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Molecular Scale Description of Anion Competition at Amine Functionalized Surfaces William Rock, Muhammed Enes Oruc, Ross J. Ellis, and Ahmet Uysal Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03479 • Publication Date (Web): 07 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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Molecular Scale Description of Anion Competition at Amine Functionalized Surfaces William Rock,1 Muhammed E. Oruc,2 Ross J. Ellis1 and Ahmet Uysal1,* 1

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne IL 2

Department of Chemical Engineering, Yildiz Technical University, Istanbul Turkey

Many industrial and biological processes involve the competitive adsorption of ions with different valencies and sizes at charged surfaces; heavy and precious metal ions are separated based on their propensity to adsorb onto interfaces, often as anionic ion clusters (e.g. [MClx]n-). However, very little is known, both theoretically and experimentally, about the competition of factors that drive preferential adsorption, such as charge density or valence, at interfaces in technologically-relevant systems. There are even contradictory pictures described by interfacial studies and real life applications, such as chlorometallate extractions, where charge diffuse chlorometallate ions are extracted efficiently even though charge dense chloride ions present in the background are expected to occupy the interface. We studied the competition between divalent chlorometallate anions (PtCl62- and PdCl42-) and monovalent chloride anions at positively charged amine functionalized surfaces using in situ specular X-ray reflectivity (XR). Chloride anions were present in vast excess to simulate the conditions used in the commercial separation of heavy and precious metal ions. Our results suggest that divalent chlorometallate adsorption is a two-step process, and that the divalent anions preferentially adsorb at the interface despite having a lower charge/volume ratio than chloride. These results provide fundamental insight into the structural mechanisms that underpin transport in phases of relevance to heavy and precious metal ion

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separations, explaining the high efficiency of low charge density ion transport processes in the presence of charge-dense anions.

1. INTRODUCTION The advanced energy and electronic technologies of today’s world rely on relatively rare elements, such as lanthanides, actinides and platinum group metals (PGMs). Most of these elements are refined, recycled and reprocessed via solvent extraction and ion exchange processes.1 Although some of these separation technologies can be considered mature, they are not adequate for the exponentially growing volumes of heavy metal production and recycling around the globe. A breakthrough in these technologies will require a molecular scale understanding of the processes beyond the classical pictures assumed from the macroscopic properties. A fundamental aspect of most separation technologies is that they involve preferential ion adsorption on,2-3 or transfer through, interfaces.4 For example, in solvent extraction, hydrated ions in an aqueous phase are transferred into an organic phase, usually with the aid of amphiphilic extractant molecules that form reverse micellar structures around the target ions to solubilize them in the organic phase.1,

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Very little is known, however, about the molecular

scale structures that form at interfaces during ion separations, with the exception of a few recent X-ray/neutron scattering and reflectivity studies of interfacially-adsorbed lanthanide cations at complexing amphiphiles.6-8 Even less is known about the structure of anionic species that interact electrostatically with positively-charged surfaces of relevance to separations. Unlike the adsorption of metal cations, which is driven by dative (i.e. coordinating) interactions with ligating groups on the surface, anions are extracted via a competitive ‘anion exchange’


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mechanism. For example, the production of Pt and Pd involves the preferential extraction of the PtCl62- and PdCl42- anions over Cl-, which is usually in excess by several orders of magnitude.9 Other heavy metal ions, such as the actinides, can also be separated as the chlorometallate species when in the presence of very high chloride concentrations.10 Extraction is performed using an anion exchanger, often a quaternary amine,11 which is either tethered to a solid support or in a water-immiscible organic liquid phase. Thus, extraction pivots on the preferential adsorption of the chlorometallate anion onto a solid/liquid or liquid/liquid interface consisting of positively charged organic groups. The efficiency of this process, like in all anion-exchange separations, depends on the ability of the chlorometallate anion to compete effectively with chloride for adsorption onto the charged interface. The very fast kinetics and high efficiencies of chlorometallate extraction from highly concentrated chloride aqueous phases implies that these charge diffuse ions are preferentially adsorbed at positively charged interfaces over chloride. Singly charged chlorometallate anions, such as AuCl4-, are even more charge diffuse, and thus even more preferentially extracted.9 This relationship between charge density and extraction has been known in the separations community for decades, and underpins many anion separations processes.12 However, from the interfacial perspective, this observation presents a dichotomy as ions with high charge density, such as chloride, are expected to interact more strongly with charged surfaces than ions with low charge density, such as chlorometallate.13-15 For instance, both theory and simulations suggest that the charge/volume ratio plays an important role in the competition of ions with different valencies, particularly in the high surface charge regime. This ratio roughly expresses the combined effects of electrostatic energy and entropy at the interface. Higher valence ions are


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expected to preferentially adsorb at the interface due to the electrostatic forces, and the entropic effects favor the adsorption of smaller ions. Although in theoretical and computational studies (under ideal conditions) a larger charge/volume ratio leads to stronger adsorption,13-14 in reality, hydration, van der Waals interactions and non-ideal surface properties should also play significant roles.16-17 For instance, the preferential extraction of PtCl62- (2/139 = 0.0144 e-/Å3) and PdCl42- (2/106 = 0.0189 e-/Å3) over Cl- (1/24 = 0.0417 e-/Å3),9 whose charge/volume ratio is more than twice the metallate anions, suggests that the charge/volume ratio does not always determine preferential adsorption (volumes are calculated using atomic radii). It is important to note that the extraction efficiency is ultimately related to the final structures in the organic and aqueous phases, rather than the interfacial complexes.5,

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However, efficient extraction is unlikely to occur if Cl- blocks the

interfacial adsorption of chlorometallate anions. Clarifying this apparent contradiction between the interfacial picture of the competitive adsorption of ions at a charged interface and chlorometallate extraction improves our understanding of these complex interfaces significantly, particularly those used in the separation of heavy and precious metal ions. Towards addressing this dichotomy, we probe the structure at the interface between an aminefunctionalized silicon substrate and an aqueous solution bearing high concentrations of chloride ions and minute concentrations of platinum group chlorometallate ions (reflecting the conditions in separations processes). Studying a model system at the solid/liquid interface to understand some aspects of an extraction process at the liquid/liquid interface has two advantages. Firstly, the kinetics of these separation processes can be very fast, making it very difficult to catch observable structures at the liquid/liquid interface in reasonable time scales, unless some “arrested” metastable structures are formed.7 Secondly, a real extraction system involves too


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many complex interactions, such as ion-extractant, extractant-extractant and interactions with organic and aqueous solvents. Therefore, it is usually not possible to completely isolate a certain effect. Studying fixed amine surfactants isolates the interfacial amine-anion interactions, allowing direct observation of the competition between the anions independent of other effects. The results of this study provide a benchmark to aid in the understanding of more complex systems. Also, this simple model system can be interpreted in the context of biological, geochemical, or other systems where competitive ion adsorption plays an important role.19-20 Particularly, experimental studies of the electric double-layer (EDL) structure of concentrated electrolytes are scarce, and recent studies suggest that they might be more complicated than generally assumed.21 X-ray reflectivity (XR) probes the laterally averaged interfacial structure perpendicular to the surface normal with sub-nm spatial resolution.22-24 Although XR is not element specific, the strong electron density contrast between chloride (18 e- for Cl-) and chlorometallate (116 and 182 e- for PdCl42- and PtCl62- respectively) anions allows for reasonably accurate determination of the surface chlorometallate concentrations. Despite the relatively low bulk concentrations and charge density of chlorometallate versus chloride, interfacial adsorption of PtCl62- and PdCl42- is highly favored, accounting for the high efficiency of ion transport processes that target these ions. We were also able to determine the maximum metallate coverage, providing important insights into the energy-entropy balance during the competition. 2. EXPERIMENTAL SECTION All chemicals were purchased from Sigma and used without further purification. Silicon samples were cleaned in a piranha solution (sulfuric acid: hydrogen peroxide 7:3) at 120 °C for 30


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minutes. Then they were rinsed with DI water 3 times for 5 minutes each, and finally dried with nitrogen. (3-Aminopropyl)triethoxysilane (APTES) solutions were freshly prepared in anhydrous toluene (ca. 1.0 % w/w). Clean silicon substrates were placed in these solutions for 1 h at room temperature. In order to prevent solvent evaporation during the reaction, the top of the petri dish was closed. After surface functionalization, the samples were rinsed with excess toluene and dried with nitrogen. Chlorometallate solutions were prepared from Palladium (II) chloride and hexachloroplatinic acid by dissolving appropriate amounts in background solutions (5M LiCl or 3mM NaCl at pH=1 as described in the text). The pH of the solutions was controlled with 1M HCl. While simulating industrial conditions,9 these concentrations also ensure that the chlorometallate speciation is fixed at PtCl62- and PdCl42-.

Figure 1. Depiction of the thin film sample cell and X-ray scattering geometry. The Kel-f sample cell holds the silicon substrates with the help of a kapton film (yellow lines) secured by an O-ring and compression from the sides (not shown) to prevent fluid leakage. When the fluid is injected, the kapton film puffs up, allowing quick concentration equilibration. Immediately before the measurement, the kapton film was collapsed to leave a few micron thick liquid film over the substrate.


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The XR measurements were done at beamlines 33-IDD and 12-IDD of the Advanced Photon Source (APS) at Argonne National Laboratory. A thin film sample cell25 (Figure 1) and PILATUS detector were mounted on a six-circle diffractometer (a modified Huber psi-C diffractometer at 12-IDD and a Newport Kappa diffractometer at 33-IDD). The incident beam size was focused to ~80 µm vertically. The horizontal size was set to 0.5 mm by a pair of slits. The specular reflectivity signal was recorded as a function of the vertical momentum transfer || = (4/ )sin (2/2), where λ is the wavelength of the X-rays (0.60 Å at 33ID-D and 0.73 Å at 12ID-D), and 2θ is the scattering angle. The beam damage due to the X-rays was identified carefully by repetitive measurements on the same spot, and the final measurements were limited to shorter time intervals. Samples were shifted perpendicular to the beam to a fresh spot between measurements. The X-ray data was analyzed using a box-model, which assumes laterally homogeneous layers with interfaces in the form of error functions.23,

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Density,

thickness and roughness parameters of each layer in the model were obtained by least square fitting of the calculated XR curves (according to the Parratt formalism)23, 28 to the collected data.


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Figure 2. Specular XR intensity normalized to Fresnel reflectivity as a function of the vertical momentum transfer (q) from aqueous/APTES interfaces at different bulk metallate concentrations in (a, b) 5M LiCl and (c) 3mM NaCl solutions. The solid black lines show the fits to the data (symbols) as discussed in the text.


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3. RESULTS AND DISCUSSION We measured the XR at the APTES/electrolyte interface for 0.5, 5 and 50 mM PdCl42- and PtCl62-concentrations in 5M LiCl as shown in Figures 2a and 2b respectively. Before injecting the metallate anions, we measured the XR with the 5 M LiCl background solution (black circles in each plot). We also studied 0.5mM [PtCl62-] in a 3mM NaCl background solution (Figure 2c). However, a comparable measurement with PdCl4 was not done because the speciation of PdCl4 cannot be fixed to PdCl42- at low Cl- concentrations.29 The concentration dependent measurements were done on a single APTES film for each metallate. All experiments begin with the background solution and progress from low to high chlorometallate concentration. After each measurement, the sample chamber is thoroughly flushed, by puffing up the kapton film, with the new solution and allowed to equilibrate for at least 10 minutes, and the sample is translated to a fresh spot. After solution is fully exchanged, the interfacial equilibration is expected to happen less than a few minutes.30 This provides a reliable background subtraction to identify the effects of increasing the bulk metallate concentration on the interfacial concentration. The intensity of the XR signal (R) decreases proportional to q-4 even at an ideal flat surface; this is known as Fresnel reflectivity (RF).23, 25 Normalizing R to RF enhances visualization of the Kiessig fringes (the minima and maxima in XR data due to the electron density gradients at the interface).23 Before discussing the results obtained from the model dependent fits, we investigate the qualitative behavior of the XR signal in Figure 2a, b as a function of increasing metallate concentration. Two major trends can be observed in both Pd and Pt data. The minima in the XR curves shift to higher q values in 0.5 mM metallate solutions (blue squares) compared to the


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background (black circles). However, as the concentration increases to 5 and 50 mM (green diamonds and red triangles respectively), the minima move in the opposite direction (to smaller q values). Since q represents reciprocal space, these results suggest that the interfacial structure (APTES + ions) gets thinner compared to the background solution at 0.5mM metallate, counterintuitively implying desorption of ions. However, as the metallate concentration increases further, the thickness increases as expected. Also, the amplitude of the Kiessig fringes increases significantly in the 5 and 50 mM solutions, implying that the density contrast at the interface is also increasing.

Table 1. Best fit parameters for the XR data and fits shown in Figure 2. Silicon electron density and the Si/SiO interface roughness are fixed to 0.71 e-/Å3 and 1.5 Å, respectively, for all samples. The superphase density is fixed to 0.36 e-/Å3 for the 5M LiCl background solutions and 0.33 e-/Å3 for the 3mM NaCl background solution. All the parameters were allowed to float within reasonable limits for 0 metallate concentration background samples. Then, the silicon oxide parameters and APTES layer thicknesses are fixed (marked with a) for the measurements with


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metallate ions. The uncertainties shown in the bottom row are estimated from the least square fitting of the background solutions; they are very similar, and usually smaller for other fits where some of the parameters are fixed.

3.1 APTES in 5M LiCl. We first discuss APTES films in background solutions without any metallate ions. Figure 3 shows the EDP of the APTES film structure obtained from fitting the top reflectivity data set in Figure 2a. The films used with other samples also have the same qualitative EDP with only slight differences in the densities and thicknesses (Table 1 and Figure 4). The ~6 Å lengths of each APTES layer matches well with the calculated size of the molecule by MMFF94 force field (Figure 3, inset).31 Contrary to most well-known self-assembly processes, such as octadecyltrichlorosylane (OTS),32-33 APTES self-assembly is not self-limiting after the formation of the first layer; the APTES Si has three Si-O groups that can attach to the surface or self-polymerize, and the hydrocarbon chain is too short to block the formation of APTES-APTES bonds. Therefore, the thickness of the film increases with the reaction time (Figure 3).34 Our synthesis parameters matched well with previous reports that show quasibilayer formation.34 We modeled the film with two separate boxes and allowed the parameters to vary freely. The fits show that the second layer is ~10-20% less dense than the first. A multilayered surfactant is representative of interfaces in commercial liquid-liquid extraction systems,8 where atomically-flat monolayers are unusual.


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Figure 3. Electron density profile (EDP) of APTES on silicon in 5M LiCl (solid red line). The green solid lines show the same EDP without interfacial roughness. The bottom left inset is the molecular model of APTES. The superimposed cartoon depiction of the APTES film structure with Cl- adsorption is deduced from the EDP.

A separate layer was assigned for the adsorbed Cl- ions. As shown in Figure 3, this layer has almost the same density as the previous layer and it is only ~3 Å thick. Indeed, it is possible to obtain an identical EDP using a single box (with ~8 Å thickness) for the top APTES layer and the Cl-.35 However, this extra slab allows an easier comparison between different samples with varying metallate concentrations. Nevertheless, all the results presented here can be reproduced with 3-slab models with no qualitative differences (Please see supplementary information for details). The APTES coverage, calculated from the EDP in Figure 3, is ~24 Å2/molecule in the first layer and ~29 Å2/molecule in the top layer. The same analysis of the APTES film used for PtCl62adsorption (Figure 2b) gives 22 and 27 Å2/molecule for the first and the second APTES layers,


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respectively. The APTES film used in the NaCl background solution, which was prepared on a silicon substrate from a different batch during an earlier beamtime, has ~30 and ~32 Å2/molecule coverages. Since the APTES films are prepared under ambient conditions, different relative humidity levels during the preparation are likely responsible for the slight differences in the films.36-37 These coverages are used to estimate the surface charge, assuming that all the amine groups at the top surface are protonated, and therefore positively charged in the highly acidic (pH = 1) solution.

Figure 4. Comparison of EDPs of APTES on silicon in 5M LiCl before (black dashed lines) and after (blue solid lines) addition of 0.5 mM metallate calculated by fitting the XR data in Figure 2. The PdCl42- curve is moved up by 0.1 for clarity. The top right inset shows the differential EDPs for both plots generated by the difference of the dashed black and solid blue curves. They are almost identical for both metallates – within the errors of the measurement (SI-Figure 4). The inset schematic describes a possible explanation for the drop in the EDP due to the metallate anions adsorbed in the diffuse layer disturbing the chloride anions in the Stern layer.

3.2 Chloride Desorption from the Surface with Metallate Injection. When 0.5 mM metallate is injected into the 5M LiCl background solution, the minima in the reflectivity signal (Figure 2,


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blue squares) shift towards higher q, suggesting a decrease in the thickness of the interfacial layer. A comparison of the EDPs calculated by fitting the XR data (Figure 4) shows that this is due to a decrease in the electron density of the slab assigned to the adsorbed anions. This decrease in the EDP must be caused by a net decrease in the adsorbed ions or by thinning the APTES film. The injection could dislodge physisorbed APTES molecules; this is unlikely because the film is thoroughly washed during preparation and first injected with the background solution. The presence of metallate ions could rearrange the APTES layer. This could change the shape of the EDP, but would conserve the total amount of interfacial electrons. The data shows a net decrease in interfacial electrons, making this scenario unlikely. Finally, the metallate ions could break some of the Si-O bonds, thinning the film; this is also unlikely, especially because this strong chemical interaction would need to occur without enough chlorometallate adsorption to increase the electron density. Therefore, it is most likely that these changes arise from desorption of adsorbed ions. Interestingly, the subtraction of the background EDP from the 0.5 mM EDP gives a very similar differential EDP for both metallate anions (Figure 4, inset), suggesting that the effect is approximately the same for both PdCl4 and PtCl6 – Cl- anions desorb from the surface and are not replaced by metallate anions. This behavior is reflective of the identical bulk-phase liquidliquid extraction equilibria for PdCl42- and PtCl62-:9 PdCl42- + 2[Cl.HA] = [PdCl4.HA2] + 2ClPtCl62- + 2[Cl.HA] = [PtCl6.HA2] + 2Clwhere A=amine extractant.


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Why do Cl- ions leave the surface after the introduction of the metallate? Most likely, metallate anions are attracted to the surface and disturb the Cl- ions in the Stern layer. However, metallate anions cannot get adsorbed, possibly due to their hydration (Figure 4, inset schematic). Since XR is sensitive to the gradient in the EDP, the few metallates that are not adsorbed exactly at the surface (and therefore cannot form a well-defined layer) do not create enough contrast to be directly measured. Assuming charge compensation, this is the simplest explanation. Also, the almost identical differential EDPs for both metallates (Figure 4, inset) suggests that there is no significant metallate adsorption on the Stern layer, which would result in observable differences due to their dissimilar electron densities. It’s known that, on negatively charged mineral surfaces, divalent cations adsorb as outer sphere and extended outer sphere complexes with little effect on the total electron density profile.38 Hence, it is not surprising that divalent anions behave similarly at a positive interface. In this picture ~4% of the Cl- ions leave the surface at a 0.5 mM bulk metallate concentration; it is also assumed that for every two chloride ions leaving the Stern layer, one metallate is adsorbed in the diffuse layer. However, in liquid-liquid anion extraction systems using amine/ammonium extractants, partial or complete dehydration of the anion is necessary to complete transport from one phase to another via the interface. Therefore, under conditions where ion extraction is known to proceed rapidly (i.e. at higher metallate concentrations that reflect process conditions), there should be evidence of penetration of the anion from the diffuse layer into the stern layer. 3.3 Chlorometallate Adsorption at the Stern Layer. Increasing the metallate concentration in the bulk to 5 and 50 mM causes the minima in the XR data to shift to smaller q and increases the Kiessig fringe amplitudes (Figure 2, green diamonds and red triangles); this is direct evidence of the adsorption of metallate anions on the Stern layer. Figure 5 shows the electron density profiles


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derived for these higher relative metallate concentrations, including the measurement in 3mM NaCl background (Figure 2c), which has a 0.17 relative metallate concentration.

Figure 5. (a) EDPs of chlorometallate adsorbed APTES films derived from the fits to the XR data in Figure 2a-c respectively from top to bottom. The dashed lines correspond to the EDPs in the appropriate background solutions. The top and bottom curves are shifted +0.1 and -0.1 respectively for clarity. (b) Differential EDPs for [PdCl42-] adsorption, calculated by subtracting the dashed blue line from the green and red curves for 10-3 and 10-2 [PdCl42-] / [Cl-] relative concentrations respectively. (c) Differential EDPs for [PtCl62-] adsorption, calculated by subtracting the dashed lines in the bottom two profiles from the green, red and magenta curves for 10-3, 10-2 and 1.7×10-1 [PtCl62-] / [Cl-] relative concentrations, respectively.


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The differential EDPs (Figure 5 b and c) provide important insights about the adsorption of metallate anions as a function of the relative metallate concentration. The integration of each curve gives the change in the electron density per unit area at the interface; this can be converted to the coverage of metallate anions by using the number of electrons they have. We can normalize each of the coverages to the available number of sites obtained from the APTES molecular areas, assuming a full coverage (θ = 1) when 2 APTES molecules are occupied by 1 metallate (Figure 6). It is important to note that XR provides EDPs that are convoluted with the experimental resolution (π / qmax ~ 4Å in these experiments).35 Therefore, the differential EDPs are too broad to determine the exact position of the ions. However, the area under the EDP is a more robust parameter.27

Figure 6. Plot of metallate coverage at the interface as a function of the relative metallate concentration in the bulk. The data points for the lowest bulk relative concentration (10-4) are estimated from Figure 4 according to the model


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described in the text. All the other data points are calculated by integrating the differential EDPs in Figure 5 b and c. The error bars do not represent the error in the integration (much smaller), but show the approximate range of surface concentrations based on the model used to estimate the metallate coverages. The black solid line represents the metallate coverage corresponding to the same relative concentration on the surface and in the bulk. Overlap of [PtCl62-] and [PdCl42-] results suggest that the differences in their geometries do not play a significant role in the competition with chloride.

Figure 6 summarizes all the results obtained from the XR experiments. The first remarkable result is that, after calculating the number densities and normalizing with respect to the surface charge, there is no significant difference between the PdCl42- and PtCl62- adsorption at the interface over a relative concentration range of three decades. The charge/volume ratio of PdCl42and PtCl62- are 45 and 35% of Cl-, respectively. PdCl42- has a planar shape while PtCl62- is octahedral. Apparently these factors do not create a significant difference in their competition against chloride at the APTES/water interface. This is a surprising result because computer simulations suggest that PdCl42- and PtCl62- have different hydration shells,39-40 which are expected to affect interfacial interactions,41-43 due to the differences in their molecular geometries. This multitude of seemingly contradictory phenomena underscores the importance of further research into the molecular-level interactions of highly concentrated electrolyte solutions at charged interfaces. The black solid line in Figure 6 represents the case in which the relative metallate concentrations at the surface and in the bulk are the same. The results suggest that metallate adsorption at the interface is significantly favored at a metallate to chloride ratio up to 0.1. At a concentration ratio of 0.17, the measured surface concentration is almost the same as the bulk concentration. This strongly suggests that the metallate coverage saturates significantly below the theoretical limit of 2NH3+ + M2-, which assumes the complexation of two amine groups with 1 metallate. The data


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was fit with a Langmuir isotherm (blue dashed lines), which shows that only ~1/3 of the available sites are occupied by metallate anions at high metallate concentrations. The fit does not provide absolute adsorption free energy because we use relative concentrations without any activity coefficients. However, it helps to qualitatively visualize the fact that the interfacial metallate concentration starts saturating at high relative concentrations. The preferential adsorption of the metallate anions onto the ammonium surface is qualitatively reflective of their rapid kinetics and high extraction efficiency seen in analogous liquid-liquid extraction systems.9 However, there is a great paucity of quantitative kinetic data on these systems that can give insight into the absolute interfacial mechanisms associated with the extraction of Pt and Pd under these conditions. This is in part due to the rapid kinetics, making interfacial reactions difficult to deconvolute from diffusion-controlled processes. However, with the advent of new microfluidic techniques that couple rapid mixing with well-defined interfacial areas,44 extraction studies should now be able to access absolute interfacial kinetics values in these rapidly evolving systems. Such studies would be invaluable for comparison with the surface structures presented herein. 4. CONCLUSIONS We studied the competitive adsorption of platinum and palladium chlorometallate anions on amine functionalized surfaces in highly concentrated chloride solutions; the conditions were reflective of commercial heavy and precious metal ion extractions. XR reflectivity data clearly shows that the metallate anions are able to adsorb to the surface even at 10-3 relative metallate/chloride concentrations in the bulk, suggesting that energetics due to Coulombic interactions play an important role in this regime and favor divalent over monovalent anions


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regardless of the charge/volume ratio; this is contrary to the expectations from MC simulations and mean field theories.13-14 Hydration of the ions may be mainly responsible for this difference between experiment and theory. Nevertheless, this is consistent with the well-known very rapid extraction kinetics for these metallate species. At higher relative concentrations metallate anion coverage levels off after occupying 1/3 of the available sites, indicating that entropic effects (mainly driven by the smaller volume of chloride ions)14 play a critical role in determining the maximum coverage. These results show that at small relative concentrations, a low charge/volume ratio does not prevent metallates from getting adsorbed to the interface, explaining their efficient liquid/liquid extraction. On the other hand, entropic effects keep the maximum possible coverage low at high metallate concentrations. We can speculate that in a solvent extraction process, where ions are transferred into an organic phase from the aqueous phase by forming reverse micellar structures (contrary to forming a static interfacial structure as in our experiment),18 limited surface coverage may help prevent third phase formation that occurs after high loading of metals at the interface. Such a relation could explain the inversely proportional relation between the charge/volume ratio and the extraction efficiency in chlorometallate anions. Both platinum and palladium chlorometallate adsorption have two regimes. At very low metallate concentrations, they get adsorbed on the diffuse layer without forming a well-defined structure and cause an apparent decrease in the interfacial electron density. As the concentration increases, metallate ions are adsorbed at the interface preferentially over chloride, suggesting that the electrostatic interactions dominate for most technologically relevant concentrations. At very high metallate/chloride relative concentrations, entropic effects become important and limit the maximum coverage of the metallates. These observations will help to develop a better


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understanding of interfacial processes that include the competitive adsorption of ions with different charges and sizes. ASSOCIATED CONTENT Supporting Information Details of the models used to fit XR data and atomic force microscopy (AFM) characterization of APTES films.

AUTHOR INFORMATION Corresponding Author *[email protected] (A.U.) ACKNOWLEDGMENT XR experiments were done at Sector 12-IDD and Sector 33-IDD of the Advanced Photon Source at Argonne National Laboratory. This work and the use of the Advanced Photon Source are supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences, under Contract DEAC02-06CH11357. We thank Lynda Soderholm for her comments on the manuscript. We also thank Paul Fenter and Sang Soo Lee for fruitful discussions and access to their AFM. REFERENCES 1. Tasker, P. A.; Plieger, P. G.; West, L. C. Metal complexes for hydrometallurgy and extraction. 2004.


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