Fe(III) with DBMA was strongly dependent on two key parameters: The concentration of DBMA for Nd(III) and the aqueous nitric acid concentration for Fe(III).[START_REF] Mastretta | Palladium Isolation and Purification from Nitrate Media: Efficient Process Based on Malonamides[END_REF] Therefore, the optimized conditions to reach selective Pd(II) extraction with DBMA have been established at 0,5-0,6M DBMA in toluene with a 3M aqueous HNO 3 solution. THMA showed an impressively high selective extraction of palladium in the presence of Fe(III) and Nd(III) compared to DBMA. For instance, at 0.3M THMA, S Pd/Fe and S Pd/Nd were above 11600 and 35000 respectively, and Pd(II) extraction was quantitative. Experimentally, after extraction Pd(II) could not be detected in the aqueous layer (D Pd > 70), and Nd(III) could not be detected in the organic layer (D Nd < 0.002). Fe(III) was barely quantified in the organic layer (D Fe = 0.006). In comparison, for the same concentration for DBMA, S Pd/Fe and S Pd/Nd were 180 and 50, respectively, with Pd(II) distribution ratio of 4 only. The increase in extractant concentrations leads to a decrease in the selectivity regarding Fe(III) or Nd(III) with DBMA

Figure 3 .

 3 Figure 3. Pd(II) distribution ratio with mixed THMA/DHA systems in toluene. THMA concentration 0.2M, DHA molar percentage 0 mol% to 50 mol%, initial [Pd] aq 500mg/L, [HNO 3 ] aq 3M, 1 h extraction, stirring speed 1200 rpm.

  mixed THMA/DHA systems In order to precise the effect of DHA on the extraction kinetics, experiments were performed at short extraction durations (1 to 6 min). The concentration of THMA in toluene was still set at 0.2M, and controlled amount of DHA was added (10 mol% to 50 mol%). For each point in each series, a separate experiment was carried out, and the reported extraction time corresponds to the shaking time. The average absolute error in extraction time for each point was determined as the delay between the agitation turnoff and the time required to reach the maximum centrifugation speed. Assuming that afterwards during centrifugation mass transport is negligible, the time absolute errors are estimated at 6 seconds for each point, and are positive. A first-order model was used to interpret the experimental data. The rate of palladium extraction can be expressed as follows:

  , the plots of are represented for four chosen samples: (a) Pure THMA, (b) THMA + 10 mol% DHA, (c) THMA + 20 mol% DHA and (d) THMA + 50 mol% DHA. The data of each sample is in agreement with the first order model as they can be modelled by a straight line.

Figure 4 .

 4 Figure 4. First order model for the extraction of Pd(II) with pure THMA and mixed THMA/DHA systems in toluene. THMA concentration 0.2M, and 10 mol% DHA, 20 mol% DHA and 50 mol% DHA, initial [Pd] aq 500 mg/L, [HNO 3 ] aq 3M, stirring speed 1200 rpm.

Figure 5 .

 5 Figure 5. Variation of the Pd(II) distribution ratio with the aqueous nitric acid concentration with mixed THMA/DHA systems in toluene, 0.2M THMA concentration, DHA molar percentage 0% to 50%, initial [Pd] aq 500mg/L, 1 h extraction, stirring speed 1200 rpm.

Figure 6 .

 6 Figure 6. Pd(II) distribution ratio after extraction with THMA/DHA mixtures. [THMA]+[DHA] = 0.2M in toluene, initial [Pd] aq 500 mg/L, [HNO 3 ] aq 3M, 1 h extraction, stirring speed 1200 rpm.

  ) is a performing extracting molecule for Pd(II) extraction from nitrate and from chloride media. Although the pure molecule displays slow kinetics, the possibility to accelerate extraction through the use of excess dihexylamine (DHA) enabled to propose a formulation of extracting molecules in order to prepare a performing extraction solvent. Furthermore, this formulation can be directly obtained after reaction between the two reagents needed to prepare THMA, diethyl malonate (DEM) and DHA. We demonstrated that the crude reaction product can be employed directly for solvent extraction, without work-up nor purification, after sole dilution in the required diluent (eg.

  Finally, we would like to point attention to possible artefacts and pitfalls related to misinterpretation of solvent extraction results when kinetics are not properly checked: Although the results obtained on THMA/DHA mixtures sound like synergism, they should not be interpreted this way.

Figure S1 :

 S1 Figure S1: Experimental set-up for the synthesis of N,N,N',N'tetrahexylmalonamide.

Figure S2 :

 S2 Figure S2: Progress of Pd(II) extraction with 0.2M THMA in toluene as a function of time. [HNO 3 ] aq 3M, initial [Pd] aq 500mg/L, A/O=1, stirring speed 1200 rpm.

Figure S3: 1 H

 1 Figure S3: 1 H NMR and 13 C NMR spectra of THMA (400 MHz, 100 MHz, solvent CDCl 3 ).

Figure S5 :

 S5 Figure S5: Effect of stirring speed on the Pd(II) extraction with 0.2M THMA in toluene. [HNO 3 ] 3M, initial [Pd] aq 500 mg/L, 1 min of extraction.

Figure S6 :

 S6 Figure S6: Pd(II) distribution ratio with 0.2M THMA in toluene with added DHA after 1 h of extraction. DHA molar percentage 0 mol% to 100 mol% respective to THMA, initial [Pd] aq 500mg/L, [HNO 3 ] aq 3M, A/O=1, stirring speed 1200 rpm.

Figure S7 :

 S7 Figure S7: Variation of the interfacial tension upon addition of DHA in the organic phase. 0.2M THMA in toluene, DHA molar percentage 0 mol% to 50 mol% respective to THMA, [HNO 3 ] aq 3M, T = 23°C.

Figure S8 :

 S8 Figure S8: Evolution of observed rate constant k obs of Pd(II) extraction with 0.2M THMA in toluene from aqueous 3M HNO 3 solution according to the added DHA quantity. Initial [Pd] aq 500 mg/L, [HNO 3 ] aq 3M, stirring speed 1200 rpm.

Figure S9 :

 S9 Figure S9: Effect of HNO 3 on Pd(II) extraction with 0.2M THMA in toluene at equilibrium (24 h). Initial [Pd] aq 500 mg/L, A/O=1.

Figure S10 :

 S10 Figure S10: Pd(II) distribution ratio with 0.2M THMA in toluene with added DHA at equilibrium (24 h extraction duration). DHA molar percentage 0 mol% to 50 mol% respective to THMA, initial [Pd] aq 500mg/L, [HNO 3 ] aq 5M, A/O=1.

Figure S11: 1 H

 1 Figure S11: 1 H NMR spectra (400 MHz, 100 MHz, solvent CDCl 3 ) of (a) crude THMA and (b) Dihexylamine (DHA).

Table 1 .

 1 Distribution ratios of Pd(II), Fe(III) and Nd(III) and Pd/Fe, Pd/Nd selectivity S Pd/Fe , S Pd/Nd after extraction using THMA and DBMA in toluene, from an aqueous 3M HNO 3 phase. Initial [Pd] aq 500 mg/L, 1 h extraction, stirring speed 2000 rpm.

		THMA	DBMA	THMA	DBMA	THMA	DBMA
		0.3M	0.3M	0.5M	0.5M	0.6M	0.6M
	D Pd	>70 *	4.0	>70 *	10	>70 *	15
	D Fe	0.006	0.02	0.05	0.2	0.09	0.31
	D Nd	<0.002 *	0.08	0.038	0.37	0.069	0.83
	S Pd/Fe	>11600	200	>1400	50	>778	48
	S Pd/Nd	>35000	50	>1840	27	>1014	18

Table 2 .

 2 [START_REF] Fröhlich | Valuable Metals-Recovery Processes, Current Trends, and Recycling Strategies[END_REF] H NMR spectrum (see SI, FigureS11). Other diluents can be employed, such as aromatic cuts, eg. Solvesso TM 150 (vide infra). Comparison of the results obtained during Pd(II) extraction between purified THMA and crude THMA in toluene are presented in Table2: Extraction

	of Pd(II) with purified THMA and crude THMA synthesized with an
	excess of DHA (10 mol%). THMA concentration 0.2M in toluene, initial [Pd] aq 500mg/L,
	[HNO 3 ] aq 3M, stirring speed 2000 rpm, A/O = 1.
		Purified THMA	Crude THMA
		C = 0.2 mol/L	C = 0.2 mol/L
	Experiment duration	24 h	1 h
	D Pd	28	34

Table 3 )

 3 . So far, results obtained on Pd(II) extraction with malonamides were mitigated: The best results were obtained in 1,2-dichloroethane at 8M HCl,[START_REF] Paiva | The solvent extraction performance of N,N'-dimethyl-N,N'-dibutylmalonamide towards platinum and palladium in chloride media[END_REF] with specific malonamides, with distribution ratios close to 4. In our hands, extraction of Pd(II) with DBMA from chloride media was very low, so that we employed a sodium chloride solution as stripping reagent in the Pd purification process we developed. The possibility to extract Pd(II) with excellent efficiency from chloride media (D Pd = 35.4 at [HCl] = 1M) is particularly striking, as only the hydrophobic part of the molecule was modified, the coordinating head is still the same diamide moiety. Using same conditions, Pd(II) extraction

	yield with DBMA (0.6M in toluene) was only 2%. Also, extraction of Pt(IV) was evaluated
	and gave successful results (Table 3). Distribution ratios of both metals strongly depend on
	the aqueous hydrochloric acid concentration. The distribution ratio of Pd(II) decreased from
	35.4 to 1 with HCl concentration increasing from 1M to 6M, whereas that of Pt(IV) increased
	from 10.3 to 27.4. The separation of Pt(IV) from Pd(II) is efficient at 6M aqueous
	hydrochloric acid concentration, where the separation factor S Pt/Pd was 27.4.

Table 3 .

 3 Pd(II) and Pt(IV) distribution ratios (D Pd and D Pt ) and Pt/Pd selectivity (S Pt/Pd ) with crude THMA in Solvesso TM 150 from hydrochloric acid media. THMA concentration 0.2M, initial [Pd] aq 500mg/L, initial [Pt] aq 500mg/L, 1 h extraction, A/O = 1, stirring speed 2000 rpm.

		[HCl] = 1M	[HCl] = 6M
	D Pd	35.4	1
	D Pt	10.3	27.4
	S Pt/Pd	0.29	27.4

Table 4 .

 4 Preliminary cost analysis of THMA synthesis. France), thus at a 56 €/kg price, to be compared with the 134 €/kg price for THMA based on reagents employed. In other words, the cost for THMA is expected to be in the same order of magnitude as that of TBP. From same supplier, and based on same packaging, dioctyl sulfide (DOS) is 10-20 times as expensive as TBP. Thus, THMA is one order of magnitude cheaper than DOS, an extracting molecule than can be employed at industrial scale for PGM refining. Altogether, this analysis and the obtained results suggest that THMA should be investigated with careful attention for PGM refining.

	Costs based on Sigma-Aldrich

Table S2 :

 S2 Variation of the aqueous concentration of Pd(II) at equilibrium (24 h) after extraction with 0.2M THMA in toluene with added DHA. DHA molar percentage 0 mol% to 50 mol% respective to THMA.

	DHA molar		k obs (min -1 )	t 1/2 (min)
	percentage
	0 mol%	16.7	0.028	25
	10 mol%	15.2	0.052	13
	20 mol%	13.2	0.070	10
	30 mol%	14.7	0.079	9
	40 mol%	13.6	0.104	7
	50 mol%	15.4	0.116	6

Acknowledgements

The authors thank the Commissariat à l'Energie Atomique et aux Energies alternatives (CEA) that has funded and supported this work.

Supporting information

Solvent extraction of Palladium(II) using malonamides: A