Chemical Modification of the Surface of a Sulfonated Membrane by

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Langmuir 2004, 20, 4989-4995

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Chemical Modification of the Surface of a Sulfonated Membrane by Formation of a Sulfonamide Bond Gwenae¨l Chamoulaud and Daniel Be´langer* De´ partement de Chimie, Universite´ du Que´ bec a` Montre´ al, Case Postale 8888, succursale Centre-Ville, Montre´ al (Que´ bec), Canada H3C 3P8 Received December 4, 2003. In Final Form: March 5, 2004 This paper describes a novel approach for the surface modification of a cation-exchange membrane, bearing sulfonate groups, by a cationic layer. The modification procedure involved the chlorosulfonation of the sulfonate groups of the base membrane with thionyl chloride, followed by a reaction with a diamine to yield a sulfonamide bond and a terminal amine. The latter could be quaternized by reaction with methyl iodide or protonated by soaking in acidic media. The membranes were characterized in detail by attenuated total reflectance Fourier transform infrared and X-ray photoelectron spectroscopies as well as elemental analysis to confirm that the above reactions occurred. The selectivity of these membranes toward the electrochemically assisted transport of protons versus Zn2+ metallic cations was determined during an electrodialysis in a two-compartment electrochemical cell. The data indicate a significant decrease of the transport of the metallic cations following modification of the membrane with the cationic layer. The later allows for the transport of protons from the catholyte to the anolyte compartment with much improved selectivity since the divalent cations are excluded from the membrane due to the electrostatic barrier of the cationic layer.

Introduction Polymers make up an important class of materials due to their numerous potential applications as membranes.1-8 Such membranes can be used in biochemical reactors as immobilization matrix for enzymes and to prevent fouling of the surface and improve biocompatibility, as biosensors, in the fields of ultrafiltration, pervaporation, and affinity separations. A special category of membranes used in electrochemically assisted separation science (electrodialysis) is the ion exchange membrane which has the ability to selectively transport ions only of positive or negative charges and reject ions of the opposite charge, under the influence of an electric field.9-11 For example, electrodialysis can be used for the treatment of spent acids containing heavy metals where the aim can be either the recovery of metals or acids.12 Unfortunately, the efficiency of electrodialysis processes is commonly limited by the nonideal selectivity of the commercial membranes and the fairly poor selectivity of most cation-exchange mem* To whom correspondence should be addressed. E-mail: [email protected]. (1) Ritchie, S. M. C.; Bachas, L. G.; Olin, T.; Sikdar, S. K.; Bhattacharyya, D. Langmuir 1999, 15, 6346-6357. (2) Bluhm, E. A.; Bauer, E.; Chamberlin, R. M.; Abney, K. D.; Young, J. S.; Jarvinen, G. D. Langmuir 1999, 15, 8668-8672. (3) Chen, T.-Y.; Leddy, J. Langmuir 2000, 16, 2866-2871. (4) Dauginet, L.; Duwez, A.-S.; Legras, R.; Demoustier-Champagne, S. Langmuir 2001, 17, 3952-3957. (5) Suendo, V.; Minagawa, M.; Tanioka, A. Langmuir 2002, 18, 62666273. (6) Wavhal, D.-S.; Fisher, E. R. Langmuir 2003, 19, 79-85. (7) Kou, R.-Q.; Xu, Z.-K.; Deng, H.-T.; Liu, Z.-M.; Seta, P.; Xu, Y. Langmuir 2003, 6869-6875. (8) Nie, F.-Q.; Xu, Z.-K.; Huang, X.-J.; Ye, P.; Wu, J. Langmuir 2003, 9889-9895. (9) Davis, T. A.; Genders, J. D.; Pletcher, D. A First Course in Ion Permeable Membrane; The Electrochemical Consultancy: Romsey, England, 1997; p 202. (10) Paquay, E.; Clarinval, A. M.; Delvaux, A.; Degrez, M.; Hurwitz, H. D. Chem. Eng. J. 2000, 79, 197. (11) Tzanetakis, N.; Taama, W. M.; Scott, K.; Jachuck, R. J. J.; Slade, R. S.; Varcoe, J. Sep. Purif. Technol. 2003, 30, 113. (12) Green, T. A.; Roy, S.; Scott, K. Sep. Purif. Technol. 2001, 22-23, 583.

branes to protons compared to divalent cations. A successful approach along those lines consists of the deposition of a thin layer of an anion exchange layer at the surface of the cation-exchange membrane. In these instances, the positive charges of the thin anion exchange layer will limit the penetration of the divalent cations with respect to protons due to the enhancement of the electrostatic repulsion.13 Several routes have been proposed for the surface modification of membrane and more specifically cationexchange membrane: formation of a thin anion exchange layer by immersion of the membrane in a cationic polyelectrolyte solution, electrodeposition, photoinduced polymerization on the surface of the membrane, oxidative chemical polymerization of a conducting polypyrrole or polyaniline layer,14,15 and amide-acid bonding.16-19 For the latter case, a base membrane without any ion exchange sites was chlorosulfonated and aminated before their use in electrodialytic concentration of seawater. In a recent study, a Nafion sulfonyl chloride membrane was prepared by reaction of the perfluorinated ion-exchange Nafion membrane with a phosphorus pentachloride/phosphorus oxychloride mixture.20 This modified membrane was then reacted with an amine to form a sulfonamide bond at the membrane surface. The proton-coupled competitive transport of alkali metal cations was found to be strongly affected by the modification procedure. In this work, the surface of a commercial cationexchange membrane bearing sulfonate groups was chemically modified by a cationic layer to improve its selectivity (13) Sata, Toshikatsu; Sata, Tomoaki; Yang, W. J. Membr. Sci. 2002, 206, 31. (14) Sata, T.; Ishii, Y.; Kawamura. K.; Matsusaki, K. J. Electrochem. Soc. 1999, 146, 585. (15) Tan, S.; Laforgue, A.; Be´langer, D. Langmuir 2003, 19, 744. (16) Sata, T.; Izuo, R.; Takata, K. J. Membr. Sci. 1989, 45, 197. (17) Sata, T.; Izuo, R. Angew. Makromol. Chem. 1989, 171, 101. (18) Sata, T.; Izuo, R. J. Appl. Polym. Sci. 1990, 41, 2349. (19) Takata, K.; Yamamoto, Y.; Sata, T. J. Membr. Sci. 2000, 179, 101. (20) Hayashita, T.; Lee, J. C.; Bartsch, R. A. J. Membr. Sci. 1996, 116, 243.

10.1021/la036285l CCC: $27.50 © 2004 American Chemical Society Published on Web 05/04/2004

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for protons versus metallic cations such as Zn2+. This modification procedure involved the chlorosulfonation of the sulfonate groups with thionyl chloride, followed by a reaction with a diamine to yield a sulfonamide bond and an amine. The latter could be quaternized by reaction with methyl iodide or protonated by soaking in acidic media. The membrane was characterized at various stages of the modification by elemental analysis, attenuated total reflectance Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The electrochemically assisted transport properties of both the nonmodified and modified membranes were evaluated. More specifically, the selectivity of these membranes toward protons versus metallic cations such as Zn2+ was determined during electrodialysis in a two-compartment electrochemical cell. The data indicate that the transport of the metallic cations is decreased significantly following modification of the membrane with the cationic layer. Experimental Section Chemicals and Materials. NaOH (Anachemia, ACS 97%), NaCl (BDH, ACS), HCl (EM Science, ACS), H2SO4 (EM Science, ACS), Zn2SO4‚7 H2O (Anachemia, CAS), and N,N-dimethylethylenediamine (Aldrich, 95%) were used as received. Toluene (BDH, 99.5%) and thionyl chloride (BDH, 99%) were distilled once prior to use. Precautions were taken with thionyl chloride which reacts with alcohol, amine, and violently with water. Millipore water (18 MΩ, obtained from a Sybron/ Barnstead Nanopure system), was used for the preparation of all solutions. Prior to use, the commercial cation exchange membranes, NEOSPTA CMX, produced by Tokuyama Corp., were stored in a 0.5 M sodium chloride solution. Procedure for the Chemical Modification of the Membranes. CMX membrane, stored in 0.5 M sodium chloride solution, was rinsed with water and stirred, in water, under ultrasonic bath, to remove sodium chloride. The surface of the membrane was dried with a tissue, rinsed with toluene, and sonicated in a toluene bath to remove water. Then, the CMX membrane was conditioned into fresh toluene solution for at least one week. The chlorosulfonation reaction was carried out under reflux for a period of 3 h in 70 mL of a toluene solution containing 10% (v/v) of thionyl chloride. Then the membrane was rinsed three times with fresh toluene. The amination reaction was realized into 100 mL of a toluene solution containing 10% (v/v) of N,N-dimethylethylenediamine. After 1 h, the reaction was stopped, and the membrane was rinsed with freshwater, sonicated in an aqueous 0.1 M NaCl solution to rehydrate and stabilize the membrane. Then the membrane was hydrolyzed, under stirring, into 200 mL of an aqueous 1 M NaOH solution during 24 h. Finally, the membrane was rinsed with freshwater and stored in 0.1 M HCl aqueous solution, for a least 2 weeks, prior to use in electrodialysis. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR). Prior to the measurements, the samples were dried under vacuum at room temperature for 36 h in the presence of P2O5. ATR-FTIR analyses were performed with a Thermo Nicolet FT-IR spectrometer/analyzer model 205, using an IR source and a MCT/B detector. Due to CMX membrane low reflection, multireflection method, using a ZnSe/45° ATR plate, was preferred. Data were recorded, under low pressure, between 650 and 3200 cm-1 (200 scans). X-Ray Photoelectron Spectroscopy (XPS). After being conditioned in 0.5 M NaCl, the membranes were rinsed with water and dried under vacuum at room temperature for 36 h in the presence of P2O5. XPS analyses were performed using a VG Escalab 220i-XL system equipped with an hemispherical analyzer and an Al anode (monochromatic KR X-rays at 1486.6 eV) used at 10 kV and 15 mA. The data were recorded at room temperature and at a pressure below 10-8 Torr. To compensate for charging effects, binding energies were corrected for covalent Cl 2p3/2 at 200.6 eV. The elementary analysis was done using the appropriate sensitivity factors: C 1s (1.00), N 1s (1.80), O 1s (2.93), Cl 2p1/2 (0.775), Cl 2p3/2 (1.51), S 2p1/2 (0.567), and S 2p3/2 (1.11). The curve fitting procedure for the Cl 2p and S 2p core level spectra,

Figure 1. FTIR-ATR spectra of (a) unmodified CMX membrane, (b) CMX membrane after chlorosulfonation, and (c) chlorosulfonated CMX membrane after amination reaction with N,N-dimethylethylenediamine. as well as peak integration, were carried out using the CasaXPS software (version 2.2.68). The Cl 2p and S 2p spectra were fitted by assuming a Gaussian line shape. The full-width at halfmaximum (fwhm) was kept constant and the peak area ratio fixed at 1:2 for each sets of Cl 2p1/2-Cl 2p3/2 and S 2p1/2-S 2p3/2 peaks. In the case of the Cl 2p spectra, the peak separation between Cl 2p1/2 and Cl 2p3/2 was allowed to vary within (1.6 ( 0.1) eV. Elemental Analysis. Prior to measurements, 2 mg samples were dried under vacuum at room temperature for 36 h in the presence of P2O5. Elemental analyses were performed using a Fisons Instruments SPA system (model EA1108). Ion Exchange Capacity. The membranes were washed and stabilized several times alternatively in 1 M HCl, H2O, and 1 M NaCl for 1 h in each solution for three cycles. The membranes are then soaked for 24 h in a 1 M NaCl solution to ion-exchange H+ with Na+. After removing excess sodium chloride (by immersion in water for 30 min and rinsing with water), the membranes were dried for 1 h at 65 °C in order to measure their weight in the Na+ form. The Na+ ions were ion-exchanged with H+ by immersion in a 1 M HCl solution during 24 h. In some cases, a longer immersion time of 7 days was used in order ensure that full exchange was achieved. The sodium concentration in the latter solution was determined by atomic emission spectroscopy (Instrumentation Laboratory AA/AE Spectrophotometer model 257). Electrodialysis. Electrodialyzes were performed in a two compartment cell containing, 150 mL of 15 000 ppm Zn in 0.5 M H2SO4 in the anodic compartment, and 33 mL of 0.5 H2SO4 in the cathodic compartment. A current density of 50 mA/ cm2 was applied between two platinum plates during 3 h using a M273 Potentiostat/ Galvanostat (EG & G Princeton Applied Research). The zinc concentration in both compartments was determined by atomic adsorption spectroscopy (Instrumentation Laboratory AA/AE Spectrometer model 257).

Results and Discussion Characterization of the Modified Membranes by ATR-FTIR, XPS, and Elemental Analysis. Figure 1 shows ATR-FTIR spectra of CMX membrane surfaces at several stages during the modification process. The spectrum for an unmodified CMX membrane is characterized by a band at 1170 cm-1 corresponding to the -SdO bond, and those corresponding to -SO3- groups were observed at 1006 and 1037 cm-1.21 After chloro(21) Edwards, H. G. M.; Brown, D. R.; Dale, J. R.; Plant, S. J. Mol. Struct. 2001, 595, 111.

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Scheme 1. Modification of the CMX Membrane by Chlorosulfonation (Reaction 1), Amination (Reaction 2), and Quaternization (Reaction 3)

sulfonation of the CMX membrane with thionyl chloride (Scheme 1, reaction 1), the intensity of the bands associated to -SO3- decreased significantly, and a new band corresponding to -SO2Cl group appeared at 1368 cm-1. This observation is in agreement with previous ATR-FTIR studies on chlorosulfonated membranes.16 After reaction with N,N-dimethylethylenediamine, the -SO2Cl band totally disappeared and a new band at 1317 cm-1, corresponding to -SO2N- group, confirmed the acidamine bonding (Scheme 1, reaction 2).16 For the aminated CMX membrane, the -SO3- bands at 1005 and 1030 cm-1 are observed again together with a shift of the -SdO band from 1170 to 1148 cm-1. A similar variation was also observed by Sata who obtained a band at 1177 cm-1 for the chlorosulfonated membrane and at 1160 cm-1 following amination.16 Figure 2 presents XPS survey spectra of membranes at several stages of the CMX membrane modification process. The bare CMX membrane spectrum exhibits peaks at 1072 eV (Na 1s), 978 eV (O KLL Auger peak), 531 eV (O 1s), 498 eV (Na KLL Auger peak), 228 eV (S 2s), and 169 eV (S 2p) characteristic of the sulfonate (SO3-) groups together with those at 271 eV (Cl 2s) and 200 eV (Cl 2p) attributed to the poly(vinyl chloride) support.15-19 The small N 1s peak also detected at 400 eV on the unmodified CMX membrane is due to the presence of nitrogen-based compounds used for the fabrication of CMX membranes.22 Following reaction with thionyl chloride, an increase of the Cl 2p peak is clearly observed on the survey spectrum. After amination with N,N-dimethylethylenediamine, the Cl 2p peak decreased, whereas an increase of the N 1s peak intensity is observed. Finally, following hydrolysis with NaOH, a small decrease of the N 1s peak and an increase of the Cl 2p peak are observed. The Cl 2p core level spectrum for the unmodified CMX membrane is characterized by the 2p3/2 (200.6 eV) and 2p1/2 (202.2 eV) doublet with the expected 2:1 ratio of the peak intensities (Figure 3, curve a). These peaks are attributed to the PVC structure of the membrane.23 After (22) Mizutani, Y.; Tesima, W.; Akiyama, S.; Yamane, R.; Ihara, H. U.S. Patent 3,451,951, 1969.

chlorosulfonation, the peak envelope became broader and ill-defined (Figure 3, curve b). Following amination, the Cl 2p component decreased significantly (Figure 3, curve c). The spectrum of the hydrolyzed membrane is characterized by an increase in the peaks intensity and also by an additional component at lower binding energy (maximum at 198.7 eV) that is attributed to ionic chloride species (Figure 3, curve d). The N 1s spectrum of the unmodified membrane (Figure 4, curve a) contains a small quantity of nitrogen (vide supra). An increase of the N 1s signal is observed for the chlorosulfonated membrane with appearance of peaks at 400.2 and 402.8 eV (Figure 4, curve b). The reaction of the chlorosulfonated membrane with the amine induced a significant increase of the nitrogen concentration. After hydrolysis, a decrease of the N 1s signal is observed (Figure 4, curve d).

Figure 2. XPS survey spectra of the (a) unmodified CMX membrane, (b) CMX membrane after chlorosulfonation, (c) chlorosulfonated CMX membrane after amination with N,Ndimethylethylenediamine, and (d) aminated CMX membrane after hydrolysis.

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Table 1. Composition of the CMX Membranes into SO3-, SOCl2, and SO2N Calculated from the Core Level S 2p XPS Spectraa,b

-SO3- type I -SO3- type II -SO2Cl -SO2N

unmodified CMX (in water)

unmodified CMX (in toluene)

chlorosulfonated (in toluene)

aminated (in toluene)

hydrolyzed (in water)

100 ( 1 -

65 ( 5 35 ( 6 -

36 ( 7 22 ( 5 42 ( 9 -

34 ( 1 20 ( 2 23 ( 2 23 ( 2

57 ( 7 14 ( 3 29 ( 4

a The S 2p 3/2 peaks appeared at 169.0 (-SO3 type I), 168.2 (-SO3 type II), 169.6 (-SO2Cl), and 167.4 (-SO2N) eV. For more detail, see Supporting Information. b The uncertainties in the peak areas were based on CasaXPS software Monte Carlo simulations.

Table 2. Composition of the CMX Membrane Surface (by XPS), the Bulk of the CMX Membrane (by elemental analysis), and Ion-Exchange Capacity (IEC) for Various Unmodified and Modified Membranesa Surfaceb

CMX

CMXSOCl2

CMX10%

CMXhyd

%C %S %N % Cl %O

66.7 5.0 0.7 8.4 19.2

61.0 3.3 1.4 23.2 11.1

56.4 6.2 11.5 7.2 18.7

66.9 4.2 5.4 11.3 12.2

Membranec %C %S %N %H IEC (meq/g)d

CMX 42.2 5.0 0.6 5.2 1.64 ( 0.02

CMXSOCl2 43.4 3.4 0.7 4.7 1.35 ( 0.02

CMX10% 43.4 3.8 5.6 5.9 -

CMXhyd 46.2 3.4 3.0 5.7 0.26 ( 0.01

a

Figure 3. XPS Cl 2p spectra of the (a) unmodified CMX membrane, (b) CMX membrane after chlorosulfonation, (c) chlorosulfonated CMX membrane after amination with N,Ndimethylethylenediamine, and (d) aminated CMX membrane after hydrolysis.

Figure 4. XPS N 1s spectra of the (a) unmodified CMX membrane, (b) CMX membrane after chlorosulfonation, (c) chlorosulfonated CMX membrane after amination with N,Ndimethylethylenediamine, and (d) aminated CMX membrane after hydrolysis.

The S 2p core level spectrum of the CMX membrane shows the characteristic doublet of the sulfonate groups with the 2p3/2 and 2p1/2 peaks centered at 168.6 and 169.5 eV, respectively (Figures SI 1 and SI 2).24,25 The chlorosulfonation step, which generated the -SO2Cl species, (23) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers, The Scienta ESCA300 Database; John Wiley & Sons: New York, 1992; pp 240-241 and 266-267. (24) See the Supporting Information.

It should be noted that the atomic content was not determined for all elements present in the membrane. Thus, only a few selected elements were determined by each technique since the aim was not to obtain a complete elemental analysis but rather to get some insight into the effect of the treatment on selected elements and to demonstrate that a reaction is really occurring at the membrane surface. b Calculated values from XPS measurements. c Calculated values from elemental analysis. d Ion-exchange capacity, meq/g, of dry membrane.

induced a shift of the S 2p spectrum to higher binding energy (Figure SI 1, curve b). To get some insight into the effect of the modification procedure on the sulfonate groups, the S 2p spectra were deconvoluted by considering four main sulfur species.24 These species are -SO3- (two types), -SO2Cl, and -SO2N groups, and their respective fractions for various modified membranes are reported in Table 1 (see also Figures SI 2-6). For the membrane that was conditioned in toluene, the S 2p spectrum could not be properly fitted with only one -SO3- species. This seems to suggest that this treatment led to some structural change of the membrane. Presumably, soaking the membrane in toluene led to its partial dehydration and had an effect on the local environment of a fraction of the sulfonate groups. So, a second component attributed to another form of sulfonate groups (type II) has been considered for the fitting of the S 2p spectrum of the membrane treated in toluene. The chemical composition of the bulk and the surface of the modified membranes were determined by elemental analysis and XPS, respectively, and the results of these analyses can be found in Tables 2 and 3. The variation of these atomic contents will be discussed in detail below. The chlorosulfonation reaction which led to the formation of S-Cl bond on the CMX membrane surface was evidenced by ATR-FTIR. The intensity of the -SO3- peaks decreased significantly and the peaks associated to -SO2Cl groups appeared (Figure 1). Those observations are in accordance with XPS results that showed an increase of (25) Yoo, S-e.; Gong, Y.-D.; Seo, J.-s.; Sung, M. M.; Lee, S. S.; Kim, Y. J. Comb. Chem. 1999, 1, 177.

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Table 3. Composition of the Bulk of the CMX Membrane (Measured by Elemental Analysis) for Various Unmodified and Modified Membranes membrane

%N

%C

%H

%S

IEC

CMX CMX + toluene (A)a CMX + toluene + amine10% (B)a CMX + toluene + heat (C)a CMX + toluene + heat + amine 10% (D)a CMX + toluene + heat + SOCl2 (E)a CMX + toluene + heat + SOCl2 + hydrolysis (F)a CMX + toluene + heat + SOCl2 + amine 1% (G)a CMX + toluene + heat + SOCl2 + amine 10% (H)a CMX + toluene + heat + SOCl2 + amine 10% + hydrolysis (I)a

0.6 0.7 0.8 0.7 2.1 0.7 5.0 5.6 3.0

42.2 42.6 43.0 42.9 43.8 43.4 43.3 43.4 46.2

5.2 5.2 5.1 4.9 5.2 4.7 5.8 5.9 5.7

5.0 5.4 4.6 4.9 3.4 3.4 3.2 3.8 3.4

1.64 ( 0.02 1.45 ( 0.02 1.53 ( 0.02 1.56 ( 0.02 1.35 ( 0.02 1.56 ( 0.02 > 0.03 ( 0.01 0.26 ( 0.01

a

See Scheme 2.

the intensity of the Cl 2p peaks (Figure 2). The deconvolution of the S 2p spectrum revealed that 42% of -SO3groups near the membrane surface were transformed into -SO2Cl (Table 1). The total surface Cl content increased from 8.4 to 23.2 at. % (Table 2). These results and the slight decrease of the ion exchange capacity (Table 2) confirmed that thionyl chloride essentially reacted with -SO3- groups at the CMX membrane surface, and about half of these -SO3- groups were transformed into -SO2Cl according to reaction 1 (see Scheme 1). The new peaks on the N 1s core level spectrum following the reaction with thionyl chloride can be explained by a structural modification of the membrane which could lead to a variation of the nitrogen content at the membrane surface and also by a chemical reaction involving the N-species of the membrane. The peak at lower binding energy appeared following treatment of the CMX membrane under reflux in toluene whereas the one at higher binding energy is noticed upon exposure to thionyl chloride (Figure SI 7). The nature of the chemical transformation is difficult to determine, considering the unknown composition of the CMX membrane. Subsequently, the chlorosulfonated membrane was placed in a N,N-dimethylethylenediamine solution, and the ATR-FTIR results show that -SO2Cl groups were transformed into -SO2N, and also that a part of those -SO2Cl groups were hydrolyzed back to -SO3- (Figure 1). These transformations were also observed by XPS that revealed a decrease of the intensity of the Cl 2p peaks and an increase of the N 1s peak (Figures 2-4). The Cl 2p peaks corresponding to the PVC structure disappeared due to the presence of the N,N-dimethylethylenediamine on the membrane surface which block the Cl 2p PVC signal. This hypothesis is in accordance with the increase of N content at the membrane surface (determined by XPS) and for the bulk of the membrane (determined by elemental analysis). The curve fitting of the S 2p spectrum suggests that approximately 25% of -SO3- groups (or 55.5% of -SO2Cl groups) were transformed into SO2N. The adsorption of N,N-dimethylethylenediamine on the membrane surface is also established from the composition of the membrane following the hydrolysis step. This step permitted the removal of the diamine that was adsorbed on the surface and not chemical bonded to the membrane surface. This is confirmed by the decrease of the intensity of the N 1s peak (Figures 2 and 4) and of the N content at the membrane surface from 11.5 to 5.4% and into the membrane from 5.6 to 3.0% (Table 1). Moreover, the reappearance of Cl 2p PVC peaks (Figure 3) that were previously masked by the adsorbed diamine can be also noticed. Following the hydrolysis, the modified membrane N content is due to the grafted amine. This was also confirmed by the observation of a nearly constant proportion of -SO2N species (about 25%) before and after hydrolysis of the membrane (Table 1). On the other hand,

the hydrolysis of unreacted -SOCl2 into -SO3- is highlighted by an increase of the Cl content at the membrane surface. This new contribution on the Cl 2p XPS spectrum at 198.7 eV is due to the formation of HCl (chloride counterions for the positively charged amine) that occurred during the amination according to reaction 2 of Scheme 1. The XPS data of Table 1 also show that -SO2Cl groups were partially hydrolyzed and that 29% of sulfonate groups were transformed into -SO2N for the modified membrane. The small value of 0.26 meq/g for the ion exchange capacity of the aminated membrane is surprising. This low value is probably due to the fact that membrane was modified on its two sides and that the Na+ exchange with the sulfonate groups of the membrane is hindered by the cationic layer surrounding the membrane. This is demonstrated by the low limiting current recorded in the current-voltage curve for this membrane in the presence of a sodium salt in comparison to that obtained in acidic media.26 These current-voltage curves will be presented and discussed in a forthcoming publication. Electrodialyzes. The modified CMX membranes were tested in electrodialysis according to the procedure described in the Experimental section to determine the effect of the various modification steps on the Zn2+ leakage. The results of these electrodialyzes are summarized in the flowchart of Scheme 2 that reports the percent metal leakage after each step. For the unmodified CMX membrane, previously stored into NaCl aqueous solution, the Zn2+ leakage is 6.8%. This value compares well with that of 10.1% recently reported for a CMX membrane when a higher current density of 100 mA/cm2 was used for the electrodialysis.15 Scheme 2 shows that the overall modification procedure led to a significant decrease of the metal leakage to values lower than 1%, demonstrating the improvement of the selectivity for the transport of protons in the presence of Zn2+ in highly acidic media. The optimized modified membrane (J) is characterized by a metal leakage of only 0.3% and is comparable with that recently reported for polyaniline-modified CMX membrane.15 The low zinc leakage through the modified membrane of this study compares favorably with those reported with a commercially available cation-exchange membrane (Neosepta CMS), designed specifically for high permselectivity to monovalent cations,27 and a polyethyleneimine (PEI)-modified Nafion cation-exchange membrane 28 for which the zinc leakage was about 2-5 and below 1%, respectively. It should be noted that in these cases, other metallic cations (Mg2+ and Mn2+) were present (26) Chamoulaud, G.; Be´langer, D. Manuscript in preparation. (27) Boucher, M.; Turcotte, N.; Guillemette, V.; Lantagne, G.; Chapotot, A.; Pourcelly, G.; Sandeaux, R.; Gavach, C. Hydrometallurgy 1997, 45, 137. (28) Sistat, P.; Pourcelly, G.; Gavach, C.; Turcotte, N.; Boucher, M. J. Appl. Electrochem. 1997, 27, 65.

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Scheme 2. Evolution of Zn2+ Leakage during Electrodialysis for Various Modified CMX Membranes

in the acidic solution and that their presence might influence the amount of leaking Zn2+ when compared to an acidic solution containing only Zn2+. Scheme 2 shows that the metal leakage is strongly affected by the various treatments of the modification procedure. When the CMX membrane is conditioned in toluene (A), it gave a lower metal leakage of 6.1%, which could be assigned to the structural modification induced by soaking the membrane in toluene (vide supra). This modification of the membrane structure is also suggested by the decrease of the IEC from 1.64 to 1.45 meq/g, which indicates a modification of the environment of the -SO3groups (Table 3), as was also observed by XPS. When this membrane is placed in contact with N,N-dimethylethylenediamine in toluene for 1 h, the Zn2+ leakage remained almost unchanged at 5.9% (B). This result suggests that the amine is not absorbed on the membrane or trapped within the membrane. This is confirmed by the elemental analysis of this membrane (Table 3). On the other hand, when the CMX membrane was refluxed in toluene, the Zn2+ leakage decreased to 5.4% presumably because the membrane became more dehydrated (C). After treatment of this membrane with the amine, the Zn2+ leakage slowly decreased to 5.0% due to small the adsorption of amine within the membrane (D). This was confirmed by elemental analysis that revealed an increase of the nitrogen content from about 0.7% for the membrane treated in toluene to 2.1% following soaking in the amine solution and also by its unchanged value of the IEC. Following chlorosulfonation, the Zn2+ leakage decreased to 3.6% (E). This decrease can be attributed to an increase of the resistance of the membrane due to the transformation of the ionic -SO3- groups to the neutral -SO2Cl groups that do not permit the conduction of cationic species. Indeed, the initial cell potential was 9 V but it was not stable, as it continuously decreased and reached, at the end of electrodialysis, a value of 6.5 V which is much higher than that observed (4 V) for the unmodified membrane. The hydrolysis of the SO2Cl groups back to -SO3- groups is occurring during electrodialysis and is responsible of the decrease of cell potential. The unexpected high value of 1.35 meq/g obtained by IEC measurements could be also explained by a similar hydrolysis during IEC protocol. A second electrodialysis with the same membrane (E) revealed an increase of the leakage value from 3.6 to 5.1%. This value corresponds to the Zn2+

leakage value of 4.9% obtained by hydrolysis of the -SO2Cl groups into ion conducting -SO3- by the treatment with NaOH (F). This alkaline hydrolysis also led to a cell potential during electrodialysis and to IEC values similar to that recorded for a CMX membrane (C). Following amination with 1 and 10% (in volume) of N,Ndimethylethylenediamine (G and H), the Zn2+ leakage of these modified CMX membranes decreased to 2.2% and 0.7%, respectively. For these membranes, the cell potential was around 6 V, indicating that their resistance was affected by the modification procedure. Finally the hydrolysis of the aminated membranes reduced the cell potential to the value of the unmodified membrane and without change in the Zn2+ leakage which remained at 0.6% for the membrane aminated in the presence of the 10% amine solution. Thus, the grafting of N,N-dimethylethylenediamine on chlorosulfonated CMX membrane really improved their permselectivity for the transport of protons with respect to Zn2+. For all those membranes, IEC measured values are under 0.3 meq/g (Table 3). Here again, the cationic layer formed by quaternization of the grafted amine surrounding the membrane do not allow for a quantitative evaluation of the IEC membrane values.26 Optimization of the Modification Procedure. The differences observed between amination with the 1 and 10% of N,N-dimethylethylenediamine solution demonstrates that the amine concentration is an influential parameter on membrane permselectivity. Figure 5 shows the effect of the amine concentration on the Zn2+ leakage. The leakage is about 5% for low amine concentrations and then decreased to reach a constant value lower than 0.5% for amine concentrations larger than 5.5% (v/v). An amine concentration of at least 7.5% (v/v) was also needed to obtain the optimum modified membrane for electrodialysis of brine.15 It was also shown that under our experimental conditions the amination is very fast as the minimum leakage is found after an amination of only 5 min. (Figure SI 8).24 This result is to be contrasted with that reported for the amination of a chlorosulfonated membrane with polyethyleneimine, where the reaction time was found to be much longer due to the relatively higher viscosity of a polyethyleneimine solution compared to the amine used in this work.17 It was also shown that Zn2+ leakage decreased when the thionyl chloride concentration (Figure 5) and the

Surface of a Sulfonated Membrane

Figure 5. Variation of Zn2+ leakage as a function of the thionyl chloride concentration and the N,N-dimethylethylenediamine concentration used for the amination reaction.

chlorosulfonation time increased (Figure SI 8).24 A thionyl chloride concentration of at least 10% (v/v) and a chlorosulfonation time longer than 3 h are required to obtain the lowest Zn2+ leakage values. The decrease is attributed to the fact that more sulfonated groups are chlorosulfonated when the reaction time and the concentration are increased, thus leading to N,N-dimethylethylenediamine that could be grafted and transformed into cationic sites. Conclusion Commercially available sulfonated membranes were chlorosulfonated prior to an amination with N,N-dimethylethylenediamine to generate cationic groups at the surface of the membrane. The cationic groups are generated by soaking the modified membrane in acidic media that was used for the electrodialysis experiments. The modification procedure differs from those previously reported in that the sulfonated groups of the base membrane are chlorosulfonated instead of generating directly the -SO2Cl group in a neutral membrane. The modified membranes were characterized by several techniques, including XPS, which was used to investigate the changes that are occurring at the membrane surface during each steps of the modification procedure. This procedure was found to be very effective in improving the

Langmuir, Vol. 20, No. 12, 2004 4995

permselectivity of the membrane for the transport of protons versus divalent metallic cations. The improvement is due to the electrostatic repulsion between the surface cationic layer and the Zn2+ cations that prevent their transport through the cationic ion-exchange membrane. This novel modification procedure which consists of the formation of a covalent bond between the base membrane and the cationic layer is an attractive alternative to other approaches which rely on electrostatic interactions to retain the cationic layer at the base membrane surface. This is the case for polyethyleneimine on Nafion28 and polyaniline on CMX.15 The stability of the polyethyleneimine layer was found to be insufficient for long-term use in an electrodialysis cell. Presumably, the stability and the adhesion of this anion-exchange layer can be improved by plasma treatment of the base membrane.29 On the other hand, it will be interesting to investigate the long-term stability of the new modified membranes prepared by the procedure described in this work. In principle, these membranes should be more stable due to the presence of a covalent bond between the base membrane and the cationic layer. Finally, this new modification procedure could be also useful for the preparation of new membrane that could eventually find application in areas mentioned above (see Introduction). It is now widely accepted that surface modification of membranes to form hybrid or composite membranes is as important for their applications than the base membrane itself.6 The application of these ionexchange membranes, modified by sulfonamide bonding, in fuel cell30-32 is currently being pursued in our laboratory. Acknowledgment. This research was funded by the Natural Science and Engineering Research Council of Canada (NSERC) through a strategic grant (234959-00). NSERC funding for an equipment grant for an XPS spectrometer (to D.B. and nine others) is also acknowledged as well as the financial contribution of UQAM. We also thank Prof. M. Morin for his help in the FTIR-ATR measurements. Supporting Information Available: XPS spectra, fitting results, and variation of Zn2+ leakage. This information is available free of charge via the Internet at http://pubs.acs.org LA036285L (29) Vallois, C.; Sistat, P.; Roualde`s, S.; Pourcelly, G. J. Membr. Sci. 2003, 216, 13. (30) Yoshida, N.; Ishisaki, T.; Watakabe, A.; Yoshitake, M. Electrochim. Acta 1998, 43, 3749. (31) Thompson, S. D.; Jordan L. R.; Forsyth, M. Electrochim. Acta 2001, 46, 1657. (32) Jia, N.; Lefebvre, M. C.; Halfyard, J.; Qi, Z.; Pickup, P. G. Electrochem. Solid State Lett. 2000, 3, 529.