J. Phys. Chem. 1996, 100, 16633-16640
16633
Preparation and Properties of Composite Membranes Composed of Anion-Exchange Membranes and Polypyrrole Toshikatsu Sata,* Takanori Yamaguchi, and Koji Matsusaki Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Yamaguchi UniVersity, Tokiwadai 2557, Ube City, Yamaguchi Prefecture, 755, Japan ReceiVed: April 4, 1996; In Final Form: August 5, 1996X
Composite membranes composed of commercial anion exchange membranes and polypyrrole were prepared by chemical oxidation of pyrrole in the membrane matrix. Two different procedures were used: immersing the anion-exchange membranes equilibrated with a ferric chloride solution into an aqueous pyrrole solution; immersing the anion-exchange membranes equilibrated with the pyrrole solution into the ferric chloride solution. Polypyrrole homogeneously existed throughout the membranes in the former composite membranes, and the polymer existed on the membrane surfaces as layers in the latter membranes. Electrical conductivity parallel to the plane and that across a cross section of the membrane also demonstrated different morphology of the composite membranes. After evaluating the electrochemical properties of the composite membranes, transport properties in electrodialysis, transport number of sulfate ions, nitrate ions, and bromide ions relative to chloride ions and current efficiency were evaluated. Permeation of sulfate ions through the composite membranes was difficult; permeation of nitrate ions was enhanced by the existence of polypyrrole on and in the membranes and the relative transport number of bromide ions decreased depending on morphology of the membranes. The polypyrrole in the anion-exchange membranes markedly affected the transport number of anions relative to chloride ions.
Introduction Ion-exchange membranes are one of the advanced separation membranes that have been used in various industries: electrodialytic concentration of seawater to produce edible salt, a separator for production of chlorine, sodium hydroxide, and hydrogen by electrolysis, desalination of saline water by electrodialysis, separation of ionic materials from non-ionic materials by electrodialysis, recovery of acid and alkali from waste acid and alkali by diffusion dialysis,1 dehydration of water-miscible organic solvent by pervaporation,2 etc. Thus, various ion-exchange membranes for wide utilization have been developed according to the requirements. At the same time, as the utilization becomes wider, various new functions are required for the ion-exchange membranes. Polypyrrole is one of the conducting polymers that have been studied as functional polymers.3 Among various conducting polymers, pyrrole is easily polymerized to polypyrrole by chemical4 or electrochemical oxidation,5 and the polypyrrole has a tight, rigid structure with weakly basic anion-exchangeable groups. It is well-known that a free-standing film of polypyrrole has not only electronic conductivity but also ionic conductivity. Various studies have been reported: a study of the anionexchange behavior of polypyrrole membranes,6 and anion/ cation-permeable polypyrrole membranes in which large organic anions such as dodecyl sulfate,7 catechol, hydroquinone, melanin, anthraquinone-2-sulfonate,8 etc., were incorporated as a dopant anion. On the other hand, because polypyrrole has a good affinity for ion-exchange membranes, the composite membranes, which were prepared from both cation- and anionexchange membranes and pyrrole, and their properties were studied from various aspects: the composite membranes prepared from the anion-exchange membranes show high acid retention,9 the composite membranes from anion-exchange membranes show anti-organic fouling properties,10 an EMF is X
Abstract published in AdVance ACS Abstracts, September 15, 1996.
S0022-3654(96)01024-6 CCC: $12.00
generated from the composite membrane which is dependent on humidity,11 the preparation of a free-standing conducting film,12 etc. Although ion-exchange membranes have excellent permselectivity for counterions, it is difficult to separate ions with the same charge and same valence. Several modification methods for the ion-exchange membranes to control permselectivity between ions with the same charge have been proposed and industrially used: formation of a highly cross-linked layer on the membrane surface,13 the introduction of specific ionexchange groups into the membrane,14 the formation of a thin layer of opposite charge relative to the ion-exchange groups on the membrane surface,15 controlling hydrophobicity of the anionexchange groups according to anion species,16 etc. When a thin polypyrrole layer was formed on a cation-exchange membrane, monovalent cations were found to selectively permeate through the membrane due to the synergistic effect of the difference in the electrostatic repulsion force of weakly basic anionexchangeable groups in the polypyrrole layer relative to monovalent and multivalent cations and sieving of large ions due to the rigidity of the polymer.17 It is interesting to examine the permselectivity among various anions using the composite membrane composed of anion-exchange membranes and polypyrrole. In this work, the composite membranes were prepared by two different procedures, and the permselectivity between two anions in electrodialysis was examined after evaluation of their basic properties. Experimental Section (a) Materials. Anion-Exchange Membranes. Anion-exchange membranes used to prepare composite membranes were supplied from Tokuyama Corp. NEOSEPTA AM-1, AM-2, and AM-3. The anion-exchange groups of the membranes were trimethylbenzylammonium groups. The ion-exchange capacity and water content of these membranes are different. Before preparing the composite membranes, the membranes were © 1996 American Chemical Society
16634 J. Phys. Chem., Vol. 100, No. 41, 1996
Sata et al.
TABLE 1: Characteristics of Anion-Exchange Membranes Used in This Work name, backing
NEOSEPTA AM-1, poly(vinyl chloride)
NEOSEPTA AM-2, poly(vinyl chloride)
NEOSEPTA AM-3, poly(vinyl chloride)
thickness (mm) electric resista transport numberb ion-exchange capacityc water contentd
0.135 1.4 >0.98 2.25 0.31
0.128 2.2 >0.98 2.12 0.23
0.110 4.9 >0.98 1.55 0.18
a Ω cm2; measured with 1000 Hz ac at 25.0 °C after equilibration with 0.500 N sodium chloride solution. b Measured by electrodialysis of 0.50 N sodium chloride solution at 20 mA/cm2. c Mequiv/g of dry membrane of Cl- form. d g of H2O/g of dry membrane of Cl- from; measured after equilibration with pure water.
equilibrated with aqueous 1.0 N hydrochloric acid solution and aqueous 0.5 N ammonia solution alternately several times and then equilibrated with aqueous 1.0 N hydrochloric acid solution. The characteristics of the anion-exchange membranes used are shown in Table 1. Reagents. Pyrrole, FeCl3‚6H2O, sodium sulfate, sodium nitrate, sodium bromide, sodium chloride, hydrochloric acid (36%), ammonia solution (29%), methyl alcohol, and other reagents, which were of reagent grade, were obtained from Wako Pure Chemical Industries, Ltd. Deionized water was used throughout all experiments. (b) Preparation of the Composite Membranes. Two different procedures were used. (1) To prepare the composite membranes in which polypyrrole homogeneously existed across a cross section of the membranes, pyrrole was polymerized with ferric ions adsorbed in the anion-exchange membranes. To produce much polypyrrole in the membrane phase, a substantial amount of ferric ions, an oxidative agent, should be adsorbed in the membrane. Thus, after the anion-exchange membranes had been equilibrated with aqueous 0.5, 1.0, 2.0, and 3.0 mol/L ferric chloride solutions, ferric ions in the membrane were eluted with 1.0 N hydrochloric acid solution after the membrane had been blotted with filter paper and were determined by a colorimetric method using 1,10-phenanthroline after reducing the ferric ions to ferrous ions by adding ascorbic acid to the solution.18 Subsequently, the adsorbed ferric chloride in the membranes was maximum (5.4 × 10-4 mol/g of dry membrane) when the membrane had been equilibrated with the 2 mol/L ferric chloride solution. After the anion-exchange membrane of chloride ion form had been equilibrated with aqueous 2.0 mol/L ferric chloride solution (the solution renewed several times), the membrane was immersed in aqueous 0.745 mol/L pyrrole solution for 12 h with or without blotting the surfaces with filter paper (4 h was sufficient to polymerize pyrrole in the membrane matrix.). The composite membranes prepared by this procedure were designated as Fe-Py membranes. (2) To prepare the composite membranes with thin polypyrrole layers on the membrane surfaces, after the anion-exchange membranes had been immersed in aqueous 0.745 mol/L pyrrole solution for 40 h to attain equilibrium, the membranes were immersed in aqueous 2.0 mol/L ferric chloride solution for 12 h. The composite membranes prepared by this procedure were designated as Py-Fe membranes. The weight increase of the anion-exchange membranes after the pyrrole polymerization was also measured. To confirm the polymerization of pyrrole in the membrane matrix, the electrical conductivity was measured. For this purpose, the composite membranes were prepared under different conditions: the Fe-Py membranes with and without blotting with filter paper after equilibration with 2 mol/L ferric chloride solution before immersing the membranes into 0.745 mol/L pyrrole solution; the Py-Fe membranes which were immersed in 0.5, 1.0, or 2.0 mol/L ferric chloride solution after equilibration with 0.745 mol/L pyrrole solution.
(c) Measurement of Basic Properties of the AnionExchange Membranes and the Composite Membranes. The electrochemical properties of the anion-exchange membranes and the composite membranes, from which ferric and ferrous ions had been completely removed by washing the membranes with 1.0 N hydrochloric acid solution, were evaluated. The electrical resistance of the membranes was measured at 1000 cycle ac, using a Hewlett-Packard LCR meter 4263A, at 25.0 °C after equilibration with 0.500 N sodium chloride solution. The transport number was evaluated by electrodialysis of 0.50 N sodium chloride solution at 20 mA/cm2 for 60 min. The ionexchange capacity and water content of the membranes were measured according to conventional methods (based on dry membrane weight). The amount of weakly basic anionexchange groups was measured by the same method as that in the previous paper.19 The characteristics of the composite membranes are shown in Table 2. Also, the cross sections of both composite membranes were observed by microscope. (d) Evaluation of Transport Properties of the Composite Membranes. (1) Apparatus. A four-compartment cell with two kinds of silver-silver chloride electrodes, which were for current supply and measurement of the voltage drop across the membrane, was used to measure the transport properties of the composite membranes, which was the same as that used in the previous paper.16 The effective membrane area was 10 cm2 (2 cm × 5 cm), and the capacity of each compartment was 100 cm3. Electricity passing through the membrane was measured by a coulometer, Nokko Digital Coulomb Meter NDCM-4, Nikko Keisoku, Ltd. The voltage drop across the membrane was measured with Ag-AgCl probe electrodes and recorded on an X-t recorder, Toa Electronics Co., EPR-2T. (2) Measurements of Transport Properties of Anion-Exchange Membranes. The measured transport properties of the anionexchange membranes were the relative transport number between two anions, the current efficiency, and the voltage drop across the membrane. The relative transport number between two anions was defined as follows:
PClA )
tA/tCl CA/CCl
where tA and tCl are the transport numbers of anion A and chloride ions in the membrane, and CA and CCl are the average concentrations of anions A and chloride ions before and after electrodialysis. PClA is the permeated equivalent of anion A through the membrane where 1 equiv of chloride ions permeates through the membrane (because a 1:1 mixed salt solution was used). (3) Electrodialysis Procedure. After an anion-exchange membrane or composite membrane had been placed at the middle of the cell, the two middle compartments were filled with 100 cm3 of the mixed salt solution and both anolyte and catholyte were filled with 100 cm3 of sodium chloride solution. (The concentration of sodium ions was the same as that in the
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J. Phys. Chem., Vol. 100, No. 41, 1996 16635
TABLE 2: Characteristics of Composite Membrane Prepared in This Work membrane species composite membrane a
weight increase after immersion in pyrrole (∆m/g of dry membrane × 100) weight increase after polymerization (∆m/g of dry membrane × 100) electric resistanced transport numbere ion-exchange capacityf water contentg thickness (mm)
NEOSEPTA AM-1 Py-Fe
NEOSEPTA AM-2
Fe-Py
45.1
Py-Fe
Fe-Py
36.7
NEOSEPTA AM-3 Py-Fe
Fe-Py
32.6
32.0b
13.6c
28.8b
14.8c
26.9b
12.5c
3.1 >0.98 2.16 0.31 0.151
1.7 >0.98 2.20 0.16 0.141
4.9 >0.98 2.13 0.23 0.153
2.4 >0.98 1.99 0.22 0.147
16.8 >0.98 1.77 0.19 0.134
5.7 >0.98 1.18 0.13 0.128
a After Cl- form dry membranes had been equilibrated with an aqueous 0.745 mol/L pyrrole solution (40 h), weight increase of the membranes (∆m) was measured after blotting the membrane surfaces with a filter paper. bThe weight increase of the membranes (∆m) after the membranes equilibrated with the pyrrole solution were immersed in a 2 mol/L ferric chloride solution for 12 h. c The weight increase of the membranes (∆m) after the membranes equilibrated with a 2 mol/L ferric chloride solution were immersed in a 0.745 mol/L pyrrole solution for 12 h. d Ω cm2; measured with 1000 Hz ac at 25.0 °C after equilibration with 0.500 N sodium chloride solution. e Measured by electrodialysis of 0.50 N sodium chloride solution at 20 mA/cm2. f Mequiv/g of Cl- dry form membrane. gH2O/g of Cl- form dry membrane; measured after equilibration with pure water.
middle compartments.) The middle compartments and the electrode compartments were separated by cation-exchange membranes (NEOSEPTA CM-2, made by Tokuyama Corp.; the transport number of sodium ions in the membrane is more than 0.99 in the electrodialysis of 0.5 N sodium chloride solution at a current density of 20 mA/cm2; the electrical resistance is 2.8 Ω cm2). The two middle comparments were filled with a 1:1 mixed salt solution: sodium sulfate and sodium chloride, sodium nitrate and sodium chloride, or sodium bromide and sodium chloride. Their concentration was also changed: 0.01, 0.04, 0.15, and 0.50 N as total anions. Electrodialysis was carried out under vigorous agitation (1500 rpm with stirrers) at 25.0 °C. In the cases of NEOSEPTA membranes and the Fe-Py membranes, the current density of electrodialysis was changed according to the concentration of the salt solutions: 0.5 N: 10 mA/cm2 for 1 h; 0.15 N: 3.0 mA/cm2 for 1 h; 0.04 N: 1.0 mA/cm2 for 1 h; and 0.01 N: 1.0 mA/cm2 for 1 h. However, because the Py-Fe membranes easily underwent the concentration polarization (pH change of the solution) due to the existence of tight layers having weakly basic anion-exchangeable groups on the membrane surfaces, the current density was decreased to half of the above value: 0.5 N: 5 mA/cm2 for 2 h; 0.15 N: 1.5 mA/cm2 for 2 h, etc., to maintain the pH of the solution after electrodialysis below 5 in the desalting side solution and above 7.5 in the concentrating side solution. If the pH of the concentrating side solution became basic, the accurate effect of the polypyrrole layer on the relative transport number could not be evaluated (hydroxide ions permeated through the membrane together with the anions to be measured). In cases of the anion-exchange membranes and the Fe-Py composite membranes, the pHs of the anolyte and catholyte after electrodialysis did not change appreciably. After electrodialysis, the solutions were analyzed by the Mohr method (for Cl-), conventional chelate back-titration (for SO42-), or ion chromatography (TOHSO CCPD, IC-8010, Chromatocorder 21). Thus, the relative transport number was calculated from the change in the concentration of each anion, the current efficiency was calculated from the change in the concentrations of anions in each compartment to the electricity measured with a coulometer. (4) Measurement of Electrical Conductivity of the Composite Membranes. The electrical conductivity of the dry composite membranes was measured to determine the degree of the polymerization of pyrrole in the membrane matrix. A strip of the composite membrane (5.0 cm × 1.0 cm × membrane thickness) was used to measure the conductivity parallel to the plane of the membrane. Also the conductivity across a cross section of the membrane was measured using a piece of the
membrane (1.0 cm × 1.0 cm). These strips and pieces of the membrane were cut from several places (original size: 5.0 cm × 5.0 cm). When the conductivity parallel to the plane of the membrane was measured, a coating of silver conductive paste (Electroconductive DOTITE D-550 made by Fujikura Kasei Co., Ltd.) was placed on both sides of the ends of the strips (10 mm length each), and a 1.0 cm × 1.0 cm window at the center of each strip was not coated. To measure the conductivity, the coated parts of the strip were tightly clamped between two platinum plates, and voltage was applied to the platinum plates using a potentiostat/galvanostat HA-303 from Hokuto Denko, Ltd. The current which passed through the strip was then measured with an HM-105 Zero Shunt Ammeter from Hokuto Denko, Ltd. The voltage was increased from 0.1 to 0.3 V, which was measured with an HE-104 electrometer from Hokuto Denko, Ltd., and the current was recorded. The conductivity was calculated from the slope of the current-voltage curve relationship. When the conductivity across the cross section of the membrane was measured, both surfaces of the membrane (1.0 cm × 1.0 cm) were coated with the same silver conductive paste and clamped between two platinum plates. The conductivity was calculated by the same current-voltage curve. The measurements were carried out in desiccator at less than 20% relative humidity at 25 ( 1 °C. (5) Measurement of Permeability Coefficient of Urea through the Membranes. To estimate difference of pore size of the anion-exchange membranes from the composite membranes, the permeability coefficient of urea was measured using a two-compartment cell under vigorous agitation (1500 ( 100 rpm) at 25.0 °C. After an aqueous 2 mol/L urea solution had filled the concentrated compartment of the cell (250 cm3) and pure water (120 cm3) the dilute compartment, the permeated amount of urea into the water was analyzed after 48 h of dialysis. Urea was analyzed by HPLC (Hitachi L-6000) and the permeability coefficient was calculated by the following equation:
P ) ∆m/At(CC - CD) where ∆m is the permeated amount of urea, A the effective membrane area (20 cm2), CC the average concentration of the concentrated compartment, and CD the average concentration of the dilute compartment. Results and Discussion (1) Preparation of the Composite Membranes Composed of Anion-Exchange Membranes and Polypyrrole. It is well-
16636 J. Phys. Chem., Vol. 100, No. 41, 1996
Figure 1. Micrograph of a cross section of a composite membrane (Py-Fe membrane).
known that pyrrole is easily polymerized by chemical oxidation, and the resultant polypyrrole is rigid and tight, in addition to being electrically conductive. Pyrrole and polypyrrole have a good affinity for both cation- and anion-exchange membranes. Thus, the composite membranes, which have different morphology, were prepared from the anion-exchange membranes and pyrrole using two different procedures. When the anionexchange membranes equilibrated with an aqueous pyrrole solution were immersed in the ferric chloride solution, polypyrrole existed on the membrane surfaces as layers as shown in Figure 1 (Py-Fe membrane). (The black part is polypyrrole, and the white part the anion-exchange membrane.) This is due to the fact that because the ferric chloride solution was acidic and contained an oxidizing agent, ferric ions, pyrrole molecules adsorbed in the membrane were protonated, excluded by anionexchange groups (Donnan exclusion) and polymerized on the membrane surfaces by the ferric ions. On the other hand, when the anion-exchange membranes equilibrated with the ferric chloride solution were immersed in the aqueous pyrrole solution, polypyrrole homogeneously existed throughout the membrane matrix as shown in Figure 2 (Fe-Py membrane), because pyrrole molecules diffused into the membrane matrix were polymerized by the ferric ions adsorbed in the membrane. Table 2 shows the weight increase after polymerization of pyrrole and the characteristics of the composite membranes. Though the weight increase before polymerization in Table 2 was due to the adsorbed water and pyrrole (the anion-exchange membranes were immersed in the aqueous 0.745 mol/L pyrrole solution), these values were higher than those of the water content of the membranes, which means that pyrrole has a good affinity for the membranes. Table 2 also shows that the weight increase in the Py-Fe membranes was larger than that of the Fe-Py membranes and the water content of the membranes was markedly low in the Fe-Py membranes. This is reasonable
Sata et al.
Figure 2. Micrograph of a cross section of a composite membrane (Fe-Py membrane).
because little ferric ions were adsorbed in the membranes; therefore, the weight increase in the Fe-Py membranes was low. It is thought that because polypyrrole existed on the membrane surfaces as layers in the Py-Fe membranes, water adsorption by the anion-exchange membranes sandwiched with polypyrrole layers was not restricted by the polypyrrole. The ion-exchange capacity of both Py-Fe and Fe-Py membranes did not appreciably increase, though the membrane weight increased in the presence of the polymer having weakly basic anion-exchange groups. The amount of quaternary ammonium groups decreased due to the increase in the total membrane weight: 2.16 mequiv/g of dry membrane of the Py-Fe membrane from NEOSEPTA AM-1 was broken down quaternary ammonium groups of 1.71 mequiv/g of dry membrane and weakly basic anion-exchange groups of 0.45 mequiv/g of dry membrane. The amount of the weakly basic anion exchange groups was calculated to be 3.09 mequiv/g of dry membrane from the weight increase (calculated anion-exchange capacity of polypyrrole is 9.65 mequiv/g of dry resin and the weight increase 32.0%). Only a part of the secondary amino groups of the polypyrrole in the composite membranes acted as anion exchange groups. It was reported on the basis of STM analysis that polypyrrole has two types of helical structures with diameters of 1.5-1.8 nm, a simple helix, and 5-6 nm, a superhelix.20 It is thought that dissociation of the secondary amino groups of the polypyrrole on and in the composite membrane might be restricted due to the rigidity of the polymer. The electrical resistance of both composite membranes increased compared with the respective anion exchange membranes; especially, the Py-Fe membranes and the thickness of the PyFe membranes increased considerably. Because polypyrrole is a conducting polymer, the electrical conductivity of the dry composite membranes was measured to determine the degree of polymerization of pyrrole. Tables 3 and 4 show the electrical conductivity of the Fe-Py membranes and the Py-Fe membranes both parallel to the plane
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J. Phys. Chem., Vol. 100, No. 41, 1996 16637
TABLE 3: Electrical Conductivity of the Composite Membranes Prepared from Anion-Exchange Membranes and Pyrrole (Fe-Py Membrane)a direction parallel to plane
with blotting × 105 (σ cm-1) without blotting × 105 (σ cm-1)
across cross section
NEOSEPTA AM-1
NEOSEPTA AM-2
NEOSEPTA AM-3
NEOSEPTA AM-1
NEOSEPTA AM-2
NEOSEPTA AM-3
24490 14000
20510 8350
5195 3050
29.5 31.3
10.6 7.40
4.79 2.68
a After anion-exchange membranes had been equilibrated with a 2.0 mol/L ferric chloride solution, the membranes were immersed in an aqueous 0.745 mol/L pyrrole solution.
TABLE 4: Electrical Conductivity of the Composite Membranes Prepared from Anion-Exchange Membranes and Pyrrole (Py-Fe Membrane)a direction parallel to plane
across cross section
membrane
NEOSEPTA AM-1
NEOSEPTA AM-2
NEOSEPTA AM-3
NEOSEPTA AM-1
NEOSEPTA AM-2
NEOSEPTA AM-3
0.5 mol/L FeCl3 (σ cm-1) × 105 1.0 mol/L FeCl3 (σ cm-1) × 105 2.0 mol/L FeCl3 (σ cm-1) × 105
55960 48190 27800
27780 23940 9660
17180 13290 6060
0.028 0.020 0.016
0.020 0.016 0.016
0.010 0.008 0.006
a After anion-exchange membranes had been equilibrated with an aqueous 0.745 mol/L pyrrole solution, the membranes were immersed in respective ferric chloride solutions for 12 h.
of the membrane surfaces and across the cross section of the membranes. In the case of the Fe-Py membranes, conductivity existed across the cross section of the membrane, which means that the polypyrrole existed in the interior of the membranes (about 1/1000 of the conductivity parallel to the plane). The conductivity of the composite membranes after blotting with filter paper was higher than that without blotting. This might be due to the fact that excess ferric ions adhering on the membrane surfaces restricted the adsorption of pyrrole molecules on the membrane surfaces and the pyrrole polymerized in the solution. On the other hand, because the Py-Fe membranes were almost insulators across the cross section as shown in Table 4, the conductivity of the strip of the Py-Fe membranes arose from the polypyrrole layers on the membrane surfaces. The conductivity decreased with increasing concentration of the ferric chloride solution. It is thought that because the anionexchange membranes in which pyrrole was adsorbed should shrink on being immersed in a highly concentrated ferric chloride solution, many pyrrole molecules were excluded from the membranes into the solution and polymerized in the solution. Thus, the thickness of the polypyrrole layers might be decreased. The results of the conductivity measurements agreed with the difference in morphology of both composite membranes which were observed in a micrograph. On the basis of these results, polypyrrole existed mainly on the membrane surfaces in the Py-Fe membranes. To examine the pore size of the anion-exchange membranes and the composite membranes, the permeability coefficient of neutral molecules, such as urea (Stokes radius: 3.3 Å), through anion-exchange membranes and the corresponding Fe-Py and Py-Fe membranes was measured (Figure 3). Though it is reasonable that the permeability coefficient decreased with decreasing ion-exchange capacity of the anion-exchange membranes, the coefficients of the Fe-Py membranes was lower than those of the corresponding anion-exchange membranes and especially those of the Py-Fe membranes remarkably decreased. Because thin polypyrrole layers existed on the membrane surfaces in the Py-Fe membranes, this implied that pore size range of the polypyrrole membrane was very narrow. (2) Electrodialytic Transport Properties of the Composite Membranes. It is expected that the relative transport number between two anions changes due to the existence of polypyrrole
Figure 3. Change in permeability coefficient of a neutral solute (urea) with ion-exchange capacity. (0) NEOSEPTA AM-1, AM-2, AM-3; (O) Fe-Py membranes; (4) Py-Fe membrane A 2.0 mol/L urea solution was dialyzed for 2 days at 25.0 °C. Ion-exchange capacity on the horizontal axis represents values before preparation of the composite membranes.
Figure 4. Effect of concentration of sodium chloride and sodium sulfate solution on PClSO4. (0) NEOSEPTA AM-1; (O) Fe-Py membrane from NEOSEPTA AM-1.
on and in the anion-exchange membranes. Figure 4 shows the change in the relative transport number of sulfate ions to chloride ions of NEOSEPTA AM-1 and the corresponding Fe-Py membrane with the concentration of the mixed salt solution. (The membranes were used after equilibration with 1.0 N hydrochloric acid solution before equilibration with the solution
16638 J. Phys. Chem., Vol. 100, No. 41, 1996
Figure 5. Effect of ion-exchange capacity of anion exchange membranes used on PClSO4. (0) NEOSEPTA AM-1, AM-2, AM-3; (O) FePy membranes; (4) Py-Fe membranes. Concentration of the mixed salt solution, 0.04 N. Ion-exchange capacity on the horizontal axis represents values before preparation of the composite membranes.
Figure 6. Effect of concentration of sodium chloride and sodium nitrate solution on PClNO3. (0) NEOSEPTA AM-1; (O) Fe-Py membrane from NEOSEPTA AM-1; (∆) Py-Fe membrane from NEOSEPTA AM-1.
to be used in electrodialysis to dissociate the secondary amino groups of polypyrrole.) Apparently, the relative transport number of the Fe-Py membrane decreased compared with that of NEOSEPTA AM-1. The hydrophobicity of anion-exchange groups was reported to remarkably affect the permselectivity of anions through the membrane.16 It is thought that because sulfate ions are large and hydrophilic compared with chloride ions,21 the decrease in permeation of sulfate ions was due to sieving of smaller anions from the larger by the rigid polypyrrole and lower affinity of sulfate ions for the composite membranes as a result of increasing hydrophobicity of the membrane due to the polypyrrole. Thus, the relative transport number of the Fe-Py and Py-Fe membranes prepared from anion-exchange membranes with different ion-exchange capacity (with different hydrophobicity) was compared. Though the relative transport number of conventional anion-exchange membranes increased slightly with increasing ion exchange capacity (increasing hydrophilicity), the values of both Py-Fe and Fe-Py membranes were almost constant and independent of ion exchange capacity as shown in Figure 5. Permeation of sulfate ions through the Py-Fe membranes was especially difficult. The fact that the transport number of sulfate ions relative to chloride ions was constant regardless of the ion-exchange capacity means that permeation of sulfate ions was controlled only by polypyrrole, which existed mainly on the membrane surfaces in both the Fe-Py and Py-Fe membranes. The polypyrrole layers on the membrane surfaces are too tight to be permeable to sulfate ions and less hydrophilic for ion exchange of sulfate ions. Figure 6 shows the change in transport number of nitrate ions relative to chloride ions with the concentration of the mixed salt solution. In general, conventional anion-exchange membranes are selectively permeated by nitrate ions relative to
Sata et al.
Figure 7. Effect of ion-exchange capacity of anion-exchange membranes used on PClNO3. (0) NEOSEPTA AM-1, AM-2, AM-3; (O) FePy membranes; (4) Py-Fe membranes. Concentration of the mixed salt solution, 0.04 N. Ion-exchange capacity on the horizontal axis represents values before preparation of the composite membranes.
chloride ions due to selective ion exchange of nitrate ions with the anion-exchange membrane.22 Both Fe-Py and Py-Fe membranes permitted selective permeation of nitrate ions compared with the conventional anion exchange membranes. Especially, the relative transport number was about 5 in the Py-Fe membranes. Because polypyrrole on and in the membranes enhanced the selective permeation of nitrate ions through the membranes, the effect of the ion-exchange capacity of the membranes on the relative transport number was examined. Figure 7 shows that nitrate ions permeated more selectively through the Py-Fe membranes than the Fe-Py membranes, independent of the ion-exchange capacity of the membrane similar to the transport number of sulfate ions relative to chloride ions (The relative transport number of conventional anion exchange membranes decreased with increasing ionexchange capacity; permeation of nitrate ions decreased with increasing hydrophilicity of the membrane due to less hydration of nitrate ions compared with chloride ions.21 It seems that the polypyrrole controlled the permeation of nitrate ions through the membranes. It is well-known that the relative transport number is a product of the ion-exchange equilibrium constant and the ratio of the mobilities of two ions in the membrane phase. Nitrate ions are bulkier than chloride ions; therefore, the mobility of nitrate ions was lower than that of chloride ions. (The electrical resistance of the Py-Fe membrane from NEOSEPTA AM-1 was 3.1 Ω cm2, measured in a 0.50 N sodium chloride solution, and 5.4 Ω cm2, measured in a 0.50 N sodium nitrate solution.) The selective permeation of nitrate ions through the composite membranes was due to selective ion exchange of nitrate ions with the composite membranes compared with chloride ions. (The ion-exchange equilibrium constant, KClNO3, of the Py-Fe membrane from NEOSEPTA AM-1 was 4.84 and that of NEOSEPTA AM-1 was 3.63 when the membranes had been equilibrated with a 1:1 solution of sodium chloride and sodium nitrate; concentration of sodium ions: 0.04 N.) Because nitrate ions are less hydrated than chloride ions, nitrate ions were selectively ion-exchanged with increasing hydrophobicity of the ion-exchange membranes.16 Because the polypyrrole, which mainly existed on the membrane surfaces in both Fe-Py and Py-Fe membranes, has secondary amino groups, the composite membranes were hydrophobic compared with the anionexchange membrane with quaternary ammonium groups. In fact, the water content of the Fe-Py membranes remarkably decreased. Figure 8 shows the relationship of the relative transport number between bromide ions and chloride ions to the concentration of the mixed salt solution in the composite membranes
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Figure 8. Effect of concentration of sodium chloride and sodium bromide solution on PClBr. (0) NEOSEPTA AM-1; (O) Fe-Py membranes; (4) Py-Fe membranes.
Figure 10. AMF image of NEOSEPTA AM-1.
Figure 9. Effect of ion-exchange capacity of anion-exchange membranes used on PClBr. (0) NEOSEPTA AM-1, AM-2, AM-3; (O) FePy membranes; (4) Py-Fe membranes. Concentration of the mixed salt solution, 0.04 N. Ion-exchange capacity on the horizontal axis represents values before preparation of the composite membranes.
Figure 11. AFM image of the Fe-Py composite membrane from NEOSEPTA AM-1.
prepared from NEOSEPTA AM-1. The relative transport number decreased in both composite membranes (Fe-Py and Py-Fe). Bromide ions have a larger ionic diameter and are less hydrated than chloride ions.21 If the decrease in bromide ion permeation is due to the sieving of bromide ions by the polypyrrole, the relative transport number of the Py-Fe membranes, in which the effect of the polypyrrole becomes remarkable, should be lower than that of the Fe-Py membrane (Figure 3). However, the relative transport number of the PyFe membrane was higher than that of the Fe-Py membrane. Figure 9 shows the change in transport number of bromide ions relative to chloride ions with the ion-exchange capacity of the membranes in conventional anion-exchange membranes, the Py-Fe membranes, and the Fe-Py membranes. PClBr decreased with increasing ion-exchange capacity in the conventional anionexchange membranes, which means that bromide ions easily permeated through the membranes with high hydrophobicity. Polypyrrole is a hydrophobic polymer compared with the anionexchange membrane due to less dissociation of the secondary amino groups of the polypyrrole. It is thought that though the rigidity of the polypyrrole predominantly affected the decrease in the relative transport number (sieving effect), the hydrophobic affinity between the polypyrrole, and bromide ions partially cancelled the sieving effect. Thus, PClBr of the Py-Fe membranes was higher than that of the Fe-Py membranes. It is considered that there is an optimum amount of the polypyrrole on and in the membrane which decreases or increases the permeation of bromide ions. The current efficiency of the composite membranes and the voltage drop across a membrane was measured by electrodialysis. The current efficiency was more than 99% in all composite membranes, and the electrical resistance calculated from the voltage drop across a membrane
corresponded to the electrical resistance of the membranes measured at 1000 Hz ac. The Fe-Py membranes were prepared by immersing the membranes into an aqueous pyrrole solution without blotting with filter paper after the membrane had been equilibrated with 2.0 N ferric chloride solution. Because ferric chloride solution adhering on the membrane surfaces acted as an oxidizing agent to polymerize pyrrole, polypyrrole should exist on the membrane surfaces, though the micrograph did not show the layer in the Fe-Py membrane. Figures 10 and 11 show AFM images of NEOSEPTA AM-1 and the Fe-Py membrane prepared from the NEOSEPTA AM-1. Though the membrane surfaces were flat before introduction of anion-exchange groups, a convex surface was observed in the anion-exchange membrane as shown in Figure 11 (because a copolymer membrane of chloromethylstyrene and divinylbenzene exists in the membrane forming a microdomain, the domain swelled after introduction of the anion-exchange groups and the convex surface was formed).16 After polymerization of pyrrole (Fe-Py membrane), a mountainlike convex surface was observed (Figure 11). Because ferric ions which adhered on the membrane surfaces acted as an oxidizing agent, the Fe-Py membrane also had a polypyrrole layer, which was revealed by AMF. Conclusions Composite membranes with polypyrrole layers and in which polypyrrole homogeneously existed throughout the membranes were prepared by chemical oxidation of pyrrole in commercial anion-exchange membranes. This was confirmed by measurements of electrical conductivity parallel to the plane and across a cross section of the membrane. The permeability coefficient of urea through the membranes revealed that the pore size range of the composite membranes
16640 J. Phys. Chem., Vol. 100, No. 41, 1996 became very narrow compared with that of the anion-exchange membranes. Measurements of the relative transport number of sulfate ions, nitrate ions, and bromide ions relative to chloride ions in electrodialysis suggested that permeation of large and hydrophilic anions, sulfate ions, through the composite membranes was difficult, and the less hydrated anions, nitrate ions, selectively permeated through the membranes compared with chloride ions. Though permeation of bromide ions through the composite membranes became difficult, the formation of very tight polypyrrole layers on the anion-exchange membranes allowed permeation of bromide ions compared with the membranes with less tight layers. A remarkable change in the relative transport number between various anions and chloride ions arose due not only to a sieving effect by the rigid polypyrrole but also to a change in the hydrophilicity of the anion-exchange membranes. References and Notes (1) Sata, T. Macromolecules; Kahovec, J., Ed.; VSP: Utrecht, The Netherlands, 1992; p 451. (2) Maeda, Y.; Tsuyumoto, M.; Karakane, H.; Tsugaya, H. Polym. J. 1991, 23, 501. Hirotsu, T.; Arita, A. J. Appl. Polym. Sci. 1991, 42, 3225. Simons, R.; Zuccon, J.; Dickson, M. R.; Shaw, M. J. Membr. Sci. 1993, 78, 63. (3) Diaz, A. F.; Kanazawa, K. K.; Gardini, G. P. J. Chem. Soc., Chem. Commun. 1979, 653. (4) Khulbe, K. C.; Mann, M. S. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 1089.
Sata et al. (5) Kanazawa, K. K.; Diaz, A.; Geiss, R. H.; Gill, W. D.; Kwark, J. K.; Logan, J. A.; Rabolt, J. F.; Street, G. B. J. Chem. Soc., Chem. Commun. 1979, 854. (6) Tsai, E. W.; Pajkossy, T.; Rajeshwar, K.; Reynolds, J. R. J. Phys. Chem. 1988, 92, 3560. (7) Ehrenbeck, C.; Juttner, K. Electrochim. Acta 1996, 41, 511. (8) Hepel, M. Electrochim. Acta 1996, 41, 63. (9) Sata, T.; Ogura, S.; Kishimoto, F. J. Membr. Sci. 1993, 84, 259. (10) Sata, T. J. Phys. Chem. 1993, 97, 6929; J. Chem. Soc., Chem. Commun. 1993, 1122. (11) Sata, T.; Saeki, K. J. Chem. Soc., Chem. Commun. 1989, 230. (12) Sata, T. Electrochim. Acta 1994, 39, 131. (13) Onoue, Y.; Mizutani, Y.; Yamane, R.; Takasaki, Y. Denki Kagaku 1961, 29, 544. (14) Soga, K.; Tomizawa, S. Bull. Soc. Sea Water Sci. Jpn. (Nippon En Gakkai-Shi) 1962, 16, 24. (15) Sata, T.; Yamane, R.; Mizutani, Y. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 2071. (16) Sata, T.; Yamaguchi, T.; Matsusaki, K. J. Phys. Chem. 1995, 99, 12875. (17) Sata, T.; Funakoshi, T.; Akai, K. Macromolecules 1996, 29, 4029. (18) Muto, G. Hishoku-Bunseki-Ho (Colorimetric Analysis Method); Kyoritsu Shuppan: Tokyo, 1955; p 161. (19) Sata, T.; Teshima, K.; Yamaguchi, T. J. Polym. Sci., Polym. Chem. Ed. 1996, 34, 1475. (20) Yang, R.; Evans, D. F.; Christensen, L.; Hendrickson, W. A. J. Phys. Chem. 1990, 94, 6117. (21) Ohtaki, H. Hydration of Ions; Kyoritsu Shuppan: Tokyo, 1992; 30. (22) Helfferich, F. Ion Exchange; McGraw-Hill: New York, 1960; p 175.
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