J. Phys. Chem. 1995,99, 12875-12882
12875
Effect of Hydrophobicity of Ion Exchange Groups of Anion Exchange Membranes on Permselectivity between Two Anions 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: January 12, 1995; In Final Form: May 15, 1 9 9 p
To prepare anion exchange membranes which permeate specific anions selectively in electrodialysis, anion exchange membranes with various anion exchange groups were prepared and the relative transport number of various anions compared with chloride ions was examined. Anion exchange groups introduced in the membranes were trimethylbenzylammonium, triethylbenzylammonium, tri-n-propylbenzylammonium, tri-nbutylbenzylammonium, and tri-n-pentylbenzylaonium, in order of increasing hydrophobicity. The relative transport number of various anions compared with chloride ions changed remarkably with increasing chain length of alkyl groups (increasing hydrophobicity of the groups). The relative transport number of highly hydrated anions, such as sulfate ions and fluoride ions, compared with chloride ions decreased with increasing hydrophobicity of the groups. On the other hand, less hydrated anions such as bromide ions and nitrate ions compared with chloride ions permeate selectively through the membrane with increasing hydrophobicity of the groups. For example, the relative transport number of nitrate ions compared with chloride ions increased from 1.58 (membrane with trimethylbenzylammonium groups) to 16.5 (membrane with tri-n-pentylbenzylammonium groups) in electrodialysis of a 1:1 mixed solution of sodium nitrate and sodium chloride (0.04 N as sodium ion concentration). However, the increase in hydrophilicity of the membrane by further reaction of the remaining chloromethyl groups of the membrane with trimethylamine caused the relative transport number between them to decrease.
Introduction Ion exchange membranes have been widely used in various industrial fields: electrodialysis to concentrate or deionize aqueous electrolyte solutions, diffusion dialysis to recover acid or alkali from waste acid or alkali solutions, as a separator for electrolysis as in chlor-alkali production, etc. One of unsolved problems of the ion exchange membrane is poor permselectivity for specific ions. Namely, when given specific ions should be concentrated or removed from a solution, conventional ion exchange membranes are ineffective. For this reason, several specific ion exchange membranes were developed and used in industry, e.g., monovalent cation or anion permselective membranes, which have been used in electrodialytic concentration of sea water to produce edible salt,' antiorganic-fouling membra ne^,^.^ through which large organic ions like ionic surface active agents cannot permeate, proton permselective ion exchange membrane: etc. In recent years, requirements for ion exchange membranes having selectivity for specific ions have been increasing. For example, the concentration of nitrate ions in groundwater has greatly increased in parts of some European countries because of excess use of artificial fertilizer. This is a serious problem because nitrate ions are hannful to human health. European Community proposed that the concentration of nitrate ions in drinking water should be below 25 ppm. However, the concentration of nitrate ions in groundwater of some places exceeds 50 ppm. To solve this problem, various methods to remove nitrate ions from groundwater have been proposed and A requirement for this water treatment is to preserve the properties of natural water as much as possible. Electrodialysis is one of the promising methods if a nitrate ion permselective anion exchange membrane is developed. @Abstractpublished in Advance ACS Absfructs, August 1, 1995.
0022-365419512099- 12875$09.00/0
Recently also, separation membranes, which perform separations of high level, for resolution of racemic compounds, for isotope separation, for separation of blood plasma, etc., have been actively studied. Separation between ions with the same sign and the same valence also requires similar research. Methods to separate ions with the same sign that have been studied are classified as follows:8 (1) sieving of small ions from large ions by highly cross-linked membranes or formation of a highly cross-linked layer on the membrane surface (based on the difference of ion size), (2) utilization of the difference of electrostatic repulsion forces between different ions by an oppositely charged ion exchange group layer on the membrane surface, and (3) specific interaction between specific ion exchange groups or the membrane matrix and particular ions. Ions migrate in the solution and permeate through the membrane accompanied by hydrated water. Thus, it is important for this research to examine the interrelation between hydration of ions and hydrophobicity of the membrane. After we had examined permselectivity between anions through various anion exchange membranes, we found a new method to control the permeation of specific ions through the ion exchange membrane, Le., control of hydrophobicity of anion exchange groups. In this work, anion exchange membranes having various anion exchange groups with different hydrophobicity were prepared and the permselectivity of various anions compared with chloride ions was examined.
Experimental Section Materials. a. Chemicals. Vinyl monomers for membrane preparation, chloromethylstyrene,obtained from Seimi Chemical Co., Ltd., and divinylbenzene (the purity of divinylbenzene is 55%: a mixture of 0-, p-divinylbenzene, ethylvinylbenzenes, and diethylbenzenes), from Sankyo Kasei Kogyo Co., Ltd., were used without further purification. Acrylonitrile-butadiene 0 1995 American Chemical Society
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TABLE 1: Characteristics of Anion Exchange Membranes Used in This Study
electric resist." transport no.b ion exchange capacityC water contentd thickness (mm) reinforcing
M- 1 trimethyl-benzyl
M-2 triethyl-benzyl
M-3 tri-n-propyl-benzyl
M-4 tri-n-butyl-benzyl
M-5 tri-n-pentyl-benzyl
1.1 >0.98 2.20 0.28 0.136 PVC
2.5 >0.98 1.62 0.23 0.130 PVC
7.9 >0.98 1.14 0.13 0.137 PVC
12.2 >0.98 1.11
143 >0.98
0.08 0.138 PVC
0.70 0.07 0.1 10 PVC
51 cm2; measured with 1000 Hz ac at 25.0 "C after equilibration with 0.500 M sodium chloride solution. Measured by electrodialysis with 0.50 M sodium chloride solution at 2 A/dm2. milliequiv/g dry membrane in C1- form. g HzO/g dry membrane.
TABLE 2: Characteristics of Anion Exchange Membranes Used in This Study (after Further Amination with Trimethylamine)
electric resist." transport no.b ion exchange capacityC water contentd thickness (mm) reinforcing
M-1' trimethyl-benzyl
M-2' triethyl-benzyl
M-3' tri-n-propyl-benzyl
M-4' tri-n-butyl-benzyl
M-5' tri-n-pentyl-benzyl
1.1 '0.98 2.20 0.28 0.135 PVC
2.5 '0.98 1.88 0.25 0.145 PVC
4.1 >0.98 1.61 0.26 0.143 PVC
6.7 >0.98 1.51 0.22 0.141 PVC
13.1 '0.98 1.58 0.18 0.118 PVC
C2 cm2; measured with 1000 Hz ac at 25.0 "C after equilibration with 0.500 M sodium chloride solution. Measured by electrodialysis with 0.50 M sodium chloride solution at 2 A/dm2. milliequiv/g dry membrane in CI- form. g Hz O/g dry membrane. (I
rubber (NBR) was obtained from Japan Synthetic Rubber Co., Ltd. Benzoyl peroxide (an initiator to polymerize the monomers), amines such as trimethylamine, triethylamine, tri-npropylamine, tri-n-butylamine, and tri-n-pentylamine, and solvents such as methyl alcohol and ethyl alcohol were obtained from Wako Pure Chemical Industries Ltd. Sodium chloride, sodium sulfate, sodium nitrate, sodium bromide, sodium thiocyanate, sodium fluoride, hydrochloric acid, ammonia solution, sodium hydroxide, and other reagents, obtained from Wako Pure Chemical Industries Ltd., were of reagent grade. These were also used without further purification. Deionized water was used in all experiments. b. Preparation of Anion Exchange Membranes. Anion exchange membranes with different anion exchange groups were prepared by reaction of the membranous copolymer composed of chloromethylstyrene and divinylbenzene with various amines, trimethylamine, triethylamine, tri-n-propylamine, tri-n-butylamine, and tri-n-pentylamine. The copolymer membrane was prepared by copolymerizing chloromethylstyrene with divinylbenzene in the presence of acrylonitrile-butadiene rubber and backing fabric. Chloromethylstyrene (82 wt %) and divinylbenzene (10 wt %) (0-and p-divinylbenzene) were mixed, and 5 wt % of acrylonitrile-butadiene rubber was dissolved in the vinyl monomer mixture to give mechanical strength to the membrane. After 3 wt % of benzoyl peroxide had been added to the mixture, the obtained pasty material was coated on a woven fabric made of poly(viny1 chloride), Teviron cloth, Teijin Co., Ltd., and polymerized at 80 "C for 16 h under nitrogen atmosphere after covering with a polyester film on both sides (the cross-linking degree of the membrane was 10%). After polymerization,the obtained copolymer membrane reacted with the amines: an aqueous 1 M trimethylamine solution for 24 h at 30 "C, a 1 M triethylamine methyl alcohol solution for 48 h at 50 "C, a 1 M tri-n-propylamine methyl alcohol solution for 48 h at 50 "C, a 1 M tri-n-butylamine methyl alcohol solution for 72 h at 50 "C, and a 1 M tri-n-pentylamine ethyl alcohol solution for 96 h at 50 "C. The reaction was carried out until the ion exchange capacity attained a constant value in each amine. After reaction, the membranes were thoroughly washed with water, methyl alcohol or ethyl alcohol, and then a 1.0 N hydrochloric acid solution. Then the membranes were equilibrated with an aqueous 1.0 N hydrochloric acid solution and a
Figure 1. Apparatus for measurement of transport properties of the anion exchange membrane: C, cation exchange membrane; A, anion exchange membrane to be measured; X, Sod2-, N03-, F-, Br-; 1, 4, Ag-AgC1 electrodes; 2, 3, Ag-AgC1 wire probe electrodes; effective membrane area, 10 cm2; capacity of each compartment, 100 cm3.
0.5 N ammonia solution altemately, and then stored in a 0.5 N sodium chloride solution. Other types of membranes were prepared: the membranes except the membrane reacting with trimethylamine were immersed in the aqueous 1.0 M trimethylamine solution in order to make the remaining chloromethyl groups in the membrane react with trimethylamine. The characteristics of the membranes which reacted with various amines are shown in Table 1. Table 2 shows the characteristics of the membranes which reacted with various amines and then with trimethylamine. Before use, the membranes were equilibrated with a mixed solution to be used in electrodialysis. c. Appnratus. A four-compartment cell with two kinds of Ag-AgC1 electrodes, which are for current supply and measurement of voltage drop by the membrane, was used to measure transport properties of the anion exchange membranes (Figure 1). The effective membrane area of the cell was 10 cm2 (2 cm x 5 cm), and the capacity of each compartment was 100 cm3. The area of the Ag-AgC1 electrodes for current supply was 4.0 cm x 10.0 cm; the electrodes were bent in a wave-like shape in the cell. Wire probe electrodes of Ag-AgC1 were placed close to the membrane surfaces (about 2 mm from the membrane surfaces). Electrodialysis can be carried out at the current density of 200 mA for 2 h without any pH change of the solution using this cell. The sodium chloride solution was used as an anolyte and a catholyte, which were separated by cation exchange membranes (NEOSEFTA CM-2, made by Tokuyama Corp.; transport number of sodium ions in the membrane is more than 0.99 in electrodialysis of a 0.5 N sodium chloride solution at the current density of 20 mA/cm2; electric resistance is 2.8 P cm2). The two middle compartments were filled with a 1:l
J. Phys. Chem., Vol. 99, No. 34, 1995 12877
Hydrophobicity of Ion Exchange Groups mixed salt solution: sodium sulfate and sodium chloride, sodium nitrate and sodium chloride, sodium fluoride and sodium chloride, sodium bromide and sodium chloride, and sodium thiocyanate and sodium chloride. Their concentration was 0.04 N as sodium ion concentration. The anolyte and the catholyte were 0.04 N sodium chloride solutions. Electricity was measured by a coulometer, Nikko Digital Coulomb Meter NDCM-4, Nikko Keisoku Ltd. Voltage drop by the membrane was measured with Ag-AgC1 probe electrodes and recorded by an X-t recorder, Toa Electronics Co., EPR-2T. d . Measurements of Transport Properties of Anion Exchange Membranes. The transport properties of the anion exchange membranes measured were relative transport number between the two anions, current efficiency, and voltage drop by the membrane. The relative transport number between two anions was defined as follows:
where t~ and tcl are transport numbers of anion A and chloride ions in the membrane and CAand CCIare average concentrations of anion A and chloride ions before and after electrodialysis. PclA means the permeated equivalent of anion A through the membrane when 1 equiv of chloride ions permeates through (because a 1:l mixed solution was used). e. Procedure. After an anion exchange membrane had been placed in the cell, the two middle compartments were filled with 100 cm3 of the mixed salt solution and both anolyte and catholyte were filled with sodium chloride solution (100 cm3). Electrodialysis was carried out at the current density of 1 mA/ cm2 under vigorous agitation (1500 rpm with stirrers) for 60 min at 25.0 "C. After electrodialysis, the solutions were analyzed by the Mohr method (for Cl-), conventional chelate back titration (for Sod2-), or ion chromatography (TOSHO 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 concentrations of anions in each compartment, and the electricity was measured with a coulometer. f. Measurement of Permeability of Urea. To estimate the change in pore size of the membranes, the permeability of urea, a neutral molecule, was measured. The measurement was carried out using a two-compartment cell under vigorous agitation (1500 rpm) at 25.0 "C. After an aqueous 2 M urea solution had filled the concentrated compartment of the cell (250 cm3) and pure water had filled the dilute compartment, the permeated amount of urea into the water was analyzed after 48 h. The urea was analyzed by HPLC (Hitachi L-6000, RI Monitor L-3350), and the permeability was calculated with the following equation:
P=
Am
- D' )
where Am is the permeated amount of urea; A , the effective membrane area (20 cm2); t, the diffusion period (s); Cc, the average concentration of the concentrated compartment during dialysis; and CD,the average concentration of the dilute compartment. g. Analysis of Anion Exchange Membranes. Morphological analysis of the membranous copolymer (before introduction of anion exchange groups) was performed by scanning transmission electron microscope (STEM), JOEL JEM-200 CX. The sample was prepared by ultramicrotomy, LBK 4800, after having been stained by osmium tetraoxide, as described in a previous paper.9
0.3
0.1
0
C,
CZ
C3
C4
Caibon Number of Alkyl Chains bonded to Ammonium Groups
Figure 2. Effect of species of anion exchange groups on PcP04: (0) reacted with various amines; (0)reacted with various amines and then with trimethylamine. Concentration of the mixed salt solution is 0.04 N as sodium ions.
The membrane surface before and after introducing anion exchange groups to the membranous copolymer was also observed by AFM (atomic force microscope), Digital Instrument Corp. Nanoscope 11,to visualize introduction of anion exchange groups to the membrane.
Results and Discussion 1. Relative Transport Numbers of Various Anions Compared with Chloride Ions in Anion Exchange Membranes with Different Anion Exchange Groups. It is important to clarify the interrelation between hydrophobicity of anion exchange groups of the membrane and hydration of anions, and permselectivity between tuso anions. In this work, chloride ions were selected as a reference anion. In general, when the carbon number of alkyl groups of the surface active agent increases, the hydrophobicity of the agent increases. This is defined as a hydrophilic-lipophilic balance (HLB value):I0 HLB = E(hydrophilic group numbers) - E(hydrophobic group numbers) 7.l' HLB values of amines that were bonded to the membranous copolymer as anion exchange groups, trimethylamine, triethylamine, tri-n-propylamine, tri-n-butylamine, and tri-n-pentylamine, are 15.0, 13.6, 12.0, 10.7, and 9.3, respectively, which x e calculated from HLB groups numbers (N(tertiaq amine) 9.4; -CH;?- 0.475). When these amines with different HLB values are introduced in the membranous copolymer as anion exchange groups, it is expected that the hydrophobic effect of anion exchange groups on the relative transport number between two anions will become clear. Figure 2 shows the change in Pcls04 with the carbon number of the alkyl groups bonded to quaternary ammonium groups. The permeation of sulfate ions through the membranes becomes difficult with increasing hydrophobicity. Since sulfate ions are bulky and hydrophilic compared with chloride ions, it is reasonable that the hydrophilic ions are difficult to permeate through the hydrophobic membranes. It is apparent from Figure 2 that the hydrophobic effect on the relative transport number becomes remarkable starting with propyl groups. Since the ion exchange capacity of the membranes decreased with increasing chain length of alkyl groups because of the bulkiness of amines (steric hindrance), as shown in Table 1, unreacted chloromethyl groups in the membrane reacted with trimethylamine further. As shown in Figure 2, PclSo4 increased with reaction with trimethylamine. This result shows clearly the effect of hydrophilicity of the membrane on permselectivity of sulfate ions compared with chloride ions. There are many excellent studies on hydration of ions.'* From these studies, the Gibbs energy of hydration of various anions was cited in Table 3.13 AGO of sulfate ions is -1000 kJ/mol, which is the largest among anions
+
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TABLE 4: Ion Exchange Equilibrium Constant between Nitrate Ions and Chloride Ions in Anion Exchange Membranes with Various Ion Exchange Group# ion exchange groups KclN03 trimethylbenzylammonium 3.63 triethylbenzylammonium 7.39 tri-n-propylbenzylammonium 7.75 tri-n-butylbenzylammonium
8.20
After the anion exchange membranes had been equilibrated with a 1:l mixed solution of 0.02 N sodium nitrate and 0.02 N sodium chloride (concentration of sodium ions: 0.04 N), ions in the membranes were eluted with 0.2 N sodium nitrate solution and chloride ions in the eluent were analyzed by the Mohr method. The amount of nitrate ions was calculated by subtracting the amount of chloride ions from the ion exchange capacity of the membranes. &lN03 = (CNO,~/CCI~)/(CNO?/ CC?). C N O , ~the , amount of nitrate ions ion-exchanged in the , amount of chloride ions ion-exchanged in the membrane; C C I ~the , ~ CC?,concentrations of nitrate ions and chloride membrane; C N O and ions in the solution in which the membranes were equilibrated. a
C!
I
I
I
I
C2
C,
C 4
CS
Cnrbon Nuinber of Alkyl Chains bonded to Ammonium Groups
Figure 3. Effect of species of anion exchange groups on P c I ~ O ~(0) : reacted with various amines; (0)reacted with various amines and then with trimethylamine. Concentration of the mixed salt solution is 0.04 N as sodium ions. TABLE 3: Gibbs Hydration Energy of Various Anions13 ionic species -AGho (kJ/mol)
c1FBrNO,SO42-
317 433.9 303 270 1000
used in the measurement. Sulfate ions are strongly hydrated: hydrophilic anions. Figure 3 shows the change in PclN03 with the carbon number of alkyl groups bonded to the ammonium groups. PclN03 increased remarkably with increasing chain length of the alkyl groups. Kedem et al.I4 reported that the anion exchange membranes with tertiary amino groups show nitrate ion permselectivity. However, the tertiary amino groups do not dissociate at the pH of a neutral salt solution, for example, the pH of natural water. Such membranes show high electric resistance and low transport number in the neutral salt solution. Even if a thin layer with tertiary amino groups is laminated on a thick anion exchange membrane having quatemary ammonium groups to decrease the electric resistance, concentration polarization occurs at an interface between the two layers. However, the anion exchange membranes having anion exchange groups with long alkyl chain groups did not show any pH change of the solution during electrodialysis and kept high current efficiency, although electric resistance was high (the voltage drop by the membrane also did not change during electrodialysis). High permselectivity of nitrate ions is due to hydrophobicity of nitrate ions compared with chloride ions (Table 3). Accordingly, PclN03 decreased after the membranes reacted with trimethylamine to introduce trimethylbenzylammonium groups to the remaining chloromethyl groups in the membranes. Table 4 shows the ion exchange equilibrium constant between nitrate ions and chloride ions in the membranes with various anion exchange groups. KclN03 increases with increasing hydrophobicity of anion exchange groups. The relative transport number is a product of the ratios of mobilities of the two anions to the ion exchange equilibrium constant between two anions ( P a A = KciA(U~/Ucl), where UA is the mobility of anion A; Ucl, mobility of C1-; KclA, ion exchange equilibrium constant between anion A and C1-). Since the mobility of nitrate ions in the membrane was slightly lower than that of chloride ions, the increase in KclN03 with increasing hydrophobicity of anion exchange groups plays an important role of selective permeation of nitrate ions, Le., selective uptake of nitrate ions by hydrophobic anion exchange groups. This suggests that formation of a thin layer of hydrophobic anion exchange groups on the desalting side of the membrane with low electric resistance
-
UU
a
0.1
0
CI
C2
c,
c4
Carhon Number of Alkyl Chains bonded to Ammonium Groups
Figure 4. Effect of species of anion exchange groups on P c ~ :(0) reacted with various amines; (0)reacted with &ous amines and then with trimethylamine. Concentration of the mixed salt solution is 0.04 N as sodium ions. provides the membrane with high nitrate ion permselectivity and low electrical resistance. Figure 4 shows the change in relative transport number of fluoride ions, which are highly hydrated anions, compared with chloride ions. The relative transport number decreased with increasing hydrophobicity of the anion exchange groups of the membranes, especially, from propyl-. The permeated amount of fluoride ions increased by reacting with trimethylamine, which has a tendency similar to the case of sulfate ions. It was reported that fluoride ions are difficult to permeate through conventional anion exchange membranes because the ion exchange equilibrium constant of fluoride ions compared with chloride ions is extremely The relative transport number between bromide ions and chloride ions was also measured using these membranes. Since bromide ions are hydrophobic compared with chloride ions, it can be predicted that bromide ions become easy to permeate through the membranes with increasing hydrophobicity, which was the similar tendency of PclNo3. Figure 5 shows the same result as expected, and PclBrdecreased with increasing hydrophilicity by reaction with trimethylamine. As mentioned above, the change in the relative transport number of a given anion compared with chloride ions is classified as two types: (1) PclA decreases with increasing carbon number of alkyl groups bonded to ammonium groups (PcT04 and P c ~ )(2) , PclA increases with increasing chain length (PA~O and ~ PaBr). This tendency is consistent with the value of hydration energy of each anion against that of chloride ions shown in Table 3. However, Table 1 shows that ion exchange capacity and water content of the membranes decrease with
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Hydrophobicity of Ion Exchange Groups
.-
9
lo7
aL -
ou
0-
a 5
64-
3
2-
c:
CI
C1
2
"
c5
c4
Carbon Number of Alkyl Chains bonded to Ammonium Groups
Figure 5. Effect of species of anion exchange groups on &le': (0) reacted with various amines; (0)reacted with various amines and then with trimethylamine. Concentration of the mixed salt solution is 0.04 N as sodium ions.
cc C2 Ca c4 Carbon Number of Alkyl Chains bonded t o Ammonium Groups
Figure 7. Change in relative transport number between nitrate ions and chloride ions by formation of anionic polyelectrolyte layers on the membrane surfaces: (0)without anionic polyelectrolyte layer; (0)after immersing the membrane into an aqueous Demol N (polycondensation product of sodium naphthalene and formaldehyde)solution (lo00 ppm, 17 h). Concentration of the mixed salt solution is 0.04 N as sodium ions. 0.4
0.3
"a" 0.8
0.9
1 .o
1.1
0.2
0.1
I o n exchange capacity ( meq/g.dry membrane )
Figure 6. Effect of ion exchange capacity of the membranes on P C I ~ O ~ . Anion exchange membranes with different ion exchange capacities (reacted with tri-n-butylamine) were used in electrodialysis of a 0.04 N mixed salt solution. TABLE 5: Relative Transport Number between Thiocyanate Ions and Chloride Ions in Anion Exchange Membranes with Different Anion Exchange Group@ anion exchange membrane PClSCN with trimethylbenzylammoniumgroups 2.6 with tri-n-butylbenzylammonium groups 5.6 a
Concentration of a mixed salt solution: 0.04 N as sodium ions.
increasing alkyl chain length. To examine the effect of the ion exchange capacity on P c I ~anion , exchange membranes with different ion exchange capacity were prepared and P a A was measured. Namely, the membranes were prepared by reaction of the copolymer membranes of chloromethylstyrene and divinylbenzene with different chloromethyl group content with tri-n-butylamine. Figure 6 shows the relationship between PclN03 and the ion exchange capacity of the membrane. Although PCINOjincreased slightly with decreasing ion exchange capacity because of less hydrophilicity, a remarkable increase in PclNo3 was not observed, which means that the increase in hydrophobicity of anion exchange groups is effective with a remarkable increase in PcINoj. The relative transport number between thiocyanate ions, typically hydrophobic anions, and chloride ions was measured (using the membrane with tri-nbutylamine). Table 5 shows that the permeation of thiocyanate ions was enhanced by giving hydrophobic properties to the membrane. Since nitrate ions are hydrophobic, PclN03 increased with increasing hydrophobicity of the membranes. On the other hand, it was reported in the previous work that formation of an anionic polyelectrolyte layer on the surface of the anion exchange membrane enhances permeation of nitrate ions versus
n C,
C2
C3
C4
Carbon Number of Alkyl Chains bonded lo Ammonium Groups
Figure 8. Change in relative transport number between fluoride ions and chloride ions by formation of anionic polyelectrolyte layers on the membrane surfaces: (0)without anionic polyelectrolyte layer; (m) after immersing the membrane into an aqueous Demol N solution (1000 ppm, 17 h). Concentration of the mixed salt solution is 0.04 N as sodium ions. chloride ions because of the difference of electrostatic repulsion force between them.' Therefore, anionic polyelectrolytes, the polycondensation product of sodium naphthalenesulfonate and formaldehyde (Demol N, obtained from Kao Chemicals Co., Ltd., molecular weight is about lOOO), were adsorbed on the membrane surface to obtain higher PCIN0j. Each membrane was immersed in an aqueous 1000 ppm Demol N solution for 17 h (until equilibrium). Although PclNo3 increased slightly in the membranes with trimethylbenzylammonium groups and triethylbenzylammonium groups, that of the membranes with tri-npropylbenzylammonium groups and hi-n-butylbenzylammonium groups decreased, as shown in Figure 7. This means that adsorption of the anionic polyelectrolyte on the membrane surface of the hydrophobic membranes decreased the strong hydrophobicity of the membranes. Namely, the electrostatic repulsion force by the anionic charge on the membrane surface toward nitrate ions is weaker than the hydrophobic affinity between nitrate ions and hydrophobic anion exchange groups. Figure 8 shows the change in PclF of the anion exchange membranes with different anion exchange groups after the membranes had been immersed in the anionic polyelectrolyte (Demol N) solution. Contrary to the case of PclNo3, PcrF increased with formation of the anionic charged layer on the membrane surface. It is thought that this is due to the synergistic effect of the weaker electrostatic repulsion force toward fluoride ions than that toward chloride ions, and the change of the membrane surface from hydrophobic to hydrophilic.
Sata et al.
12880 J. Phys. Chem., Vol. 99, No. 34, 1995
-
-
0
1
/
14
12
5 10 0
2 -
E
fa
.
Cl
8 6 4
2 0 0.05
0.10
0.20
0.15
0.25
0.30
Water content
Figure 9. Relationship between permeability of urea and water content of anion exchange membranes: (0)reacted with various amines; (0) reacted with various amines and then with trimethylamine. 16
s 0
t-
I
14
2 12 v
7- 10 2
8
2
6
e
6 4 2 0 0.6
1 .o
1.4
2.2
1.8
Ion exchange capacity ( meqlg-dry membrane
)
Figure 10. Relationship between permeability of urea and ion exchange capacity of anion exchange membranes: (0) reacted with various amines; (0)reacted with various amines and then with trimethylamine.
2. Microstructure of Anion Exchange Membranes. It is interesting to know the change in microstructure of the anion exchange membranes by introducing hydrophobic anion exchange groups. First of all, to estimate the change in pore size of each membrane, the permeability of the neutral molecule, urea (Stokes radius 3.3 A), through the membranes was measured. Figure 9 shows the relationship of the permeability of urea to the water content of the membranes. With decreasing water content (with increasing carbon number of the alkyl chain bonded to the ammonium groups), the permeability decreased. However, after these membranes reacted with trimethylamine further, the permeability decreased abruptly from the membrane with tri-n-propylbenzylammonium groups. Although the membrane with triethylbenzylammonium groups has almost the same water content as the membrane with tri-n-butylbenzylammonium groups, the permeability of the latter is lower than that of the former. Figure 10 shows the relationship between the permeability and ion exchange capacity of the membranes. The same tendency was also observed in the relationship between the permeability and ion exchange capacity in the membranes after reacting with trimethylamine. The permeability decreased remarkably in the membrane with tri-n-propylbenzylammonium groups in both relations between the permeability and water content and between the permeability and ion exchange capacity. This means that the hydrophobic effect of anion exchange groups becomes remarkable from tri-n-propylbenzylammonium groups; that is, the pore size of the membrane becomes remarkably narrow from the tri-n-propylbenzylaonium groups, although there is no significant change between triethylamine and tri-n-propylamine from the HLB value. Figure 11 shows the morphology of the copolymer membrane (before introducing anion exchange groups), which was mea-
Figure 11. Scanning transmission electron micrograph of a cross section of the base membrane (before introduction of ion exchange groups).
Hydrophobicity of Ion Exchange Groups
J. Phys. Chem., Vol. 99, No. 34, I995 12881
Nnnoscope
w
I1
Parameters:
z
m
E
hmples
0
n
Base membrane.
w E
E
0
0
n
n
b Amination with trimethylamine. c Amination with tributylamine. 8
e
Figure 12. AFM images of the base membrane and anion exchange membranes.
sured by a scanning transmission electron microscope. The copolymer of chloromethylstyrene and divinylbenzene (large and small white spherical parts), which becomes the ion exchange resin part, appears to be distributed as a distinct disperse polymer phase in the inactive polymer phase. Apart from inside the copolymer membrane, the membrane surface was observed by an atomic force microscope. Figure 12a,b,c, show AFM images of three different membranes: (a) copolymer membrane (base membrane), (b) anion exchange membrane with trimethylbenzylammonium groups, and (c) anion exchange membrane with tri-n-butylbenzylammonium groups. Before introducing anion exchange groups, the copolymer membrane has a relatively flat surface. However, after introducing anion exchange groups, the membrane surface is uneven, as shown in Figure 12b,c. The size of the convex parts of the membrane with trimethylbenzylammonium groups almost corresponds to that of the small white parts in Figure 11. This means that trimethylamine reacted with domains of the copolymer of chloromethylstyrene-divinylbenzene and the domains expanded. Figure 12c shows the AFM image of the surface of the membrane with tri-n-butylbenzylammonium groups. Degree of the convex of the membrane surface becomes high compared with that of Figure 12b in spite of lower ion exchange capacity. When a bulky amine reacts with the membrane, a hydrophobic domain is formed in the membrane matrix, in which ion exchange groups exist. Since it is thought that the atmosphere of the ion exchange site becomes strongly hydrophobic, the permeation of hydrophilic ions through the domains is difficult, and hydrophobic ions are easily adsorbed and permeate through the membrane.
Conclusion Three different methods to modify the anion exchange membrane have been used in order to separate specific ions from a mixed salt solution by electrodialysis. To clarify the effect of hydrophobicity of ion exchange groups and hydration ,of anions on permselectivity between two anions, the relative transport number of various anions compared with chloride ions was measured using anion exchange membranes having anion exchange groups with different hydrophobicity. Consequently, the permeation of hydrophobic anions is enhanced with increasing hydrophobicity of the anion exchange groups. On the other hand, the permeation of hydrophilic anions becomes difficult in the membranes with the hydrophobic groups. The hydrophobic effect appears clearly in the membrane with tri-npropylbenzylammonium groups and becomes strong with increasing carbon number of the alkyl chain bonded to the ammonium groups. Changing the hydrophobicity of the anion exchange groups of the membrane is one of the effective methods to control permselectivity between two anions. This is a new method to change the permselectivity between ions in the ion exchange membranes used in electrodialysis.
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