When Salt-Rejecting Polymers Meet Protons: An Electrochemical

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When salt-rejecting polymers meet protons: an electrochemical impedance spectroscopy investigation Noga Fridman-Bishop, and Viatcheslav Freger Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04263 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017

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When salt-rejecting polymers meet protons: an electrochemical impedance spectroscopy investigation Noga Fridman-Bishop and Viatcheslav Freger* Wolfson Department of Chemical Engineering, Technion – Israel Institute of Technology, Haifa, 32000, Israel; *corresponding authors, E-mails: [email protected]

Abstract Polymeric membranes are widely used for salt removal, but mechanism of ion permeation is still insufficiently understood. Here we analyze ion transport and scaling in polymers relevant to desalination (dense aromatic polyamide Nomex and cellulose acetate, CA) using impedance spectroscopy (EIS), focusing on the effects of the salt type, concentration and pH. The results highlight the role of proton uptake in ion permeation. For Nomex the exceptionally high affinity to proton results in a power-low dependence of conductivity on salt concentrations with an unusual exponent 1/2. The results for CA suggest dominance of pore transport, with pore charge increasing with decreasing pH, which contradicts previous view of CA as a weakly acidic polymer and points to proton uptake as possible pore-charging mechanism. The observed effects may have farreaching consequences in desalination, as even at neutral pH they may both enhance and suppress salt permeation and affect pH changes. Keywords:

Electrochemical

impedance

spectroscopy,

desalination,

polymeric

membranes, ion affinity, ion transport, power-law scaling, proton adsorption, salt rejection

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Introduction Polymeric membranes are widely used for removal and separation of salts and small organic molecules, e.g., using nanofiltration (NF) and reverse osmosis (RO).1 It is, however, recognized that current membranes still have potential for improvement, stimulating search for novel ultra-permeable nanomaterials and biomimetics.2,3 One factor that impedes such development is, physics and thermodynamics of ion and salt permeation in such membranes is still insufficiently understood. At present, various mean-field type models combining steric, dielectric and Donnan exclusion in an idealized nanoporous or pseudo-homogeneous membrane, are common.4,5 Unfortunately, they often fail to predict and even correctly describe the observed trends, e.g,, effects of


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solution composition, pH, and ion type and charge on salt permeation.6–12 Model predictions also contradict some independently obtained data, e.g., surprisingly large salt uptake by RO membranes.13–15 Ion-specific or ion affinity effects may be an important missing ingredient unsatisfactorily addressed by the current models. A simple way to account for ion affinity is through the Born self-energy of ion solvation,16 however, actual difference between ions and membranes may be more significant. Ion-ion interactions, e.g., ion sorption or association, may also play a larger role than mean-field treatments assume.17– 20

The present paper seeks insights into such effects using electrochemical impedance

spectroscopy (EIS) applied to planar polymer films on a solid electrode immersed in different electrolyte solutions. The ionic conductivity of the film Gm, deducible from EIS spectra, shows dependence on salt type, concentration, and pH, which are highly indicative of the ion exclusion and permeation mechanism. Curiously, only very few materials have been found selective and permeable enough to make successful RO membranes, most notably, aromatic polyamides (PA) and cellulose acetate (CA).17 It was suggested that low dielectric properties combined with presence of multiple hydrogen-bonding sites could be key to achieving both high salt rejection and reasonable water permeability,21,22 yet many questions remain. The present study focuses on these polymers to reveal features that make them unique. Note that, while CA is soluble and can be cast in a thin film form, cross-linked PA in actual desalination membranes is insoluble and contains fixed charges and nanovoids,23–26 making interpretation of EIS challenging. We therefore chose to use instead a soluble, linear and uncharged chemical analogue of cross-linked PA, poly (m-phenylene isophtaloyl amide),


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commercially available under the trade name Nomex, to avoid complications related to heterogeneity and fixed charges of genuine polyamide membranes. Ultimately, it turns out that Gm of Nomex and CA films reveal previously unobserved features that shed light on ion-specific effects in this polymers may strongly and no-trivially affect salt rejection.

Experimental Experimental setup Polymer films for present EIS experiments were directly drop-cast from appropriate solutions on a PEEK-shrouded glassy carbon rotating-disc electrode of active area 7.07 mm2 (Pine Instruments) used as a working electrode immersed in an appropriate single salt solution. A Pt plate was used as a counter electrode, and Ag/AgCl/3 M KCl electrode with a porous Vicor glass plug as a reference. The film thickness was adjusted to ensure Gm was deducible from EIS spectra. The thickness for the data reported below was ~70 nm thick for Nomex and ~4 µm for CA based on amount per coated area, but effective thickness could be smaller, as polymer precipitation upon drying could leave some porosity. Measured EIS spectra for different solutions were fitted to an equivalent circuit described previously27,28 to deduce Gm (see Supporting Information for details and examples). Film preparation and thickness evaluation Nomex stock solution was prepared by dissolving 5 g of commercial Nomex wool and 1 g of LiCl in 50 mL of DMF at 90ºC for 3 h. Thereafter the solution was diluted 100 times and 10 µL of the dilute solution was drop-cast on the glassy carbon tip of the working


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electrode. The electrode was placed under a laboratory beaker turned upside down to slow down DMF evaporation. The Nomex-coated electrode was then dried in a vacuum oven at 25ºC and 0.2 bar for 1 h. Before and between EIS measurements the coated electrode was kept in a salt solution in order to prevent changes in membrane resistance. Nomex density ( ρ NOMEX ) was evaluated by immersing a wad of the Nomex wool of known mass ( mNOMEX ~ 3 g) in a volumetric flask filled with petroleum ether (PE, 100120 C boiling point) and measuring the displaced volume of PE (VPE). Thus estimated Nomex density ρ NOMEX = mNOMEX VPE was 1.23 g/mL. Based on this value, the volume of solution used for drop-casting the film, and the coated area, Nomex film thickness was estimated to be about 70 nm, assuming no porosity. Film thickness was confirmed by independent measurements using atomic force microscopy (AFM). Importantly, AFM revealed that, while some films appears completely dense, some showed a porous morphology with a much thinner fragments present (see Figs S2 and S3 in SI). This could yield a significantly smaller resistance, i.e., smaller effective thickness, consistent with the results in Fig. 1a below. CA solution was prepared by dissolving overnight 0.6 g of a CA ultrafiltration membrane (YMGESP3001, Sterlitech), containing 0.45 g CA and 0.15 g non-woven polyester backing, in 20 ml acetone. After CA dissolution, non-soluble polyester backing was removed, yielding a 22.5 g/L CA solution. 70 µl of the CA solution was then placed on an RDE electrode tip with a pipette and the tip was let dry covered with a beaker turned upside down for 20 minutes. Thereafter the tip was annealed overnight under a beaker next to an open vial containing a water-acetone solution to slow down drying. Since acetone was the solvent, acetone vapor-rich atmosphere should have slowed down drying


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and enable polymer relaxation. Finally, the electrode was dried in open air for one hour, conditioned in a salt solution for an hour and dried in air for 1.5 h. First drying in air was then supposed to remove all acetone from the film and fix its structure before immersing it in aqueous solution. Second drying was performed in order to condition, further relax the film and ensure its firm adherence to the electrode. Between EIS measurements the coated electrode was kept in a salt solution in order to prevent changes in membrane resistance due to relaxation The film thickness was evaluated in a way similar to Nomex to be 3.6 µm, assuming CA density 1.3 g/cm3 and zero porosity. Direct thickness measurements for similarly prepared CA films using a micrometer (5.3±1.0 μm) and optical microscopy (6.6±1.4 µm) showed somewhat higher values. This suggests that the film apparently contained some porosity (see Section 3.4.).

Results and Discussion Conductivity of Nomex films in different electrolyte solutions The film’s ionic conductivity of a planar polymer film on a solid electrode is related to individual ion permeabilities as follows27

F2A Gm = ∑ Ciωi RT i

(1)

where Gm is the membrane conductance, RT the thermal energy, F the Faraday constant, A the membrane area, Ci and ωi are the solution concentration and permeability of ion i, respectively, summation being over all mobile ions. For a homogeneous film ion permeabilities may be expressed as


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ωi =

Di K i

δ

,

(2)

where K i = Ci Ci and Di are the ion partitioning coefficient and diffusivity in the film, Ci ion oncentration in the film, and δ is the membrane thickness. Figure 1 displays the Gm results for Nomex. Several surprising features are immediately notable. All data follow the generic power-law scaling Gm ∝ Csα ,

(3)

where Cs is the salt concentration, with an unusual exponent α ~ 0.5 (except for Fig. 1d). Figure 1a presents Gm of several similarly prepared Nomex films in CaCl2 solution with added 0.2 mM HCl (pH 3.7). Notably, the values of Gm in Figure 1a vary for the different films, which is explained by variations of the effective thickness due to some porosity and presence of thinner fragments (see Figure S3 in SI). Thus, the most conductive films may have an effective thickness less than superficial thickness 70 nm, yet all films show the same scaling with concentration, suggesting that differences were indeed due to effective thickness of otherwise identical polymer. Figure 1b compares Gm of the same film for four different chloride salts, NaCl, KCl, CaCl2 and MgCl2 at pH 3.7. The plots are nearly identical with a similar exponent α = 0.51 ± 0.06 for all salts tested in the entire Cs range except for the highest concentration 1 M.


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Figure 1. NOMEX membrane measured conductance as function of Cl- concentration for: (a) three separate films in CaCl2 solutions with 0.2 mM HCl added; (b) four different salt solutions with 0.2 mM HCl added for the same membrane sample (c) two different HCl acid concentrations in one film sample with KCl and MgCl2 solutions (d) two different films in HCl solutions.

Figure 1c further examines the effect of pH, i.e., added HCl, for MgCl2 and KCl. In both cases, a lower conductance was obtained at higher pH. Again, MgCl2 displays a


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Gm ∝ Cs0.5 dependence at both pH, but for KCl without added HCl the exponent increases

and at 0.5-1 M KCl the effect of added HCl on Gm vanishes. Results for MgCl2 and KCl are essentially identical to those for CaCl2 and NaCl, respectively (see Supporting Information). Finally, Fig. 1d shows Gm dependence for HCl as a sole electrolyte, which, unlike metal chlorides, displays a “regular” linear behavior with α ≈ 1.

Interpretation of Nomex data If the polymer contained weakly acidic charges, the added acid could affect the fixed charge density (X) via protonation of acidic charges, however protons were not expected to contribute directly to conductivity, since proton concentration in solution in Figs 1a to c was negligible compared to salt. Assuming the polymer strongly excludes salt, ions in films should be dilute and their activity coefficients vary weakly with Cs.7,29 In such case the classical Donnan model with affinity corrections predicts (taking 1:1 salt for simplicity) 0.5 0.5  X  X 2   X  X 2  2 2 Gm 2 = D+ C+ + D−C− = D+  +  + k+ k−Cs   + D− − +  + k + k − Cs   F A 2  4 2  4      

δ RT

,

(4)

where D± and k ± are respective ion diffusivities in the polymer and ion “affinities”, defined as the ratio of ion activity coefficients in solution and in the polymer. Equation 4 predicts two distinct limiting regimes depending upon the relation between X and crossover concentration Cs* = X ( 4k + k− )

−0.5

. The “neutral membrane” regime is obtained if

X > C s* , the polymer will exclude salt and mainly contain counter-ions, yielding a

conductivity

Gm

δ RT F2A

≈ D+ X ,

(6)

which is eq. 3 with α ≈ 0. Overall, the expected scaling should follow eq. 3 with either α ≈ 0 or α ≈ 1 with a relatively sharp transition between these regimes around Cs* . The results in Fig. 1 with exponent α ≈ 1/2 show no resemblance of this classical picture, except for HCl (Fig. 1d). These results collectively point to an extremely high affinity of neutral Nomex to protons, thereby it uptakes more H+ than metal cations, despite the much lower concentration. This occurs, when

C H + k H + >> Cs k + for M+ cations or

13 for M2+ cations. Since Nomex is nominally neutral, protons still C H + k H + >> 2Cs k22/3 + k−

require Cl- uptake to keep electroneutrality, i.e., CH + ≈ CCl − >> CM , therefore eq. 5 is replaced with

Gm

δ RT 2

F A

(

≈ DH + C H + + DCl − CCl − ≈ DH + + DCl −

)( k

H

+

C H + kCl − CCl −

)

0.5

∝ C H0.5+ Cs0.5 ,

(7)

in agreement with observed Cs dependence. Equation 7 also agrees with the observation that the cation type has little effect in most of the Cs range. Figure 1c also shows that added HCl, i.e., decrease of pH = − log C H + by 2 units, increases Gm by an order of magnitude, as eq. 7 predicts. On the other hand, with HCl as a single electrolyte, eq. 5


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turns to Gm ∝ CHCl , as indeed observed in Fig. 1d. Finally, based on eq. 7 one may expect a strong effect of anion type on Gm, which agrees with the fact that for Na2SO4 solutions the conductance became extremely low and non-deducible from EIS. This is consistent with the double charge of SO42-, for which Born equation predicts k- much lower than for chloride. However, for monovalent cations without added HCl the slope seems to approach the “neutral” α ≈ 1 and pH effect disappears at the highest salt concentrations. Such transition should occur when Cs surpasses Cs** = CH + k H + kM + for monovalent (M+) 13 cations (cf. eqs 4 and 5) or Cs** = CH + k H + k M2/32+ kCl for divalent ones (M2+). Figure 1 −

suggests that for monovalent cations the transition concentration Cs** is about 1 M for CH + = 2×10-4 M (pH 3.7) and 10-2 M for pH 5.7 ( CH + ~ 10-6 M). For divalent cations the

transition is not observed, therefore Cs** > 1 M, even for pH 5.7. This yields

kH + kM +

~ 103

and

k

k

H+ 2/3 1 3 M 2+ Cl −

k

> 106 or

kH + k M 2+

> 107.5 ,

where the last relation assumes similar affinity for monovalent cations and anions, i.e., k H + kCl − ~ k H + k M + ~ 103 . These estimates clearly indicate that affinity to the protons

exceeds that of "regular” monovalent cations by three orders of magnitude and that for divalent cations by more than seven orders of magnitude. The nature of enhanced affinity of protons to Nomex is unclear and cannot be explained by a smaller Born solvation energy, inversely proportional to ion radius. Even if proton is viewed as H3O+, its radius barely exceeds the smallest cation used, Na+,30 and cannot explain 103 times


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larger affinity, equivalent to ~7 k BT solvation energy difference. An intriguing possibility is that, when dielectric permittivity is low enough, partial atomic charges spaced sufficiently apart may become independent ion-binding sites. For instance, electronegative atoms of overall neutral amide groups, abundant in polyamide, may be such binding sites, to which protons or hydronium ions may bind particularly strongly due to small size. Such mechanism, resulting in a strong uptake of Na+ and Cl- even by a neutral polymer, was revealed recently by molecular dynamics simulations as a potentially important salt uptake mechanism in polyamide.25

Potential effect of proton uptake on salt permeability It is expedient to highlight consequences of a large proton affinity for salt permeation and selectivity in desalination membranes. First, it results in very high proton permeability, exceeding salt permeability by orders of magnitude. Fast proton permeation makes the transmembrane pH difference nearly instantly respond to transmembrane potential gradient induced by salt rejection, dictated by the Nernst equation, which is consistent with our recent results for RO membranes.31 Perhaps more importantly, strong proton uptake at the expense of salt cations will actively change the inter-phase potential in a way that will reduce uptake of cations and increase those of anions by a factor (1 + β ) “charged” one, where β =

CH + k H + C+ k +

0.5

for a “neutral” membrane and (1+β) for a

, and correspondingly change ion and salt

permeabilities. This may both increase and decrease salt permeation, as well as lead to


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non-monotonic variations, depending upon which ion is less permeable. In the case of neutral membrane, the relation between parameter β and salt permeability is as follows (see Supporting Information for derivation).

ωs ( β ) ≈

ωs ( 0 )

ω+ ( 0 ) ω− ( 0 ) −0.5 0.5 (1 + β ) + (1 + β ) ω+ ( 0 ) + ω− ( 0 ) ω+ ( 0 ) + ω− ( 0 )

(8)

where ω− ( 0 ) and ω+ ( 0 ) are the individual permeabilities of anion and cation for β = 0, i.e., without proton uptake. The change of ωs ( β ) with β will then depend on the ratio of ω+ ( 0 ) and ω− ( 0 ) . For instance, if ω+ ( 0 ) = ω− ( 0 ) = ωs ( 0 ) (cf. eqs. S9), −0.5 0.5 ωs ( β ) = 2ωs ( 0 ) (1 + β ) + (1 + β )  < ωs ( 0 ) , i. e., proton uptake will reduce salt





permeability. In the limiting case of ω+ ( 0 ) >> ω− ( 0 ) , ωs ( β ) will increase relative to

ωs ( 0 ) by a factor (1 + β )

0.5

and in the opposite case it will decrease by the same factor.

RO membranes usually contain negative fixed charges, supposedly making anions less permeable than cations. In such case, protons will compete with salt cations and neutralize the fixed charge without the need to uptake cations, which will increase anion uptake and enhance salt permeation. Equation 8 will be slightly modified, namely, (1+β) factor will replace (1 + β )

0.5

(see Supporting Information), yielding for a negatively

charged membrane a higher salt permeability, as follows

ωs ( β ) ≈ ωs ( 0 )(1 + β ) .

(9)

For a positively charged membrane permeability will be correspondingly lower.


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Figure 2 plots the expected effect of proton uptake on the salt permeability, i.e.,

ωs ( β ) ωs ( 0 ) vs β for a negatively charged membrane and for a neutral membrane for different values of ω+ ( 0 ) ω− ( 0 ) . It is seen that the effect can significantly vary and show opposite and even non-monotonic trends for different cases.

∞

β Figure 2. The effect of β ≡

k H + CH + k + Cs

on salt permeability for a negatively charged membrane

and for a neutral membrane for ω+ ( 0 ) ω− ( 0 ) = 0, 0.2 ,1 ,5, and ∞.

The complicated dependence of salt permeability on β and ω+ ( 0 ) ω− ( 0 ) suggests that pH-dependence of salt rejection may help identify which ion in a salt is the slower one. Note, however, that both β and membrane fixed charge vary with pH15 thus salt permeability may show a complex dependence on pH. This may explain contradictory reports on pH dependence of salt permeability.12,32,33


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Figure 3. Cellulose Acetate membrane measured conductance as function of Clconcentration for: (a) three separate films with NaCl and 0.002 mM HCl addition; (b) two different HCl acid concentrations in one film sample with NaCl and CaCl2 solutions (c) 4 different chloride salts used in one membrane sample with 0.2 mM HCl addition (d) 3 different salts used in one membrane sample with 0.2 mM HCl addition. Equivalent concentration equals the molar concentration of chloride and half that of sulfate.


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Cellulose acetate films The non-trivial behavior of Nomex raises the question whether it is common for other salt-rejecting polymers, in particular, cellulose acetate. Figure 3a presents analogous plots for Gm of several individual CA films in NaCl solution with added 0.002 mM HCl. Different films display similar trends, which are different from Nomex’s and closely follow the behavior described by eq. 4, with a cross-over between “charged” (α ≈ 0) to “neutral” (α ≈ 1) regimes at Cs* ~ 0.01 M. Also for CA films, absolute values vary for the different films due to variations in effective thickness and partial porosity. Similar trends for LiCl and KCl without added acid in CA were reported by Asaka.34 Notably, the pH dependence disappears above Cs* , while in Nomex it sustains up to much higher concentrations (see Fig. 1c). Despite much larger thickness of CA, ~ 4-6 µm vs. ~70 nm for Nomex, CA films show a much lower conductance (cf. Figures 1a and 3a). Figures 3c and d further reveal that differences in conductance of chloride salts and Na2SO4 solutions are small and correlate with conductivity of respective salts in bulk solutions.35 This suggests that linear part above 0.01 M is likely related to transport in pores with nearly bulk-like conductivity. Cellulose acetate films are indeed prone to having some porosity, which is suggested by direct thickness measurements (See Experimental). This explains strong dependence of transport properties of CA on preparation conditions36 and fairly large thickness required here to obtain a detectable Gm for CA. The linear part, representing bulk pore conductivity due to invading free salt in the "neutral membrane" regime, is then fairly uninteresting.


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In contrast, the plateau below Cs* apparently comes from the pore charge in CA, yet its pH dependence is surprising. The plateau conductivity in Figure 3b drops by an order of magnitude when pH increases from 3.7 to 5.7. This observation contradicts the common view of CA as slightly charged by weakly acidic groups,37,38 as such charge, hence conductivity, would increase with pH (eq. 6). This suggests that proton uptake rather than dissociation of acidic charges may be the actual charge-forming mechanism in CA. (Presence of fixed weak basic charges is less likely, as no suitable chemical groups can be identified in the CA structure). However, if protons dominate over salt cations, proton adsorption within the pore would require co-uptake of chloride to keep electroneutrality, which should yield for a uniform pore potential a Nomex-like scaling with α = 1/2 that is not observed. A possible explanation is that, if the mobility of protons is much higher than for other ions, the protons may dominate pore conductance, while salt cations still dominate ion uptake. Indeed, if protons constitute a small fraction of cations within the pore, which corresponds to β

When Salt-Rejecting Polymers Meet Protons: An Electrochemical

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