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On the structure–properties relationship of the AMV anion exchange membrane
Journal of Membrane Science 340 (2009) 133–140
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On the structure–properties relationship of the AMV anion exchange membrane Xuan Tuan Le a,∗ , Thi Hao Bui b , Pascal Viel a , Thomas Berthelot c , Serge Palacin a a
Laboratory of Chemistry of Surfaces and Interfaces, CEA Saclay, IRAMIS /SPCSI, F-91191, Gif-sur-Yvette Cedex, France GEOL, Department of Geology, Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium c Laboratoire des Solides Irradiés UMR 7642 CEA/CNRS/Ecole Polytechnique, CEA-DSM/IRAMIS LSI, Ecole Polytechnique, F-91128 Palaiseau Cedex, France b
a r t i c l e
i n f o
Article history: Received 1 April 2009 Received in revised form 29 April 2009 Accepted 17 May 2009 Available online 23 May 2009 Keywords: Microheterogeneous model Conductivity Surface homogeneity Electrolyte sorption Partition equilibrium
a b s t r a c t Important parameters such as ion exchange capacity, conductivity, permselectivity, quantity of sorbed electrolyte and water uptake of the Selemion AMV anion exchange membrane conditioned in KCl and NaCl solutions were determined in order to assess the applicability of the two-phase model of structure microheterogeneity to this case. In general, a good consistency between the experimental results and the theoretical approach was obtained. Electrical conductivity measurements allowed evaluating the volume fractions of two distinct internal phases inside the membrane. Aside from the chronopotentiometric behaviour of the AMV membrane, SEM and XPS techniques contributed to provide a better description of the overall membrane homogeneity. Influence of co-ions in relation with water uptakes on the conductivity and permselectivity of the studied membrane has been shown. An existence of partition equilibrium between the electrolytes sorbed in the membrane and the external solution confirmed in this work should be introduced within the microheterogeneous model. © 2009 Elsevier B.V. All rights reserved.
1. Introduction AMV commercial anion exchange membranes have been used widely for many applications of electrodialysis such as: treatment of metallic electroplating effluents [1,2], seawater desalination [3], separation of different ions from aqueous solution [4,5], or treatment of uranium leach solutions [6]. . . Notwithstanding the importance of the mentioned applications, only little fundamental work was devoted to the study of the AMV anion exchange membrane physico-chemical properties. As well known, the membrane properties are determined by the inner membrane structure. Studying the relationship between the structure and the properties of the AMV membrane will therefore be important particularly for electrodialysis but also for many other applications of ion exchange membranes. It has been recognized that, although based on a chemically homogeneous polymer, any ion exchange membrane exhibits microheterogeneity [7–12]. There are several approaches allowing description of the transport properties of these systems, e.g., capillary models [8,9], and the microheterogeneous model [10,11]. In the former approach, the membrane is described as an ideal porous medium containing parallel and cylindrical pores of the same radius. By applying the capillary models, the average radius of pores in the AMV membrane equilibrated in pure water was found to be 2.4 nm [9], but it is actually difficult to
∗ Corresponding author. Tel.: +33 1 69 08 98 74; fax: +33 1 69 08 64 62. E-mail addresses: [email protected], xuan [email protected] (X.T. Le). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.05.025
describe quantitatively a real ion exchange membrane as an array of cylindrical pores of identical radius. In the latter approach, the membrane is described as a microheterogeneous multiphase system with at least two phases (a gel phase and an interstitial solution [10,11]) and the transport modeling is based on a combination of the properties of each phase. During the last ten years, that two-phase mathematical model was mostly used for explaining transfer processes in electromembranes. Indeed, that model proved efficient for several types of electromembranes such as Nafion 117 [12] which exhibits the “cluster-type” characteristic structure, or organic–inorganic hybrid cation exchange membranes [13]. It is thus interesting to test the validity of the two-phase model with respect to the AMV anion exchange membrane by means of conductivity measurements in order to establish the correlation between the structural parameters and the overall membrane characteristics, in particular, the transport properties. The electrotransport of potassium chloride through the AMV anion exchange membrane (chronopotentiometric and current–voltage characteristics) was then investigated. Discussion of the role of the surface properties in the electrochemical behaviour of the membrane is based on the chronopotentiometry results in combination with the membrane surface morphology as probed by SEM and XPS. Finally, besides determining the main parameters like ion exchange capacity and transport number of counter-ion, we will also measure the water uptake and the amount of electrolyte sorption as a function of the concentration in the equilibrium external solution with the aim of providing a better knowledge of the “structure–properties” relationships.
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samples at 50 ◦ C for 72 h. The water content was defined as follows:
Table 1 Main properties of AMV anion exchange membrane. Name Type Structure propertya Fixed ionic groupa Thickness (cm)a Backboneb Ion exchange capacity (mmol equiv./g)b Membrane swelling in pure water (%)b Permselectivityb Membrane density (g/cm3 )b a b
W=
Selemion AMV
a
Homogeneous PS/butadiene –NR3 + , strong basic 0.011–0.015 PVC 1.85 ± 0.04 18 ≥0.98 1.13
Ref. [14]. Determined in this work.
2. Experimental 2.1. Anion exchange membranes Some main characteristics of the commercial anion exchange membranes used in this work are presented in Table 1. 2.2. Membrane conductivity The membrane specific conductivity m is calculated from the membrane resistance Rm using the following relation: m =
d Rm A
(1)
where d and A are respectively the membrane thickness and area. The resistance of membranes equilibrated in alkaline chloride solutions of given concentrations were measured by electrochemical impedance spectroscopy. Alternating current was supplied to the two-compartment mercury electrode cell. The membrane was placed in a circular hole between the compartments. The electrical resistances of samples, previously equilibrated in given solution at 25 ◦ C, were obtained from the membrane impedance diagrams plotted in a range of frequencies extending from 0.01 Hz to 100 kHz. Measurements were performed with a SOLATRON 1255 frequency analyzer equipped with an electrochemical interface (SI 1286). Details concerning this technique are given in our previous works [15]. 2.3. Solution conductivity The solution conductivity was determined by using EC 215 Conductivity Meter (Hanna Instruments) at 25 ◦ C. 2.4. Ion exchange capacity The membranes were immersed in 0.5 M KCl solution during 24 h. Then, they were rinsed with demineralized water to remove the non-exchange KCl electrolyte sorbed in the membranes and immersed in 50 ml 0.5 M NaNO3 under stirring for 12 h in order to obtain the complete exchange of Cl− ions in the membrane for NO3 − ions from the solution. The amount of Cl− ions obtained in the solution was determined by using Chloride Analyzer. Ion exchange capacity Cex of the studied membranes was calculated from the chloride amount and the weight of the dry membrane. 2.5. Water content The membranes were left to soak during 24 h at 25 ◦ C in MCl solutions at different concentrations (with M being Na+ or K+ ). Wet weight Wa was measured after removing all the solution on the membrane surface. Dry weight Wb was obtained by drying the
Wa − Wb Wb
(g H2 O/g dry membrane)
(2)
2.6. Current–voltage characteristic Current–voltage curves were obtained using a twocompartment cell. This cell was composed of two compartments of equal volume (40 cm3 ) separated by the anion exchange membrane. The apparent area of the membrane was 1.0 cm2 . The potential difference across the membrane was measured using two Ag/AgCl wire electrodes immersed into Luggin capillaries which were placed close to the membrane surface. The electrical current was supplied at a current scanning rate of 1 A/s by a potentiostat/galvanostat (AutoLab, Model PGSTAT 30) connected to Ag/AgCl electrode plates. 2.7. Chronopotentiometry The chronopotentiometry measurements were performed with the same two-compartment cell as used for current–voltage measurement. Chronopotentiometry was carried out with a SOLARTRON 1286 potentiostat/galvanostat controlled by a computer with the Corrware software. The temperature during the experiment was maintained at 25 ◦ C. All measurements were realized without solution agitation. 2.8. Spectroscopy studies The scanning electron microscopy (SEM) images were recorded by a Hitachi S4500 equipped with a Field Emission Gun (FEG-SEM). XPS studies were performed with a KRATOS Axis Ultra DLD spectrometer, using the monochromatized Al K␣ line at 1486.6 eV. Fixed analyzer pass energy of 20 eV was used for C 1s core level scans. 2.9. Apparent transport number of counter-ion Cl− in the membrane The transport number of counter-ion Cl− in the membranes was determined by the concentration cell method based on the diffusion potential measurements using Ag/AgCl reference electrodes in contact with dilute solutions (0.001 and 0.01 mol/kg) [16]: m =
RT F
(1 − 2t¯ )ln
a 2
a1
(3)
where m is the cell potential, a1 and a2 are the molal activities of MCl solutions (with M being Na+ , K+ ) at the two compartments, respectively. 2.10. Amount of sorbed electrolyte The amounts of non-exchange MCl electrolyte (with M being Na+ or K+ ) sorbed in the AMV membranes in contact with MCl solutions at different concentrations were determined by the manystage desorption method. First, the membranes were conditioned in MCl solution. After eliminating all the surface solution by simply wiping up the membrane with a clean fine paper, they were immersed into sufficiently large amounts of pure water at 25 ◦ C during 24 h for desorption of the MCl. This procedure was repeated three times in order to assure the complete desorption. Atomic absorption spectroscopy was used for quantitative analysis of the desorbed MCl.
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3. Results and discussion 3.1. Microheterogeneity of the membrane In the microheterogeneous model [10,12], two phases are distinguished: the gel phase and the interstitial neutral solution, with volume fractions f1 and f2 , respectively (the sum f1 + f2 being equal to 1). The gel phase is a neutral nanoporous medium, which contains fixed and mobile ions, polymer matrix, and water. The interstitial neutral solution, the properties of which are assumed to be the same as those of the outer equilibrium solution, fills the inner parts of meso- and macro-pores as well as fissures and cavities (=intergel spaces). Thus, both cations and anions contribute here to the conductivity. On the contrary, in the gel phase, almost exclusively the mobile counter-ions assure the conductivity. The co-ions are present there in a very low amount, increasing with bulk concentration, but negligible in the case of diluted water solutions (up to 1 M) and relatively high ion exchange capacity membrane (as in the case of the AMV membrane used in this work). Besides, with increasing solution concentration, the water uptake decreases that can result in a decrease of the counter-ion mobility. For these reasons, the specific conductivity of the gel phase varies only slightly with the external solution concentration; hence, it can be considered as constant. In practice, in the range of concentrations 0.1 ciso < c < 10 ciso , the variation in the membrane conductivity m with the external solution concentration may be approximately described by the function: log m = f1 log g + f2 log s
(4)
with g and s stand for the conductivities of respectively the gel phase and the external solution. Where ciso is the isoconductance electrolyte concentration at which the conductivities of all membrane phases and the membrane as a whole are the same: iso = g = s
(5)
As mentioned above, it is clear that the volume fraction of all phases as well as the conductivity at the isoconductance point of an ion exchange membrane may be varied with the nature of counter-ions. A good agreement with respect to the case of anion exchange membranes was observed in the literature [17,18]. However, rather little works focused on the role of the co-ions on the structural parameters of anion exchange membrane. We wish next to demonstrate as clearly as possible the influence of co-ions on the microheterogeneity of the AMV membrane. Fig. 1 presents the experiment values of the conductivities of the AMV membrane and the external solutions versus the concentration. It is observed that the membrane conductivity increases with the external electrolyte solution concentration (Fig. 1a). Increase in the conductivity of the electroneutral solution in the interstitial phase can be given here for explanation as the main reason. Otherwise, as the conductivity of KCl is quite higher than that of NaCl solution at the same concentration (Fig. 1b), the membrane conductivity in contact with KCl is therefore higher than in the NaCl solution. Eq. (4) shows that log m is linearly related to log s . Effectively, our results confirm well the theoretical predictions based on Eq. (4) (Fig. 2). From the slopes of the lines depicted in Fig. 2, one can get the volume fraction of the interstitial phase of the AMV membrane. As expected, we obtain two quasi-equal values of f2 , 0.063 and 0.064 in contact with NaCl and KCl, respectively. This, in other words, suggests that co-ions (Na+ and K+ in this case) play a minor role towards the membrane structure. Remark that the volume fraction of interstitial phase of the AMV membrane obtained in this work is reasonable in comparison with the proportion of water accommodated in the interstitial phase f . By assuming that the volume of macro- and meso-pores is totally filled with water of density w = 1,
Fig. 1. Conductivities of the AMV membrane (a) and of the equilibrium solutions (b) as a function of solution concentration.
Fig. 2. Plots of membrane conductivity m vs. solution conductivity s (in log/log scale).
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this volume fraction of water can be estimated approximately from the membrane swelling at equilibrium with pure water and the density of hydrated membrane as follows: f =
w f2 (1 + W ) m W
(6)
It amounts approximately to 40%. By taking into account the water content determined for 1 g of dry membrane, one can deduce a water amount of about 0.07 (g/g dry membrane) for the interstitial phase. Besides, our values of f2 are acceptable for the results obtained with the homogeneous membranes [19]. On the basis of the two-phase model, Zabolotsky and Nikonenko [19] suggested that the homogeneous ion exchange membranes are characterized by low volume fraction of interstitial solution, usually found to be ≤10%. Such result was obtained at equilibrium with NaCl solutions. In order to evaluate the microstructural parameters of the AMV membrane, it is needed to know the concentration of fixed ionic groups in the gel phase c¯ cx . Actually, this value can be estimated from the concentration of fixed ionic groups in the overall membrane c¯ ex and the volume fraction of gel phase f1 through the relation: c¯ cx =
c¯ ex Cex m = f1 (1 + W )f1
(mmol equiv./cm3 gel phase)
(7)
where Cex is the exchange capacity (mmol equiv./g dry membrane) and m is the density of hydrated membrane (g/cm3 ). The diffusion coefficient of counter-ion Cl− in the gel phase is then calculated from the concentration of fixed ionic groups in the gel phase c¯ cx and the conductivity iso by the following relation: D¯ Cl− =
RT iso F 2 c¯ cx
(8)
Table 2 presents the main characteristics of internal microstructure of the AMV membrane in NaCl and KCl solutions. As can be seen here, alkaline co-ions affect very slightly the value of iso thus the diffusion coefficient of counter-ion Cl− despite the difference in the isoconductance concentrations found for the NaCl and KCl solutions. On the other hand, the calculated values of diffusion coefficient of counter-ion Cl− in the gel phase, as expected, is found close to the diffusion coefficient of chloride ion within the AMV membrane published in the literature [20]. According to the two-phase model that has been adopted in this work, the AMV anion exchange membrane is characterized by a small degree of internal microheterogeneity, accepted as criteria in the literature for defining homogeneous ion exchange membranes. It is worth noting here that the two-phase model has further been developed into a three-phase model which then divides the “gel phase” into an inert phase (hydrophobic polymer) and a polyelectrolyte pure gel phase (fixed group zone) [19,21]. The inert phase in the case of the AMV membrane is poly(vinyl chloride) used as the membrane backbone through the classical paste preparation method [22]. It should also be remarked that the quantitative description of an ion exchange membrane as a combination of the Table 2 Main characteristics of internal microstructure of the AMV membrane in NaCl and KCl solutions.
Volume fraction of gel phase, f1 Volume fraction of interstitial phase, f2 Concentration of fixed ionic groups in the gel phase, c¯ cx (mmol equiv./cm3 gel phase) Isoconductance concentration, ciso (M) Conductivity at isoconductance point, iso (mS/cm) Diffusion coefficient of counter-ion, D¯ Cl− (m2 /s)
NaCl
KCl
0.937 0.063 1.89
0.936 0.064 1.89
0.046 4.0
0.031 3.8
5.6 × 10−11
5.3 × 10−11
electrically neutral solution and the gel phase containing fixed and counter-ions with hydration water is well accepted for both threephase and two-phase models [23]. However, the electrochemical behaviour of the ion exchange membranes, even the homogeneous ones, may be influenced by the presence of some non-conducting PVC regions on the membrane surface. Taking into account those facts requires testing the overall membrane homogeneity. It has been proved in the literature that electrochemical techniques like potentioamperometry or chronopotentiometry, applied to membrane systems, encompass many aspects related specifically to the membrane surface heterogeneity [15,16,24]. Further work aimed, therefore, at checking the applicability of chronopotentiometry with the object of getting some knowledge concerning the membrane homogeneity. 3.2. Overall surface homogeneity The SEM images of the surfaces as well as the cross-sections of the AMV membrane at different scales are shown in Fig. 3. As expected, the images shown here are very similar to those of the AMV membrane and the other homogeneous membranes published in the literature [16,25]. In particular, the AMV membrane surface looks mainly homogeneous but, as for ASV membranes [16], some micron-sized inert regions attributed to the poly(vinyl chloride) backbone are observed (Fig. 3a and b). The membrane surface was further characterized by XPS measurements. Fig. 4 presents the decomposed neutral Cl 2p core level spectra of the AMV anion exchange membrane. As can be seen in Fig. 4, the 2p3/2 and 2p1/2 peaks centred at 200.6 and 202.2 eV with the expected 2:1 ratio of the intensities confirmed very well the presence of PVC [26,27]. Furthermore, we also observed the characteristic doublet of the chloride counter-ions with the 2p3/2 and 2p1/2 peaks centred at 197.4 and 199.1 eV, respectively. When an electric current is applied to a membrane in an electrolyte solution, concentration polarization phenomena arise, i.e. concentration gradients are developed in the vicinity of the membrane. The process occurring near the membrane until a steady state is reached can be monitored by measuring the potential drop across the membrane as a function of time. When the electrolyte concentration approaches zero at the membrane surface, the potential varies very abruptly and the corresponding characteristic time is called the transition time . This time depends on the concentration, the diffusion coefficient of the electrolyte and the ion transport numbers in the membrane and external solution. The relationship existing between and these experimental parameters are classically provided by the Sand equation by taking into consideration a linear diffusion of the electrolytes. Detail concerning the application of the Sand equation in the membrane system has been reviewed in our recently published work [16]. The transition time as a function of applied current density i for an electrolyte solution A1 B2 → 1 Az1 + 2 Bz2 and a homogeneous membrane can be written as follows: i 1/2 =
c0 |z1 |1 F (Ds )1/2 2 t¯1 − t1
(9)
where Ds is the diffusion coefficient of the electrolyte, F the Faraday constant, c0 the electrolyte concentration; and t¯ , t are the transport numbers of the counter-ion in the membrane and in the solution, respectively. Fig. 5 presents the chronopotentiometric characteristics of the AMV membrane in contact with 0.1 M KCl solution at i = 21 mA/cm2 . The chronopotentiometric curve shown in this figure exhibits the typical shape recorded for homogeneous membranes as reported in the literature [16]. In order to apply the Sand equation to our system, we determined the transition time from the chronopotentiometric curve derivative. The transition time corresponds to the
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Fig. 3. SEM images of the surface as well as the cross-section of the AMV membrane at different scales.
Table 3 Values of i 1/2 at different current densities.
maximum point of the derivative curve. Sand equations indicates that for a given concentration, the diffusion coefficient of electrolyte and the transport number of counter-ion in solution fixed, the value i 1/2 is constant and independent of the current density. The values i 1/2 presented in Table 3 show such invariance. Further data treatment allows us to estimate the effective transport number of counter-ion in the AMV membrane if the diffusion coefficient of the electrolyte and the transport numbers of counter-ion in the bulk solution are known. With the values of 0.509 for the transport number of chloride ion and of 1.84 10−5 cm2 /s for the diffusion
coefficient in the bulk solution of KCl 0.1 M [28,29], the effective transport numbers of Cl− in the AMV membrane soaked in 0.1 M KCl solution was determined to be 0.99 ± 0.01. This figure is in a good agreement with the results published by Larchet et al. [30]. It was
Fig. 4. Decomposed Cl 2p core level spectra of the AMV anion exchange membrane.
Fig. 5. Plot of a chronopotentiometric curve of the AMV membrane and its derivative in 0.1 M KCl solution at i = 21 mA/cm2 .
i (mA/cm2 )
19.0
19.5
20.0
20.5
21.0
i 1/2
77.3
76.4
76.5
76.2
76.0
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reported that the counter-ion transport numbers in high charged ion exchange membranes, like the AMV membrane studied in this work, immerged in dilute solutions (0.1 M) are very close to 1, at least >0.98. Here, an important point to be underlined is that Eq. (9) considers a linear diffusion (1D diffusion) of the electrolyte through the adjacent layer to the membrane and current lines normal to the surface. One can deduce that the overall surface of the AMV anion exchange membrane behaves as a conducting plane with respect to the transport of KCl. Such observation seems in contradiction with the results obtained with SEM and XPS techniques. This, however, can be explained by taking into account the thickness of the diffusion layer. To determine the thickness of the diffusion layer, we evaluated the current–voltage characteristic of the AMV anion exchange membrane in 0.1 M KCl solution (Fig. 6). Three characteristic regions were observed as expected: (I) a first region of approximately ohmic behaviour transforms as voltage is increased into a second region (II), in which the current varies very little with the voltage leading to a “plateau” corresponding to the limiting current, followed by a third region (III) of marked current increase (over-limiting region). In this work, the limiting current density (15.4 mA/cm2 ) was determined from the intersection point between the two first regions as shown in Fig. 6. From the diffusion coefficient of the electrolyte, transport numbers of counter-ion and limiting current density Ilim , the thickness of the diffusion layer ı has been evaluated on the basis of Eq. (10): ı=
1 |z1 |FDs c0 (t¯1 − t1 )Ilim
(10)
and amounts to 240 m. This value is acceptable for the thickness of diffusion layer at the surface of an ion exchange membrane, from 50 to 500 m up to the different conditions. As the thickness of the diffusion layer is close to 240 m, the surface inhomogeneity of the order of several micrometers on the AMV membrane does not disturb the ion transfer, which can be described as 1D to a homogeneous surface. Sand equation is therefore validated in that case [16]. 3.3. Water content and sorption electrolyte in the membranes As it has already been mentioned in Section 3.1, the water uptake and co-ion quantity are extremely important parameters
Table 4 Water uptake of the AMV membrane as a function of the external solution concentrations. Water content (g/g dry membrane)
In NaCl In KCl
0.5
1.0
2.0
3.0
0.180 0.180
0.168 0.170
0.162 0.164
0.153 0.152
0.143 0.143
Table 5 Quantity of non-exchange electrolytes sorbed in the AMV membrane as a function of the external solution concentrations. ¯ exp (mmol/g dry membrane) N
NaCl KCl
Concentration (M) 0.0
0.5
1.0
2.0
3.0
0 0
0.0224 0.0254
0.0508 0.0582
0.1065 0.1025
0.1724 0.1745
because they directly affect the ion transport, thus the conductivity and permselectivity of the membrane. Determination of the nonexchange electrolyte amount as well as the water uptakes in the AMV membrane is therefore essential in this work. Table 4 presents the water content of the AMV membrane in contact with KCl and NaCl solutions at different concentrations. It is shown indeed that the AMV membrane swells more when the solution concentration is low. This phenomenon can be explained by the reduction of the osmotic pressure difference between the interiors of the membrane with increasing the solution concentration, thus the driving force for water uptake is smaller [31]. Table 5 presents the amount ¯ exp (mmol) of sorbed KCl and NaCl per gram of dry membrane N as a function of the external solution concentrations. Once again, the expected small quantities of co-ions presented in this table are consistent with the interpretation given above. For example, in contact with 0.5 M KCl solution, we found only 0.025 mmol of co-ion K+ which is rather minor compared to the value of ion exchange capacity of the AMV membrane, 1.85 (mmol equiv./g). The decrease in water uptake and the very low amount of co-ions assured the invariance in the conductivity of the gel phase, thus led to the agreement of the experimental conductivity results with the theoretical approach predicted in Eq. (4). ¯ exp in the overThe experimental value of sorbed MCl molality m all membrane is given by: ¯ exp = m
Fig. 6. Current–voltage curve of the AMV membrane in contact with 0.1 M KCl solution.
Concentration (M) 0.0
¯ exp N W
(mol/kg H2 O)
(11)
with W is the water content (g/g dry membrane). Fig. 7 shows that ¯ exp vary linearly as a function of the molality of exterthese values m nal solution ms with the straight-line slopes K¯ being equal to 0.38 ¯ exp /ms and 0.36 respectively for the NaCl and KCl solutions. K¯ = m can be defined as coefficient of partition equilibrium between the external solution and the overall membrane. Data concerning the molality of the external solutions were reproduced from Ref. [29]. Existence of an equilibrium partition of the electrolytes between the external solution and the membrane found in this work allows us to confirm that the conductivity of the interstitial phase increases quasi-linearly with the concentration of the external solution. Note that the slightly higher value of K¯ obtained with the case of NaCl may be due to the reduction of Donnan exclusion from the K+ to Na+ . The hydrated radius of Na+ is higher than that of K+ (0.184 and 0.125 nm, respectively). A similar finding has also been obtained with the membrane permselectivity. The apparent transport numbers of counter-ion Cl− determined by means of the concentration cell method were found to be 0.986 and 0.981 (error ≈0.4%), respectively for KCl and NaCl solutions. As discussed in our previous work [16], increase in electrostatic expulsion between the co-ions and the fixed ionic groups –NR3 + inside the membrane from Na+ to
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layer, and the deviations of the current lines from 1D uniform distribution can be neglected. Measurements of the sorption isotherms have proved an existence of partition equilibrium between the electrolytes sorbed in the membrane and the external solution, which should be taken into consideration for improved understanding of internal membrane structure. Acknowledgements The authors wish to thank Prof. C. Buess-Herman (CHANI-ULB) for providing the membrane samples. Mrs. Pascale Jegou (CEA Saclay) is acknowledged for XPS measurements.
Nomenclature Cex W d A Rm m m g s f1 f2 ciso iso w f c¯ ex c¯ cx T D¯ Cl− Fig. 7. Molality of sorbed electrolytes as a function of the molality of external solutions: (a) NaCl and (b) KCl.
K+ explains the increase of the Donnan exclusion, thus the stepup in permselectivity of the AMV membrane. However, it is easy to observe that the role of co-ions is very weak in the AMV anion exchange membrane. 4. Conclusion The physico-chemical properties of the AMV anion exchange membrane (electrical conductivity, electrolyte sorption quantity, water uptake, permselectivity) determined experimentally in this work fit the requirements for the validity of the microheterogeneous model. According to this model, the studied membrane can be well described as a two-phase system on microstructural scale: gel phase and interstitial phase with the volume fractions f1 and f2 , respectively. The typical behaviour of a homogeneous ion exchange membrane has been confirmed by means of a low volume fraction of the electroneutral solution and overall surface homogeneity. It is shown clearly that the experimentally observed surface inhomogeneity (of the order of several micrometers) does not disturb the ion transfer through the AMV membrane. Thus, when describing the ion transfer, the membrane surface can be considered as ideally homogeneous, hence, the 1D model is valid. The reason is that the size of inhomogeneities is much less than the thickness of diffusion
F Ds c0 i Ilim t¯ t ¯ exp N ¯ exp m ms K¯
ion exchange capacity (mmol equiv./g dry membrane) water content (g/g dry membrane) membrane thickness (cm) membrane area (cm2 ) membrane resistance () density of hydrated membrane (g/cm3 ) membrane conductivity (mS/cm) conductivity of gel phase (mS/cm) conductivity of interstitial solution (mS/cm) volume fraction of gel phase volume fraction of interstitial phase isoconductance concentration (M) conductivity at isoconductance point (mS/cm) water density (g/cm3 ) volume fraction of water found in the interstitial phase concentration of fixed ionic groups in the overall membrane (mmol/cm3 membrane) concentration of fixed ionic groups in the gel phase (mmol equiv./cm3 gel phase) temperature (K) diffusion coefficient of the counter-ion Cl− in the gel phase (m2 /s) transition time (s) Faraday constant diffusion coefficient of electrolyte in the bulk solution (cm2 /s) electrolyte concentration (M) current density (mA/cm2 ) limiting current density (mA/cm2 ) transport numbers of counter-ion in the membrane transport numbers of counter-ion in the solution mmol of sorbed MCl per gram of dry membrane experimental value of the sorbed MCl molality in the overall membrane (mol/kg H2 O) molality of external solution (mol/kg H2 O) coefficient of partition equilibrium between the external solution and the overall membrane
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[4] B. Van der Bruggen, A. Koninckx, C. Vandecasteele, Separation of monovalent and divalent ions from aqueous solution by electrodialysis and nanofiltration, Water Res. 38 (2004) 1347–1353. [5] M. Sadrzadeh, A. Razmi, T. Mohammadi, Separation of different ions from wastewater at various operating conditions using electrodialysis, Sep. Purif. Technol. 54 (2007) 147–156. [6] A. Lounis, C. Gavach, Treatment of uranium leach solution by electrodialysis for anion impurities removal, Hydrometallurgy 44 (1997) 83–96. [7] J.P.G. Villaluenga, V.M. Barragan, M.A. Izquierdo-Gil, M.P. Godino, B. Seoane, C. Ruiz-Bauza, Comparative study of liquid uptake and permeation characteristics of sulfonated cation-exchange membranes in water and methanol, J. Membr. Sci. 323 (2008) 421–427. [8] S. Koter, Transport of simple electrolyte solutions through ion-exchange membranes—the capillary model, J. Membr. Sci. 206 (2002) 201–215. [9] T. Malewitz, P.N. Pintauro, D. Rear, Multi component absorption of anions in commercial anion-exchange membranes, J. Membr. Sci. 301 (2007) 171–179. [10] N.P. Gnusin, N.P. Berezina, N.A. Kononenko, O.A. Dyomina, Transport structural parameters to characterize ion exchange membranes, J. Membr. Sci. 243 (2004) 301–310. [11] N.P. Berezina, N.A. Kononenko, O.A. Dyomina, N.P. Gnusin, Characterization of ion-exchange membrane materials: properties vs structure, Adv. Colloid Interface Sci. 139 (2008) 3–28. [12] L. Chaabane, G. Bulvestre, C. Larchet, V. Nikonenko, C. Deslouis, H. Takenouti, The influence of absorbed methanol on the swelling and conductivity properties of cation-exchange membranes, evaluation of nanostructure parameters, J. Membr. Sci. 323 (2008) 167–175. [13] X. Zuo, S. Yu, X. Xu, R. Bao, J. Xu, W. Qu, Preparation of organic–inorganic hybrid cation-exchange membranes via blending method and their electrochemical characterization, J. Membr. Sci. 328 (2009) 23–30. [14] T.W. Xu, Ion exchange membranes: state of their development and perspective, J. Membr. Sci. 263 (2005) 1–29. [15] L.X. Tuan, M. Verbanck, C. Buess-Herman, H.D. Hurwitz, Properties of CMV cation exchange membranes in sulfuric acid media, J. Membr. Sci. 284 (2006) 67–78. [16] X.T. Le, P. Viel, D.P. Tran, F. Grisotto, S. Palacin, Surface homogeneity of anion exchange membranes: a chronopotentiometric study in the overlimiting current range, J. Phys. Chem. B 113 (2009) 5829–5836.
[17] A. Elattar, A. Elmidaoui, N. Pismenskaia, C. Gavach, G. Pourcelly, Comparison of transport properties of monovalent anions through anion-exchange membranes, J. Membr. Sci. 143 (1998) 249–261. [18] H.J. Lee, M.K. Hong, S.D. Han, S.H. Moon, Influence of the heterogeneous structure on the electrochemical properties of anion exchange membranes, J. Membr. Sci. 320 (2008) 549–555. [19] V.I. Zabolotsky, V.V. Nikonenko, Effect of structural membrane inhomogeneity on transport properties, J. Membr. Sci. 79 (1993) 181–198. [20] R. Delimi, A. Benkada, J. Sandeaux, R. Sandeaux, G. Pourcelly, Propriétés de transport de membranes échangeuses d’anions en présence d’ions organiques. Partie I. Influence de la longueur de la chaîne aliphatique, C. R. Acad. Sci. Paris, Série IIc, Chimie: Chem. 3 (2000) 853–859. [21] A.B. Yaroslavtsev, V.V. Nikonenko, V.I. Zabolotsky, Ion transfer in ion-exchange and membrane materials, Russ. Chem. Rev. 72 (2003) 393–421. [22] Y. Mizutani, Structure of ion exchange membranes, J. Membr. Sci. 49 (1990) 121–144. [23] T.W. Xu, Y. Li, L. Wu, W.H. Yang, A simple evaluation of microstructure and transport parameters of ion-exchange membranes from conductivity measurements, Sep. Purif. Technol. 60 (2008) 73–80. [24] E. Volodina, N. Pismenskaia, V. Nikonenko, C. Larchet, G. Pourcelly, Ion transfer across ion-exchange membranes with homogeneous and heterogeneous surfaces, J. Colloid Interface Sci. 285 (2005) 247–258. [25] P. McCormick, J. Pellegrino, F. Mantovani, G. Sarti, Water, salt, and ethanol diffusion through membranes for water recovery by forward (direct) osmosis processes, J. Membr. Sci. 325 (2008) 467–478. [26] S. Tan, A. Laforgue, D. Belanger, Characterization of a cation-exchange/ polyaniline composite membrane, Langmuir 19 (2003) 744–751. [27] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers: The Scienta ESCA300, John Willey & Sons, 1992. [28] V.M.M. Lobo, J.L. Quaresma, Handbook of Electrolyte Solutions, Elsevier, 1989. [29] R.A. Robinson, R.H. Stokes, Electrolyte Solutions, 2nd edition, Butterworths Scientific Pulications, London, UK, 1959. [30] C. Larchet, L. Dammak, B. Auclair, S. Parchikov, V. Nikonenko, A simplified procedure for ion-exchange membrane characterisation, New J. Chem. 10 (2004) 1260–1267. [31] F. Helfferich, Ion Exchange, McGraw-Hill Book Company, New York, 1962.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
On the structure–properties relationship of the AMV anion exchange membrane Xuan Tuan Le a,∗ , Thi Hao Bui b , Pascal Viel a , Thomas Berthelot c , Serge Palacin a a
Laboratory of Chemistry of Surfaces and Interfaces, CEA Saclay, IRAMIS /SPCSI, F-91191, Gif-sur-Yvette Cedex, France GEOL, Department of Geology, Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium c Laboratoire des Solides Irradiés UMR 7642 CEA/CNRS/Ecole Polytechnique, CEA-DSM/IRAMIS LSI, Ecole Polytechnique, F-91128 Palaiseau Cedex, France b
a r t i c l e
i n f o
Article history: Received 1 April 2009 Received in revised form 29 April 2009 Accepted 17 May 2009 Available online 23 May 2009 Keywords: Microheterogeneous model Conductivity Surface homogeneity Electrolyte sorption Partition equilibrium
a b s t r a c t Important parameters such as ion exchange capacity, conductivity, permselectivity, quantity of sorbed electrolyte and water uptake of the Selemion AMV anion exchange membrane conditioned in KCl and NaCl solutions were determined in order to assess the applicability of the two-phase model of structure microheterogeneity to this case. In general, a good consistency between the experimental results and the theoretical approach was obtained. Electrical conductivity measurements allowed evaluating the volume fractions of two distinct internal phases inside the membrane. Aside from the chronopotentiometric behaviour of the AMV membrane, SEM and XPS techniques contributed to provide a better description of the overall membrane homogeneity. Influence of co-ions in relation with water uptakes on the conductivity and permselectivity of the studied membrane has been shown. An existence of partition equilibrium between the electrolytes sorbed in the membrane and the external solution confirmed in this work should be introduced within the microheterogeneous model. © 2009 Elsevier B.V. All rights reserved.
1. Introduction AMV commercial anion exchange membranes have been used widely for many applications of electrodialysis such as: treatment of metallic electroplating effluents [1,2], seawater desalination [3], separation of different ions from aqueous solution [4,5], or treatment of uranium leach solutions [6]. . . Notwithstanding the importance of the mentioned applications, only little fundamental work was devoted to the study of the AMV anion exchange membrane physico-chemical properties. As well known, the membrane properties are determined by the inner membrane structure. Studying the relationship between the structure and the properties of the AMV membrane will therefore be important particularly for electrodialysis but also for many other applications of ion exchange membranes. It has been recognized that, although based on a chemically homogeneous polymer, any ion exchange membrane exhibits microheterogeneity [7–12]. There are several approaches allowing description of the transport properties of these systems, e.g., capillary models [8,9], and the microheterogeneous model [10,11]. In the former approach, the membrane is described as an ideal porous medium containing parallel and cylindrical pores of the same radius. By applying the capillary models, the average radius of pores in the AMV membrane equilibrated in pure water was found to be 2.4 nm [9], but it is actually difficult to
∗ Corresponding author. Tel.: +33 1 69 08 98 74; fax: +33 1 69 08 64 62. E-mail addresses: [email protected], xuan [email protected] (X.T. Le). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.05.025
describe quantitatively a real ion exchange membrane as an array of cylindrical pores of identical radius. In the latter approach, the membrane is described as a microheterogeneous multiphase system with at least two phases (a gel phase and an interstitial solution [10,11]) and the transport modeling is based on a combination of the properties of each phase. During the last ten years, that two-phase mathematical model was mostly used for explaining transfer processes in electromembranes. Indeed, that model proved efficient for several types of electromembranes such as Nafion 117 [12] which exhibits the “cluster-type” characteristic structure, or organic–inorganic hybrid cation exchange membranes [13]. It is thus interesting to test the validity of the two-phase model with respect to the AMV anion exchange membrane by means of conductivity measurements in order to establish the correlation between the structural parameters and the overall membrane characteristics, in particular, the transport properties. The electrotransport of potassium chloride through the AMV anion exchange membrane (chronopotentiometric and current–voltage characteristics) was then investigated. Discussion of the role of the surface properties in the electrochemical behaviour of the membrane is based on the chronopotentiometry results in combination with the membrane surface morphology as probed by SEM and XPS. Finally, besides determining the main parameters like ion exchange capacity and transport number of counter-ion, we will also measure the water uptake and the amount of electrolyte sorption as a function of the concentration in the equilibrium external solution with the aim of providing a better knowledge of the “structure–properties” relationships.
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samples at 50 ◦ C for 72 h. The water content was defined as follows:
Table 1 Main properties of AMV anion exchange membrane. Name Type Structure propertya Fixed ionic groupa Thickness (cm)a Backboneb Ion exchange capacity (mmol equiv./g)b Membrane swelling in pure water (%)b Permselectivityb Membrane density (g/cm3 )b a b
W=
Selemion AMV
a
Homogeneous PS/butadiene –NR3 + , strong basic 0.011–0.015 PVC 1.85 ± 0.04 18 ≥0.98 1.13
Ref. [14]. Determined in this work.
2. Experimental 2.1. Anion exchange membranes Some main characteristics of the commercial anion exchange membranes used in this work are presented in Table 1. 2.2. Membrane conductivity The membrane specific conductivity m is calculated from the membrane resistance Rm using the following relation: m =
d Rm A
(1)
where d and A are respectively the membrane thickness and area. The resistance of membranes equilibrated in alkaline chloride solutions of given concentrations were measured by electrochemical impedance spectroscopy. Alternating current was supplied to the two-compartment mercury electrode cell. The membrane was placed in a circular hole between the compartments. The electrical resistances of samples, previously equilibrated in given solution at 25 ◦ C, were obtained from the membrane impedance diagrams plotted in a range of frequencies extending from 0.01 Hz to 100 kHz. Measurements were performed with a SOLATRON 1255 frequency analyzer equipped with an electrochemical interface (SI 1286). Details concerning this technique are given in our previous works [15]. 2.3. Solution conductivity The solution conductivity was determined by using EC 215 Conductivity Meter (Hanna Instruments) at 25 ◦ C. 2.4. Ion exchange capacity The membranes were immersed in 0.5 M KCl solution during 24 h. Then, they were rinsed with demineralized water to remove the non-exchange KCl electrolyte sorbed in the membranes and immersed in 50 ml 0.5 M NaNO3 under stirring for 12 h in order to obtain the complete exchange of Cl− ions in the membrane for NO3 − ions from the solution. The amount of Cl− ions obtained in the solution was determined by using Chloride Analyzer. Ion exchange capacity Cex of the studied membranes was calculated from the chloride amount and the weight of the dry membrane. 2.5. Water content The membranes were left to soak during 24 h at 25 ◦ C in MCl solutions at different concentrations (with M being Na+ or K+ ). Wet weight Wa was measured after removing all the solution on the membrane surface. Dry weight Wb was obtained by drying the
Wa − Wb Wb
(g H2 O/g dry membrane)
(2)
2.6. Current–voltage characteristic Current–voltage curves were obtained using a twocompartment cell. This cell was composed of two compartments of equal volume (40 cm3 ) separated by the anion exchange membrane. The apparent area of the membrane was 1.0 cm2 . The potential difference across the membrane was measured using two Ag/AgCl wire electrodes immersed into Luggin capillaries which were placed close to the membrane surface. The electrical current was supplied at a current scanning rate of 1 A/s by a potentiostat/galvanostat (AutoLab, Model PGSTAT 30) connected to Ag/AgCl electrode plates. 2.7. Chronopotentiometry The chronopotentiometry measurements were performed with the same two-compartment cell as used for current–voltage measurement. Chronopotentiometry was carried out with a SOLARTRON 1286 potentiostat/galvanostat controlled by a computer with the Corrware software. The temperature during the experiment was maintained at 25 ◦ C. All measurements were realized without solution agitation. 2.8. Spectroscopy studies The scanning electron microscopy (SEM) images were recorded by a Hitachi S4500 equipped with a Field Emission Gun (FEG-SEM). XPS studies were performed with a KRATOS Axis Ultra DLD spectrometer, using the monochromatized Al K␣ line at 1486.6 eV. Fixed analyzer pass energy of 20 eV was used for C 1s core level scans. 2.9. Apparent transport number of counter-ion Cl− in the membrane The transport number of counter-ion Cl− in the membranes was determined by the concentration cell method based on the diffusion potential measurements using Ag/AgCl reference electrodes in contact with dilute solutions (0.001 and 0.01 mol/kg) [16]: m =
RT F
(1 − 2t¯ )ln
a 2
a1
(3)
where m is the cell potential, a1 and a2 are the molal activities of MCl solutions (with M being Na+ , K+ ) at the two compartments, respectively. 2.10. Amount of sorbed electrolyte The amounts of non-exchange MCl electrolyte (with M being Na+ or K+ ) sorbed in the AMV membranes in contact with MCl solutions at different concentrations were determined by the manystage desorption method. First, the membranes were conditioned in MCl solution. After eliminating all the surface solution by simply wiping up the membrane with a clean fine paper, they were immersed into sufficiently large amounts of pure water at 25 ◦ C during 24 h for desorption of the MCl. This procedure was repeated three times in order to assure the complete desorption. Atomic absorption spectroscopy was used for quantitative analysis of the desorbed MCl.
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3. Results and discussion 3.1. Microheterogeneity of the membrane In the microheterogeneous model [10,12], two phases are distinguished: the gel phase and the interstitial neutral solution, with volume fractions f1 and f2 , respectively (the sum f1 + f2 being equal to 1). The gel phase is a neutral nanoporous medium, which contains fixed and mobile ions, polymer matrix, and water. The interstitial neutral solution, the properties of which are assumed to be the same as those of the outer equilibrium solution, fills the inner parts of meso- and macro-pores as well as fissures and cavities (=intergel spaces). Thus, both cations and anions contribute here to the conductivity. On the contrary, in the gel phase, almost exclusively the mobile counter-ions assure the conductivity. The co-ions are present there in a very low amount, increasing with bulk concentration, but negligible in the case of diluted water solutions (up to 1 M) and relatively high ion exchange capacity membrane (as in the case of the AMV membrane used in this work). Besides, with increasing solution concentration, the water uptake decreases that can result in a decrease of the counter-ion mobility. For these reasons, the specific conductivity of the gel phase varies only slightly with the external solution concentration; hence, it can be considered as constant. In practice, in the range of concentrations 0.1 ciso < c < 10 ciso , the variation in the membrane conductivity m with the external solution concentration may be approximately described by the function: log m = f1 log g + f2 log s
(4)
with g and s stand for the conductivities of respectively the gel phase and the external solution. Where ciso is the isoconductance electrolyte concentration at which the conductivities of all membrane phases and the membrane as a whole are the same: iso = g = s
(5)
As mentioned above, it is clear that the volume fraction of all phases as well as the conductivity at the isoconductance point of an ion exchange membrane may be varied with the nature of counter-ions. A good agreement with respect to the case of anion exchange membranes was observed in the literature [17,18]. However, rather little works focused on the role of the co-ions on the structural parameters of anion exchange membrane. We wish next to demonstrate as clearly as possible the influence of co-ions on the microheterogeneity of the AMV membrane. Fig. 1 presents the experiment values of the conductivities of the AMV membrane and the external solutions versus the concentration. It is observed that the membrane conductivity increases with the external electrolyte solution concentration (Fig. 1a). Increase in the conductivity of the electroneutral solution in the interstitial phase can be given here for explanation as the main reason. Otherwise, as the conductivity of KCl is quite higher than that of NaCl solution at the same concentration (Fig. 1b), the membrane conductivity in contact with KCl is therefore higher than in the NaCl solution. Eq. (4) shows that log m is linearly related to log s . Effectively, our results confirm well the theoretical predictions based on Eq. (4) (Fig. 2). From the slopes of the lines depicted in Fig. 2, one can get the volume fraction of the interstitial phase of the AMV membrane. As expected, we obtain two quasi-equal values of f2 , 0.063 and 0.064 in contact with NaCl and KCl, respectively. This, in other words, suggests that co-ions (Na+ and K+ in this case) play a minor role towards the membrane structure. Remark that the volume fraction of interstitial phase of the AMV membrane obtained in this work is reasonable in comparison with the proportion of water accommodated in the interstitial phase f . By assuming that the volume of macro- and meso-pores is totally filled with water of density w = 1,
Fig. 1. Conductivities of the AMV membrane (a) and of the equilibrium solutions (b) as a function of solution concentration.
Fig. 2. Plots of membrane conductivity m vs. solution conductivity s (in log/log scale).
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this volume fraction of water can be estimated approximately from the membrane swelling at equilibrium with pure water and the density of hydrated membrane as follows: f =
w f2 (1 + W ) m W
(6)
It amounts approximately to 40%. By taking into account the water content determined for 1 g of dry membrane, one can deduce a water amount of about 0.07 (g/g dry membrane) for the interstitial phase. Besides, our values of f2 are acceptable for the results obtained with the homogeneous membranes [19]. On the basis of the two-phase model, Zabolotsky and Nikonenko [19] suggested that the homogeneous ion exchange membranes are characterized by low volume fraction of interstitial solution, usually found to be ≤10%. Such result was obtained at equilibrium with NaCl solutions. In order to evaluate the microstructural parameters of the AMV membrane, it is needed to know the concentration of fixed ionic groups in the gel phase c¯ cx . Actually, this value can be estimated from the concentration of fixed ionic groups in the overall membrane c¯ ex and the volume fraction of gel phase f1 through the relation: c¯ cx =
c¯ ex Cex m = f1 (1 + W )f1
(mmol equiv./cm3 gel phase)
(7)
where Cex is the exchange capacity (mmol equiv./g dry membrane) and m is the density of hydrated membrane (g/cm3 ). The diffusion coefficient of counter-ion Cl− in the gel phase is then calculated from the concentration of fixed ionic groups in the gel phase c¯ cx and the conductivity iso by the following relation: D¯ Cl− =
RT iso F 2 c¯ cx
(8)
Table 2 presents the main characteristics of internal microstructure of the AMV membrane in NaCl and KCl solutions. As can be seen here, alkaline co-ions affect very slightly the value of iso thus the diffusion coefficient of counter-ion Cl− despite the difference in the isoconductance concentrations found for the NaCl and KCl solutions. On the other hand, the calculated values of diffusion coefficient of counter-ion Cl− in the gel phase, as expected, is found close to the diffusion coefficient of chloride ion within the AMV membrane published in the literature [20]. According to the two-phase model that has been adopted in this work, the AMV anion exchange membrane is characterized by a small degree of internal microheterogeneity, accepted as criteria in the literature for defining homogeneous ion exchange membranes. It is worth noting here that the two-phase model has further been developed into a three-phase model which then divides the “gel phase” into an inert phase (hydrophobic polymer) and a polyelectrolyte pure gel phase (fixed group zone) [19,21]. The inert phase in the case of the AMV membrane is poly(vinyl chloride) used as the membrane backbone through the classical paste preparation method [22]. It should also be remarked that the quantitative description of an ion exchange membrane as a combination of the Table 2 Main characteristics of internal microstructure of the AMV membrane in NaCl and KCl solutions.
Volume fraction of gel phase, f1 Volume fraction of interstitial phase, f2 Concentration of fixed ionic groups in the gel phase, c¯ cx (mmol equiv./cm3 gel phase) Isoconductance concentration, ciso (M) Conductivity at isoconductance point, iso (mS/cm) Diffusion coefficient of counter-ion, D¯ Cl− (m2 /s)
NaCl
KCl
0.937 0.063 1.89
0.936 0.064 1.89
0.046 4.0
0.031 3.8
5.6 × 10−11
5.3 × 10−11
electrically neutral solution and the gel phase containing fixed and counter-ions with hydration water is well accepted for both threephase and two-phase models [23]. However, the electrochemical behaviour of the ion exchange membranes, even the homogeneous ones, may be influenced by the presence of some non-conducting PVC regions on the membrane surface. Taking into account those facts requires testing the overall membrane homogeneity. It has been proved in the literature that electrochemical techniques like potentioamperometry or chronopotentiometry, applied to membrane systems, encompass many aspects related specifically to the membrane surface heterogeneity [15,16,24]. Further work aimed, therefore, at checking the applicability of chronopotentiometry with the object of getting some knowledge concerning the membrane homogeneity. 3.2. Overall surface homogeneity The SEM images of the surfaces as well as the cross-sections of the AMV membrane at different scales are shown in Fig. 3. As expected, the images shown here are very similar to those of the AMV membrane and the other homogeneous membranes published in the literature [16,25]. In particular, the AMV membrane surface looks mainly homogeneous but, as for ASV membranes [16], some micron-sized inert regions attributed to the poly(vinyl chloride) backbone are observed (Fig. 3a and b). The membrane surface was further characterized by XPS measurements. Fig. 4 presents the decomposed neutral Cl 2p core level spectra of the AMV anion exchange membrane. As can be seen in Fig. 4, the 2p3/2 and 2p1/2 peaks centred at 200.6 and 202.2 eV with the expected 2:1 ratio of the intensities confirmed very well the presence of PVC [26,27]. Furthermore, we also observed the characteristic doublet of the chloride counter-ions with the 2p3/2 and 2p1/2 peaks centred at 197.4 and 199.1 eV, respectively. When an electric current is applied to a membrane in an electrolyte solution, concentration polarization phenomena arise, i.e. concentration gradients are developed in the vicinity of the membrane. The process occurring near the membrane until a steady state is reached can be monitored by measuring the potential drop across the membrane as a function of time. When the electrolyte concentration approaches zero at the membrane surface, the potential varies very abruptly and the corresponding characteristic time is called the transition time . This time depends on the concentration, the diffusion coefficient of the electrolyte and the ion transport numbers in the membrane and external solution. The relationship existing between and these experimental parameters are classically provided by the Sand equation by taking into consideration a linear diffusion of the electrolytes. Detail concerning the application of the Sand equation in the membrane system has been reviewed in our recently published work [16]. The transition time as a function of applied current density i for an electrolyte solution A1 B2 → 1 Az1 + 2 Bz2 and a homogeneous membrane can be written as follows: i 1/2 =
c0 |z1 |1 F (Ds )1/2 2 t¯1 − t1
(9)
where Ds is the diffusion coefficient of the electrolyte, F the Faraday constant, c0 the electrolyte concentration; and t¯ , t are the transport numbers of the counter-ion in the membrane and in the solution, respectively. Fig. 5 presents the chronopotentiometric characteristics of the AMV membrane in contact with 0.1 M KCl solution at i = 21 mA/cm2 . The chronopotentiometric curve shown in this figure exhibits the typical shape recorded for homogeneous membranes as reported in the literature [16]. In order to apply the Sand equation to our system, we determined the transition time from the chronopotentiometric curve derivative. The transition time corresponds to the
X.T. Le et al. / Journal of Membrane Science 340 (2009) 133–140
137
Fig. 3. SEM images of the surface as well as the cross-section of the AMV membrane at different scales.
Table 3 Values of i 1/2 at different current densities.
maximum point of the derivative curve. Sand equations indicates that for a given concentration, the diffusion coefficient of electrolyte and the transport number of counter-ion in solution fixed, the value i 1/2 is constant and independent of the current density. The values i 1/2 presented in Table 3 show such invariance. Further data treatment allows us to estimate the effective transport number of counter-ion in the AMV membrane if the diffusion coefficient of the electrolyte and the transport numbers of counter-ion in the bulk solution are known. With the values of 0.509 for the transport number of chloride ion and of 1.84 10−5 cm2 /s for the diffusion
coefficient in the bulk solution of KCl 0.1 M [28,29], the effective transport numbers of Cl− in the AMV membrane soaked in 0.1 M KCl solution was determined to be 0.99 ± 0.01. This figure is in a good agreement with the results published by Larchet et al. [30]. It was
Fig. 4. Decomposed Cl 2p core level spectra of the AMV anion exchange membrane.
Fig. 5. Plot of a chronopotentiometric curve of the AMV membrane and its derivative in 0.1 M KCl solution at i = 21 mA/cm2 .
i (mA/cm2 )
19.0
19.5
20.0
20.5
21.0
i 1/2
77.3
76.4
76.5
76.2
76.0
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reported that the counter-ion transport numbers in high charged ion exchange membranes, like the AMV membrane studied in this work, immerged in dilute solutions (0.1 M) are very close to 1, at least >0.98. Here, an important point to be underlined is that Eq. (9) considers a linear diffusion (1D diffusion) of the electrolyte through the adjacent layer to the membrane and current lines normal to the surface. One can deduce that the overall surface of the AMV anion exchange membrane behaves as a conducting plane with respect to the transport of KCl. Such observation seems in contradiction with the results obtained with SEM and XPS techniques. This, however, can be explained by taking into account the thickness of the diffusion layer. To determine the thickness of the diffusion layer, we evaluated the current–voltage characteristic of the AMV anion exchange membrane in 0.1 M KCl solution (Fig. 6). Three characteristic regions were observed as expected: (I) a first region of approximately ohmic behaviour transforms as voltage is increased into a second region (II), in which the current varies very little with the voltage leading to a “plateau” corresponding to the limiting current, followed by a third region (III) of marked current increase (over-limiting region). In this work, the limiting current density (15.4 mA/cm2 ) was determined from the intersection point between the two first regions as shown in Fig. 6. From the diffusion coefficient of the electrolyte, transport numbers of counter-ion and limiting current density Ilim , the thickness of the diffusion layer ı has been evaluated on the basis of Eq. (10): ı=
1 |z1 |FDs c0 (t¯1 − t1 )Ilim
(10)
and amounts to 240 m. This value is acceptable for the thickness of diffusion layer at the surface of an ion exchange membrane, from 50 to 500 m up to the different conditions. As the thickness of the diffusion layer is close to 240 m, the surface inhomogeneity of the order of several micrometers on the AMV membrane does not disturb the ion transfer, which can be described as 1D to a homogeneous surface. Sand equation is therefore validated in that case [16]. 3.3. Water content and sorption electrolyte in the membranes As it has already been mentioned in Section 3.1, the water uptake and co-ion quantity are extremely important parameters
Table 4 Water uptake of the AMV membrane as a function of the external solution concentrations. Water content (g/g dry membrane)
In NaCl In KCl
0.5
1.0
2.0
3.0
0.180 0.180
0.168 0.170
0.162 0.164
0.153 0.152
0.143 0.143
Table 5 Quantity of non-exchange electrolytes sorbed in the AMV membrane as a function of the external solution concentrations. ¯ exp (mmol/g dry membrane) N
NaCl KCl
Concentration (M) 0.0
0.5
1.0
2.0
3.0
0 0
0.0224 0.0254
0.0508 0.0582
0.1065 0.1025
0.1724 0.1745
because they directly affect the ion transport, thus the conductivity and permselectivity of the membrane. Determination of the nonexchange electrolyte amount as well as the water uptakes in the AMV membrane is therefore essential in this work. Table 4 presents the water content of the AMV membrane in contact with KCl and NaCl solutions at different concentrations. It is shown indeed that the AMV membrane swells more when the solution concentration is low. This phenomenon can be explained by the reduction of the osmotic pressure difference between the interiors of the membrane with increasing the solution concentration, thus the driving force for water uptake is smaller [31]. Table 5 presents the amount ¯ exp (mmol) of sorbed KCl and NaCl per gram of dry membrane N as a function of the external solution concentrations. Once again, the expected small quantities of co-ions presented in this table are consistent with the interpretation given above. For example, in contact with 0.5 M KCl solution, we found only 0.025 mmol of co-ion K+ which is rather minor compared to the value of ion exchange capacity of the AMV membrane, 1.85 (mmol equiv./g). The decrease in water uptake and the very low amount of co-ions assured the invariance in the conductivity of the gel phase, thus led to the agreement of the experimental conductivity results with the theoretical approach predicted in Eq. (4). ¯ exp in the overThe experimental value of sorbed MCl molality m all membrane is given by: ¯ exp = m
Fig. 6. Current–voltage curve of the AMV membrane in contact with 0.1 M KCl solution.
Concentration (M) 0.0
¯ exp N W
(mol/kg H2 O)
(11)
with W is the water content (g/g dry membrane). Fig. 7 shows that ¯ exp vary linearly as a function of the molality of exterthese values m nal solution ms with the straight-line slopes K¯ being equal to 0.38 ¯ exp /ms and 0.36 respectively for the NaCl and KCl solutions. K¯ = m can be defined as coefficient of partition equilibrium between the external solution and the overall membrane. Data concerning the molality of the external solutions were reproduced from Ref. [29]. Existence of an equilibrium partition of the electrolytes between the external solution and the membrane found in this work allows us to confirm that the conductivity of the interstitial phase increases quasi-linearly with the concentration of the external solution. Note that the slightly higher value of K¯ obtained with the case of NaCl may be due to the reduction of Donnan exclusion from the K+ to Na+ . The hydrated radius of Na+ is higher than that of K+ (0.184 and 0.125 nm, respectively). A similar finding has also been obtained with the membrane permselectivity. The apparent transport numbers of counter-ion Cl− determined by means of the concentration cell method were found to be 0.986 and 0.981 (error ≈0.4%), respectively for KCl and NaCl solutions. As discussed in our previous work [16], increase in electrostatic expulsion between the co-ions and the fixed ionic groups –NR3 + inside the membrane from Na+ to
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layer, and the deviations of the current lines from 1D uniform distribution can be neglected. Measurements of the sorption isotherms have proved an existence of partition equilibrium between the electrolytes sorbed in the membrane and the external solution, which should be taken into consideration for improved understanding of internal membrane structure. Acknowledgements The authors wish to thank Prof. C. Buess-Herman (CHANI-ULB) for providing the membrane samples. Mrs. Pascale Jegou (CEA Saclay) is acknowledged for XPS measurements.
Nomenclature Cex W d A Rm m m g s f1 f2 ciso iso w f c¯ ex c¯ cx T D¯ Cl− Fig. 7. Molality of sorbed electrolytes as a function of the molality of external solutions: (a) NaCl and (b) KCl.
K+ explains the increase of the Donnan exclusion, thus the stepup in permselectivity of the AMV membrane. However, it is easy to observe that the role of co-ions is very weak in the AMV anion exchange membrane. 4. Conclusion The physico-chemical properties of the AMV anion exchange membrane (electrical conductivity, electrolyte sorption quantity, water uptake, permselectivity) determined experimentally in this work fit the requirements for the validity of the microheterogeneous model. According to this model, the studied membrane can be well described as a two-phase system on microstructural scale: gel phase and interstitial phase with the volume fractions f1 and f2 , respectively. The typical behaviour of a homogeneous ion exchange membrane has been confirmed by means of a low volume fraction of the electroneutral solution and overall surface homogeneity. It is shown clearly that the experimentally observed surface inhomogeneity (of the order of several micrometers) does not disturb the ion transfer through the AMV membrane. Thus, when describing the ion transfer, the membrane surface can be considered as ideally homogeneous, hence, the 1D model is valid. The reason is that the size of inhomogeneities is much less than the thickness of diffusion
F Ds c0 i Ilim t¯ t ¯ exp N ¯ exp m ms K¯
ion exchange capacity (mmol equiv./g dry membrane) water content (g/g dry membrane) membrane thickness (cm) membrane area (cm2 ) membrane resistance () density of hydrated membrane (g/cm3 ) membrane conductivity (mS/cm) conductivity of gel phase (mS/cm) conductivity of interstitial solution (mS/cm) volume fraction of gel phase volume fraction of interstitial phase isoconductance concentration (M) conductivity at isoconductance point (mS/cm) water density (g/cm3 ) volume fraction of water found in the interstitial phase concentration of fixed ionic groups in the overall membrane (mmol/cm3 membrane) concentration of fixed ionic groups in the gel phase (mmol equiv./cm3 gel phase) temperature (K) diffusion coefficient of the counter-ion Cl− in the gel phase (m2 /s) transition time (s) Faraday constant diffusion coefficient of electrolyte in the bulk solution (cm2 /s) electrolyte concentration (M) current density (mA/cm2 ) limiting current density (mA/cm2 ) transport numbers of counter-ion in the membrane transport numbers of counter-ion in the solution mmol of sorbed MCl per gram of dry membrane experimental value of the sorbed MCl molality in the overall membrane (mol/kg H2 O) molality of external solution (mol/kg H2 O) coefficient of partition equilibrium between the external solution and the overall membrane
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