Ion Transport Mechanisms in LiquidâLiquid Interface - Langmuir

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Ion Transport Mechanisms in Liquid-Liquid Interface Baofu Qiao, John V. Muntean, Monica Olvera de la Cruz, and Ross J Ellis Langmuir, Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on June 2, 2017

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Ion Transport Mechanisms in Liquid-Liquid Interface

Baofu Qiao,†,* John V. Muntean, ‡ Monica Olvera de la Cruz † and Ross J. Ellis§,*

†

Department of Materials Science and Engineering and Department of Chemistry, Northwestern

University, Evanston, IL 60208, USA ‡

Chemical Sciences & Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA

§Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

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ABSTRACT: Interfacial liquid-liquid ion transport is of crucial importance to biotechnology and industrial separation processes including nuclear elements and rare earths. A water-in-oil microemulsion is formulated here with density and dimensions amenable to atomistic molecular dynamics simulation, facilitating convergent theoretical and experimental approaches to elucidate interfacial ion transport mechanisms. Lutetium(III) cations are transported from the 5 nm diameter water pools into the surrounding oil using an extractant (a lipophilic ligand). Changes in ion coordination sphere and interactions between the interfacial components are studied using a combination of synchrotron X-ray scattering, spectroscopy and atomistic molecular dynamics simulations. Contrary to existing hypotheses, our model system shows no evidence of interfacial extractant monolayers, but rather ions are exchanged through water channels that penetrate the surfactant monolayer and connect to the oil-based extractant. Our results highlight the dynamic nature of the oil-water interface and show that lipophilic ion shuttles need not form flat monolayer structures to facilitate ion transport across the liquid-liquid interface.

Keywords: microemulsion; ion transport, extractant, liquid-liquid interface, water channel


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The transport of ions through liquid-liquid interface has wide relevance in biological,1 catalytic,2 analytical,3 environmental,4 pharmaceutical,5 and industrial applications.6 It is not surprising, then, that liquid-liquid ion transport has been rigorously studied for many years,7-8 yet there remains a paucity in molecular-level knowledge of the interfacial zone. Although some systems allow ions to migrate freely between immiscible liquids,9 most require molecular agents that bind to the ion and drive its transport. Often, lipophilic complexing agents, known as extractants, are used to bind target ions and facilitate transport from water into oil (see Scheme 1a,b). These are used frequently to separate ions for a variety of applications by partitioning them between oil and water.6 Despite the obvious importance of the interface in extractant facilitated liquid-liquid ion transport, until recently the interface was only probed using physical measurements that do not directly explore structure.10,11 Over the last two decades, new experimental techniques have provided nanoscale insight into the biphasic interface structure of liquidliquid ion transport systems.12-16 This developed in parallel with atomistic molecular dynamics (MD) simulations that have been used to explore molecular-scale structure and mechanism of ion transport in the interfacial zone.17-18 Nevertheless, the role of extractant molecules in ion transport through the interfacial zone remains unclear. Under select dilute conditions, extractant molecules appear to behave like surfactants and arrange into flat monolayer and bilayer structures,13, 16, 19 suggesting that ion complexation proceeds via direct adsorption of the extractant onto the interface. However, under most conditions, the interface is much more disordered with ‘pockets’ of interpenetrating solvent.16, 19-20 This disordered structure might suggest a dynamic diffusion model for liquid-liquid ion transport that has been proposed for exchange of chloride between water and dichloroethane (an extractant-free system),9 where “fingers” of water and oil interpenetrate and provide a route through which ions migrate.21 Similarly, the transport of chloride ion between water and nitrobenzene (extractant-free, too) has been very recently quantified.22 In particular, the presence of water “finger” remarkably decreases the free energy barrier of chloride transport from water to oil phase. We wonder to what extent this mechanism might apply to extractant facilitated ion transport and, more specifically, how the extractant interacts with interfacial water channels. Here we probe the effect of extractant on facilitating ion transport with a study that converges experimental techniques with atomistic MD simulations under completely comparable conditions. This is achieved by encapsulating the entire ion transport system inside a single phase water-in-oil microemulsion (Scheme 1c): a new approach for extractant facilitated ion transport systems. The microemulsion is deliberately formulated with appropriate surfactant/water/oil concentrations to give one water droplet with the volume of 1000 nm3 in microemulsion (Scheme 1d). This system size is accessible in its entirety using atomistic MD simulations, with numbers of molecules derived directly from


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experimental concentrations. Performing ion transport inside a single phase microemulsion has the added advantage of using spectroscopic techniques that probe intermolecular interactions that are usually inappropriate for biphasic interface studies. Our findings suggest that ions are exchanged between the aqueous core and surrounding oil through water channels, without direct adsorption of the extractant molecule into the interfacial layer.

Scheme 1. (a) HDEHP (di-(2-ethylhexyl)phosphoric acid) - an example of a ‘cation exchange’ extractant; (b) classical biphasic liquid-liquid extractant facilitated ion transport system, where extractant molecules bind to ions (white dots) and facilitate its transport from water (blue) to oil (pink); (c) an water-in-oil microemulsion system where extractant binds ions and facilitates transport from aqueous reverse micelle cores to surrounding oil; (d) microemulsion formulated to give one large reverse micelle water droplet in a 1000 nm3 volume. This nanoscale ion transport system was simulated in its entirety using approx. 10 × 10 × 10 nm3 atomistic MD to give direct comparison with experiment under conditions that match in terms of dimension and constitution.

Results and Discussion HDEHP facilitated water-oil ion transport Extractant facilitated ion transport at liquid-liquid interface is typically performed between oil and water biphases and has, to our knowledge, not been performed inside a single-phase water-in-oil microemulsion. The first step is, therefore, to demonstrate that ion transport between the water pool and surrounding oil in the microemulsion proceeds, and that it is a good model for bulk liquid-liquid systems. The extractant selected for study is the alkyl phosphoric acid known as HDEHP (Scheme 1a), which was originally designed to extract lanthanide(III) cations via a ‘cation exchange’ mechanism. 23 In biphasic liquid-liquid systems, extraction proceeds via deprotonation of the dimerized HDEHP extractant molecule (LH)2, where three mono-deprotonated dimers are known to form the 6-coordinate oil-soluble complex with lanthanide(III) ions (L.LH)3Ln according to Eq. (1): 24-26 3(LH)2(org) + Ln3+ = (L.LH)3Ln(org) + 3H+ ,

(1) 4

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where LH stands for neutral HDEHP and L for deprotonated DEHP+, and the subscript “org” indicates organic soluble species. Our aim is to transport lanthanide(III) cations from the aqueous pool to the surrounding cyclohexane oil using HDEHP, and demonstrate that the nanoscale single-phase system is an analogue for the equilibrium (1) observed in traditional biphasic liquid-liquid processes. Extraction is achieved by dissolving 1.5 mM of Lu(NO3)3 into the microemulsion, and subsequently adding 10mM HDEHP to draw the metal ion from the aqueous pool to the surrounding oil. Extended X-ray absorption fine structure spectroscopy (EXAFS) is used to probe the coordination environment around the Lu3+ ion in the microemulsion to track the extraction. The k3χ(k) EXAFS data and the corresponding Fourier transforms (FTs) for the Lu 3+-bearing microemulsions are shown in Figure 1, where the blue and red lines correspond to data collected before and after HDEHP addition. Following HDEHP addition, the phase of the k3χ(k) data function changes significantly, indicating a major change in the distance between the absorbing Lu and back-scattering (i.e. coordinating) atoms. This is confirmed in the FT-EXAFSs, where the intense peak corresponding to the inner-sphere coordinating O shifts from 2.308 to 2.218 Å upon introduction of HDEHP. The FT-EXAFS data for the Lu3+-bearing microemulsion before addition of HDEHP shows a single and symmetric intense peak 2.308 Å that is consistent with a homoleptic hydrated cation. After the addition of HDEHP a second, weaker peak appears at around 3.7 Å that might be attributed to back-scattering phosphorous atoms from the coordinated HDEHP.27

before

after

Figure 1. K3χ(k) L3-edge Lu EXAFS data and corresponding FTs collected from microemulsion before (blue) and after (red) the addition of HDEHP. The data are fitted using a 1-shell O model (blue dotted lines) and a two-shell (O, P) model (red dotted lines), respectively. Inset show the coordination structures suggested by Table 1. 5 ACS Paragon Plus Environment

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Table 1. Metrics from Shell Modelling of L3-edge Lu EXAFS Data Collected from Microemulsion Before and After HDEHP Addition. atom

r/Å

CN

σ/Å

ΔE / eV

before

O

2.308(3)

7(1)

0.006(1)

2.8(3)

after

O

2.218(4)

6(1)

0.006(1)

3.9(4)

P

3.732(9)

6(1)

0.013(2)

3.9(4)

Based on the features in the FT-EXAFS, the data for the microemulsion prior to HDEHP addition is fitted using a simple single O shell model. A good fitting of the data is achieved, yielding physically reasonable values for all parameters (Table 1). Both the Lu-O dative bond distance and O coordination number from the data fitting are consistent with a fully hydrated Lu3+ cation (Figure 1 inset) as observed in dilute bulk aqueous solutions.28-29 The EXAFS data for the system after adding HDEHP was fitted with a 2-shell (O, P) model to account for both peaks in the FT-EXAFS. The Lu-O and Lu-P distances as well as the coordination numbers obtained from the data fitting (Table 1) are consistent with the 6 coordinated HDEHP (i.e., three mono-deprotonated dimers) structure known in classical solvent extraction studies (Figure 1 inset). This kind of core-shell aggregates could only exist stably in organic phases, in the form of reverse micelle clusters, which thus suggests the existence of Lu ions in the organic environment in the presence of HDEHP. Note that due to the uncertainties in the obtained coordination numbers, it is not able to strictly exclude the probability of water coordination to Lu3+ in organic phase. The partial dehydration behavior has been very recently observed in the Cl- transport from water to nitrobenzene phase via atomistic simulation, facilitating the ion transport by decreasing the free energy barrier from the ideal, complete dehydration.22 Taken together, the EXAFS data confirm the transport of Lu ions from the water subphase to the organic environment upon the addition of HDEHP, which is representative of the ionic species in macroscopic liquid-liquid extraction experiments.30 Surface activity of HDEHP To understand the mechanism of the HDEHP facilitated water-oil ion transport of Lu ions, the effects, in particular the surface activity, of HDEHP molecules need to be addressed. To that end, atomistic MD simulation on water-in-cyclohexane system is performed in the presence of HDEHP and the absence of Lu(NO3)3. The simulation box length is chosen around 11 nm to mimic the average distance between neighbor reverse micelles experimentally obtained. The numbers of water, surfactant (CTAB),


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cosurfactant (pentanol) and extractant (HDEHP) are calculated based on experimental concentrations. See Table S1 in SI for the details. It is necessary to note that prior to the simulation study of the surface activity of HDEHP, a control system where water reverse micelle with only CTAB and pentanol (i.e., no HDHP and Lu(NO3)3) was simulated (Table S1). Good agreements have been observed between experiment and simulation in terms of the size of the water reverse micelle, the distributions of CTA+, Br- and pentanol. See Section S2 in the SI for detailed discussion. This control simulation thus justifies the feasibility of our methodology for the following studies.

Figure 2. (a) Snapshots of the last atomistic simulation frame illustrating the distribution of HDEHP (blue arrows) in the absence of Lu(NO3)3. Atoms O/N/Br/C/H are colored in red/blue/green/cyan/white, respectively. The cyclohexane molecules and the hydrogen atoms of pentanol and CTA+ are omitted for the display. A rotated movie is also provided in the SI. (b) (upper) Integral number of HDEHP phosphorus atoms as a function of the distance to the center-of-mass of the water pool; (lower) the radial density profiles, where the values of pentanol, CTA+, Br-, and HDEHP are magnified for display.

In order to visualize the distribution of the (associated) HDEHP molecules in the microemulsion system, the snapshot of the last simulations frame is provided in Figure 2a. The positions of the HDEHP molecules are highlighted with the blue arrows. It is evidenced that the majority of the HDEHP molecules (4 out of 6) are dissolved in the organic phase. Moreover, one HDEHP dimer is also observed. The HDEHP dimer is stabilized in the organic environment by means of H-bonding interactions (Figure S7) , and may break apart and reform (Figure S4). To quantitatively calibrate the surface activity of HDEHP molecules, the integral number of HDEHP phosphorus atoms is calculated as a function of the distance to the center-of-mass of water molecules (Figure 2b, upper panel). See Section S1.b in the SI for the 7 ACS Paragon Plus Environment

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definition of the water center-of-mass, and Section S1.c for the discussion of the uncertainties in the calculation of HDEHP surface activity. Additionally, the radial density profiles are presented in the lower panel of Figure 2b. The radial density of water extends to 3.5 - 3.8 nm, corresponding to the maximum extent of the water pool from the center. If we define the junction point of the water density and the cyclohexane density as the boundary of the water-oil interface,31 the radius of the water reverse micelle is around 2.85 nm. It is found that only around one out of the six HDEHP phosphorous atoms is adsorbed at the water pool surface on average if we define the cutoff distance of 3.75 nm based on the first minimum of the phosphorous radial density profile, that is, the majority of the HDEHP molecules are distributed far away from the water reverse micelle (i.e., dissolved in the cyclohexane organic environment). This thus shows that the HDEHP molecules are mostly surface inactive. This surface inactivity is ascribed to the short branched alkyl tail-groups of HDEHP (Scheme 1a), which favor dissolution in the organic phase. The distributions of surfactant CTAB (i.e., CTA+, Br-) and cosurfactant pentanol are similar with those in the control simulation. See Section S2 in the SI for the related discussion. Moreover, it is found that the influence of the presence of HDEHP in the system is negligible in terms of the relative positions of the maximum distributions as well as of the density magnitudes of surfactant and cosurfactant (Figures 2b, S5b).

Figure 3. SAXS data collected from microemulsion with HDEHP concentration increasing from 0 to 0.1 M. Note that these data are not background subtracted with respect to the cyclohexane solvent.

Furthermore, the surface activity of HDEHP is also experimentally investigated. We used SAXS measurements to investigate whether HDEHP affects the water-cyclohexane interface in the present work. 8 ACS Paragon Plus Environment

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In microemulsions, the water/surfactant ratio controls the morphology (size, shape, number) of the large water pools that cause X-ray scattering at small angles. Therefore, if HDEHP participates in the CTAB surfactant layer at the water-oil interface, we expect SAXS to show a change in aggregate morphology as the water/surfactant ratio changes with increasing HDEHP concentration. Contrary to this, the SAXS data in Figure 3 show that the scattering intensity in the small angle region remains constant across all HDEHP concentrations up to 0.1 M, which is comparable to the concentration of CTAB surfactant itself (i.e., 0.15 M). Therefore, in the microemulison system it appears that the presence of HDEHP doesn’t change the interfacial structure of the water reverse micelles, which is consistent with the simulation results above. Water-oil ion transport mechanism Performing liquid-liquid ion transport inside a microemulsion provides the opportunity to apply techniques that are usually disqualified in macroscopically separated liquid-liquid systems. Nuclear magnetic resonance (NMR) is one such tool that can reveal interactions between molecular functional groups, and we use this here to investigate how the extractant HDEHP interacts with the microemulsion on the molecular scale. The proton NMR spectrum for the microemulsion without HDEHP (Figure 4a) shows distinct peaks for the different molecular groups. Of particular interest are the protons for the CTAB molecules that make up the layer at the interface, as well as those for water. The proton NMR spectrum for HDEHP in pure deuterated cyclohexane (top panel of Figure 4b) shows a distinct peak for the acidic proton at 12.6 ppm, but this disappears when HDEHP is dissolved in the microemulsion (bottom panel of Figure 4b). There is also a slight up-field shift of the peak corresponding to the HDEHP α-protons when in the presence of the microemulsion relative to the pure deuterated cyclohexane (Hb in Figure 4b) from 4.06 to 3.90 ppm, suggesting that the electronic properties of the extractant headgroup change in the presence of the microemulsion. The proton NMR spectra for the microemulsion with increasing concentrations of HDEHP (Figure 4c) shows that the peak corresponding to the HDEHP αprotons (Hb in Figure 4b) shift from 3.90 to 3.93 ppm with a 10-fold increase in concentration. This is accompanied by a broadening and down-field shift of the peak corresponding to water protons from 4.78 to 5.04 ppm. The changes in the NMR spectra for the microemulsion with increasing HDEHP concentration are consistent with proton exchange between HDEHP and the aqueous core of the reverse micelle. Therefore, protons are exchanged between the lipophilic acid and the aqueous microemulsion cores, even in the absence of the metal ion. We are interested in identifying the molecular level mechanisms that allow the extractant to facilitate the exchange of protons (and other cations) through the interfacial zone.


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Figure 4. (a) Proton NMR spectrum for the CTAB-pentanol-water microemulsion in deuterated cyclohexane; (b) Proton NMR spectrum for HDEHP in deuterated cyclohexane (top panel) and 0.05 M HDEHP inside the CTAB-pentanol-water microemulsion in deuterated cyclohexane (bottom panel); (c) Proton NMR spectra for 0 M to 0.1 M HDEHP inside the CTAB-pentanol-water microemulsion in deuterated cyclohexane.

Previous tensiometry measurements of bulk oil-water systems containing alky phosphoric acids, as well as recent grazing incidence small angle X-ray scattering data, have suggested that protons exchange via the direct absorption of the organic acid onto the interface as a flat monolayer. 13-14, 32 As indicated by the atomistic simulation and SAXS measurement, the HDEHP molecules are basically surface inactive, 10 ACS Paragon Plus Environment

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which is ascribed to the short branched alkyl tail-groups yielding very high packing parameters that force highly negative curvature. Indeed, extractants are deliberately designed to disfavor the formation of flat lamellar like structures that cause unwanted phase properties in technological applications. It is therefore improbable that extractant molecules like HDEHP would exchange their protons via the formation of ordered, flat, interfacial monolayers. Interactions between the extractant molecule and other molecules in the interfacial zone are investigated by 1D selective gradient ROESY NMR on the microemulsion containing HDEHP. By exciting a peak corresponding to protons on a target molecular group such as those highlighted in Figure 5a, other protons in close proximity (99.9%), hexadecyltrimethylammonium bromide (CTAB, >99%), 1-pentanol (>99%) and lutetium(III) nitrate (>99.99%) were all purchased from Sigma-Aldrich (WI). 18 MΩ water was used throughout. Solutions 1-3 were prepared by weighing out appropriate quantities of solute and diluting with either cyclohexane (for EXAFS and SAXS measurements) or cyclohexane-d12 (for NMR measurements). Lu(III) was used because, as a rare earth, it is extracted by HDEHP and it is diamagnetic so that it does not interfere with 1H-NMR techniques. SAXS Data Collection: SAXS data were collected at the Advanced Photon Source (APS) at the Argonne National Laboratory using beamline 12-ID-C.34 The experimental details are the same as previously reported.35-37 EXAFS data collection and analysis: Lu L3-edge spectra were collected in fluorescence mode at beamline 12-BM-B38 at the APS at Argonne National Laboratory, using a multi-element Ge detector (Canberra). The organic solutions were injected into micro X-cells (SPEX 3577) fitted with Kapton film windows (7.5 mm gauge, Chemplex Industries, No. 440) for data acquisition. Three 1 h scans were averaged for each organic solution. Step-wise unconstrained shell fitting to the k3χ(k) EXAFS were performed with EXAFSPAK. The curve-fitting using theoretical phase and amplitude functions were calculated with FEFF 8.01.39 NMR Spectroscopy: NMR experiments were performed using a Bruker Avance III 500 MHz NMR spectrometer (11.7 T). With the use of the nitrogen pre-cooler, heater coil, and the variable temperature controller, the temperature was stable at 294.3 ± 0.1 K. Spectra were recorded with a two channel 5 mm direct-detection, single-axis gradient, variable temperature probe with 2H lock at 76.773 MHz. 1D ROESY experiments40 were performed with continuous wave spinlock for mixing using selective refocussing with a shaped pulse.41-44 Data were collected with 32 acquisitions, 2 s recycle delay, 12 μs pulse width (90°), and 3.17 seconds acquisition time (64K point in the time domain). All spectra were recorded with 200ms ROESY spinlock and 80ms 180° shaped pulse to provide the desired selective excitation bandwidth.


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Atomistic MD Simulations: Atomistic simulations were performed using GROMACS. AMBER Force Field (GAFF) was employed.

46

45

The General

The concentrations of all components are obtained from

the corresponding experiments. Initially, all the solute molecules are randomly distributed in the cubic box of edge length of 7 nm, embedded in the cyclohexane box of around 12 nm in each dimension. After the energy minimization via the steepest descent algorithm, the annealing simulation37, 47-49 was performed for up to 100 ns, followed by the production simulation of up to 280 ns. In the annealing and production simulations, the isothermal-isobaric ensemble (NTP, constant number of particles, temperature and pressure) were employed. See SI for detailed information about the systems investigated and the simulation methodology.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Atomistic simulation details; control experiment and simulation in the system without extractant and ion; supporting figures and a rotation movie.

AUTHOR INFORMATION Corresponding Authors *Email: (R.J.E.) [email protected]; (B.Q.) [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work and the use of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility at Argonne National Laboratory, are supported by the U.S. DOE, Office of Science, Office of Basic Energy Science, Division of Chemical Sciences, Biosciences and Geosciences, under contract No DE-AC02-06CH11357. We gratefully acknowledge the computing resources provided on Blues, a high-performance computing cluster operated by the Laboratory Computing Resource Center at


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Argonne National Laboratory. M.O.d.l.C acknowledges the support from U.S. Department of Energy Award DE-FG02-08ER46539.

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