- Email: [email protected]
Characteristic Proton Transport through Composite Membrane Composed of Quaternary-Amino Poly(sulfone) and Sulfonated Co-poly(styrene and divinylbenzene)
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
200, 59–65 (1998)
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Characteristic Proton Transport through Composite Membrane Composed of Quaternary-Amino Poly(sulfone) and Sulfonated Co-poly(styrene and divinylbenzene) Rie Eto and Akihiko Tanioka 1 Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama Meguro-ku, Tokyo 152, Japan Received June 26, 1997; accepted November 24, 1997
the Donnan exclusion. However, the proton selectivity is considered to be very high in a mixed solution of trivalent ions such as Al 3/ and protons if the anion exchange layer is composed of poly(sulfone) with a quaternary amino group and the thickness is about 1/10th that of the cation exchange layer which is 100 mm (4). The thickness of an anionexchange layer of 10 mm is large enough to exclude the cations. Therefore, it is very difficult to consider that the anion exchange layer has high proton selectivity. The bipolar membrane is a layered structure involving a cation-selective membrane joined to an anion-selective membrane. It is well-known that rectification is observed in the current and voltage properties and that water splitting occurs at the boundary between the anion and cation layers (5–7). The water splitting mechanism in a bipolar membrane has been discussed for a long time. Mafe´ et al. explained the water splitting using the Donnan effect and the Nernst–Planck equation; water molecules dissociate into cations (H / ) and anions (OH 0 ) in accordance with the second Wien effect (8, 9). The efficiency of the water splitting is explained as a function of the dielectric constant at the boundary surface. On the other hand, Simons suggested that the water splitting was caused by the chemical reaction of binary or ternary amino groups with water molecules (10– 14). Such discussion tells us that the intermediate boundary region is responsible for water splitting. In this study, the proton transport mechanism through the composite membrane composed of quaternary amino poly(sulfone) sulfonated copoly(styrene and divinylbenzene) was examined based on the measurements of membrane potential, current–voltage curves, and ionic permeability. The relationship existing between water splitting and proton transport through the anion exchange layer from the mixed solution will be discussed.
Proton transport phenomena across a composite membrane (PBM), where an anion exchange layer composed of poly(sulfone) modified with quaternary amino groups was pasted on a cation exchange layer composed of a sulfonated copolymer of poly(styrene) and poly(divinylbenzene), were investigated. This composite membrane shows the characteristics of a bipolar membrane based on current–voltage measurements. If the voltage is applied from the anion-exchange layer, water is dissociated into H / and OH 0 , and then OH 0 is transported to the anode through the poly(sulfone) layer, which suggests the possibility of polymer decomposition of the anion-exchange layer by the base. For the separation of H / and Al 3/ from their mixed solution by the PBM, the voltage is always applied from the anion exchange layer to the cation exchange layer to cause water splitting. However, there is no evidence of poly(sulfone) decomposition during H / and Al 3/ separation though the reverse bias voltage is applied to this membrane. H / is transported from the external solution and combined with dissociated OH 0 from the intermediate surface in the anion exchange layer to produce water. The dissociated H / in the intermediate surface is transported to the cation exchange layer side. Therefore, the poly(sulfone) layer is always in a neutral state, which implies that PBM has resistance to the alkaline atmosphere. q 1998 Academic Press
Key Words: bipolar membrane; water splitting; rectification effect; poly(sulfone); proton transport.
1. INTRODUCTION
It is well-known that an ion-exchange membrane which has a thin oppositely charged layer of less than several tens of nanometers on the surface has a high selectivity for monovalent counterions vs bivalent counterions (1–3). In this case, the bivalent counterion is repulsed on the surface layer though the monovalent counterion can penetrate it. If the thickness of the thin layer is increased, the selectivity is reduced because the monovalent ion is also excluded by
2. EXPERIMENTAL
2.1. Samples Anion exchange membranes (PAM) composed of poly(sulfone) with quaternary amino groups, anion exchange
1 To whom correspondence should be addressed. Fax: /81-3-5734-2876. E-mail: [email protected].
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TABLE 1 Characteristics of the Various Monopolar Charged Membranes
QX (mol/liter) (by HCl aq. sol.) QX (mol/liter) (by KCl aq. sol.) X (mol/liter) Electrical resistance (Vrcm2) Water content (%) Thickness (mm) v (Cl0)/v (H/) v (Cl0)/v (K/)
SCM
PAM
SAM
6.7 1 1001 4.2 1 1001 8.5 2.0 15 130 0.15 1.0
7.7 1 1002 9.4 1 1002 6.4 0.4 20 40 0.18 1.1
5.7 1 1001 5.7 1 1001 4.0 3.6 26 220 0.10 1.0
membranes (SAM) composed of poly(butadiene-costyrene) rubber with introduced positively charged groups of quaternary amine, cation exchange membranes (SCM) composed of sulfonated copolymers of poly(styrene) and poly(divinylbenzene), and composite membranes (PBM), where a solution of poly(sulfone) modified with quaternary amino groups is pasted on the SCM, were prepared for the measurements. Poly(sulfone) modified with a quaternary amino group is considered to be very weak in an alkaline atmosphere. The thicknesses of PAM, SAM, SCM, and PBM were 40, 220, 130, and 120 mm, respectively. Charge densities (X) from titlation, electrical resistances, water contents, and thicknesses of monopolar emembranes (SCM, PAM, and SAM) are shown in Table 1.
ride. I–V curves were measured at KCl concentrations of 0.025, 0.05, 0.075, and 0.1 mol/liter and at temperatures of 17, 23, and 327C. 2.3. Membrane Potential Measurements (18) The apparatus for membrane potential measurement is shown in Fig. 3. Charged membranes (PAM, SCM, and SAM) were installed in the center of the measuring cell, which had two containers on both sides of the membrane. Electrolyte solutions of different concentration were poured into these containers; the left one was varied from 0.001 to 2.0 mol/liter and the right one was kept constant at 0.001 mol/liter. An electrometer connected with Ag–AgCl glass electrodes (HS-205C, TOA) was used for the measurement of electrical potential. The measurements were performed while stirring the solutions at 237C after 10 min of membrane installation. Two Ag–AgCl glass electrodes were placed in saturated KCl solutions, and each solution was connected to each container via a salt bridge. KCl and HCl were used to determine the effective charge density and ionic mobility ratio. 2.4. Permeability Coefficient Measurement (19)
2.2. Measurement of Current–Voltage Curve under Reverse Bias Condition
The permeabilities of HCl, KCl, MgCl2 , and AlCl3 at various concentrations were measured for comparison with the results of the membrane potential at 237C. The apparatus for the permeability measurement is shown in Fig. 4. Charged membranes were installed in the center of the measuring cell, which had two containers on both
The current–voltage curves under reverse bias conditions across the membrane were measured with the Luggin capillary method as shown in Fig. 1 (15–17). The membrane (cross-sectional area A Å 0.79 cm2 ) was placed between two electrodialytic half cells in an aqueous solution of potassium chloride, two Ag/AgCl electrode plates (cross-sectional area A Å 4.0 cm2 ) were laid on both sides of the cell, and two Luggin capillaries were placed between an electrode and the membrane. The current was introduced through the two Ag/ AgCl electrode plates. The applied voltage (V ) was measured by a voltmeter (PM-16A, TOA) connected with the Luggin capillaries and the current (I) by a picoammeter (AM-271A, TOA). The voltage was measured as a function of distance between the membrane and a Luggin capillary containing agar gel mixed with 3 mol/liter KCl aqueous solution as shown in Fig. 2. The electric potential difference across the membrane was determined by extrapolating to d Å 0. Thus, I–V curves were obtained between the measuring current and the applied voltage across the membrane. Because the plate is made of Ag/AgCl, we limited the applied current to the maximum 2.5 mA/cm2 in order to prevent breakage of the electrode. Two cells were maintained by circulation of the same aqueous solution of potassium chlo-
FIG. 1. Schematic diagram of current–voltage measurement apparatus.
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FIG. 2. Measured voltage between two membrane surfaces as a function of the distance between the membrane and a Luggin capillary, where the applied current density is 1.562 (mA/cm2 ), the temperature is 177C, and the concentration of KCl solution in the cell is 0.075 mol/liter.
sides of the membrane. The electrolyte solution was poured into one side of the container and the deionized water in the opposite side. A conductivity cell ( CG-511B, TOA ) connected to a conductivity meter ( CM-20S, TOA ) was placed in the deionized water side to measure the
FIG. 3. Schematic diagram of membrane potential measurement apparatus.
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conductivity of the permeated ion as a function of time. Before measurement, the relationship between the conductivity and the concentration was determined. For the PBM, the permeability coefficients were measured in two different ways; in one the anion exchange layer faced the deionized water side, and in the other the cation exchange layer faced the deionized water side. However, the results of both measurements were quite similar, so we will discuss the former case.
FIG. 4. Schematic diagram of ionic permability measurement apparatus.
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3. RESULTS AND DISCUSSION
3.1. Current–Voltage Characteristics Figure 5 shows the current–voltage curves of the PBM at 237C, where the KCl concentrations in the measurement cell are 0.025, 0.05, 0.075, and 0.1 mol/liter. For 0.025 and 0.05 mol/liter, we can observe a clear rectification effect between 0750 and /200 mV. On the other hand, the I–V curves become ohmic for 0.075 and 0.1 mol/liter because the concentration increase in the external solution reduces the Donnan effect in the membrane. For 0.025 and 0.05 mol/ liter, a large current is again observed at more than 0750 mV of applied voltage, which indicates that water splitting is occurring in the membrane. These results show that the PBM has the characteristics of a true bipolar membrane (18, 19). Figure 6 shows the current–voltage curves of PBM at 0.05 mol/liter, where the temperatures in the measurement cell are 17, 23, and 327C. A large current density is observed at the high measurement temperature, which implies that water splitting is accelerated with the temperature increase. It is concluded that PBM has the characteristics of a bipolar membrane because the rectification effect and water splitting are observed. Usually, it is considered that PBM is not strong in alkaline environment because poly(sulfone)
FIG. 6. Current–voltage characteristics of PBM at 0.05 mol/liter, where the temperatures in the measurement cell are 177C ( h ), 237C ( L ), and 327C ( n ).
modified with quaternary amino groups is very weak in alkaline solution. If the voltage is applied from the anion-exchange layer, the water is dissociated into H / and OH 0 , and OH 0 is transported to the anode through the anionexchange layer of poly(sulfone), which suggests the decomposition of the anion-exchange layer by the base. During the separation of H / and Al 3/ from their mixed solution using the PBM, the voltage is always applied from the anion exchange layer to the cation exchange layer, which causes the water splitting (7). However, this membrane is very stable in the alkaline atmosphere in spite of the production of OH 0 . 3.2. Membrane Potential According to the Teorell–Meyer–Siever theory, the membrane potential Dfm is given by q
RT Cdq1 / (2C0 /QX ) 2 / 1 Dfm Å 0 ln F C0 1 / (2Cd /QX ) 2 / 1 q
FIG. 5. Current–voltage characteristics of PBM at 237C, where the KCl concentrations in the measurement cell are 0.025 mol/liter ( h ), 0.05 mol/ liter ( L ), 0.075 mol/liter ( n ), and 0.1 mol/liter ( 1 ).
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RT 1 / (2Cd /QX ) 2 / W 0 , W ln q F 1 / (2C0 /QX ) 2 / W
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FIG. 7. Permability coefficients of HCl (a) and KCl (b) as a function of the external solution in the high concentration side.
where C0 Å 1 1 10 03 mol/liter, Cd was varied from 0.001 to 2.0 mol/liter, QX the effective charge density, Q the parameter which indicates charge effectiveness, R the gas constant, F the Faraday constant, and T the absolute temperature. W is given by
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WÅ
v/ 0 v0 1 0 ( v0 / v/ ) Å , v/ / v0 1 / ( v0 / v/ )
[2]
where v/ and v0 are the cation and anion mobilities, respectively. If W is obtained, v0 / v/ can be calculated. After
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TABLE 2 Permability Coefficients (cm2/s) of HCl and KCl through Various Membranes at C0 Å 0.05 mol/liter
SCM PAM SAM
HCl
KCl
1.1 1 1009 1.8 1 1007 1.8 1 1007
6.0 1 1009 5.4 1 1009 2.2 1 1008
measurements of the membrane potential as a function of the external solution concentration, Eq. [1] was applied to the experimental results and nonlinear regression was carried out. The parameters W and QX were then obtained, and finally from Eq. [2] the ionic mobility ratio v0 / v/ was calculated. QX, which were measured in HCl and KCl aqueous solutions, and the ionic mobility ratios of Cl 0 to H / and Cl 0 to K / for SCM, PAM, and SAM are listed in Table 1. The charge density of SCM and PAM are similar and that of SAM is 10 times larger. v0 (Cl 0 )/ v/ (H / ) for all membranes is less than one. Even if the membrane is an anion exchange membrane, the proton mobility is about six times that of chloride ion. On the other hand, the potassium mobility is nearly equal to that of chloride for all membranes.
coefficients of KCl are different from those of HCl. They have minimum values around C0 Å 0.1 mol/liter. An increase in P at low concentration indicates the effect of membrane swelling as cited above for all membranes. These permeability coefficients are close to each other between C0 Å 0.1 and 2 mol/liter. However, the differences are slightly larger between C0 Å 0.01 and 0.1 mol/liter because of the membrane swelling. In this case, P through SAM is larger than that through PAM. In Table 2 the permeability coefficients P of HCl and KCl for SCM, PAM, and SAM at C0 Å 0.05 mol/liter are given. If we apply the following equation for a theoretical permeability coefficient based on the Nernst–Planck equation and Donnan equilibrium to the experimental data for HCl and KCl with Q Å 0.047 for SCM, Q Å 0.011 for PAM, and Q Å 0.18 for SAM which were determined from membrane potential and titration measurements (21), PÅ
1
d d/ d0 Å / , P P/ P0
[3]
where P is the permeability coefficient of SCM / PAM if SCM and PAM are superimposed, P/ the permeability coefficient of PAM, P0 that of SCM, d the membrane thickness of SCM / PAM, d/ that of PAM, and d0 that of SCM. The permeability coefficients of HCl through PAM increase with an increase in the concentration of the external solution, and those through SAM have a minimum value around C0 Å 0.02 mol/liter. A decrease of P is caused by the membrane swelling effect and an increase of P is based on the reduction of the Donnan effect in the membrane. For both membranes, the permeability coefficients are similar through this concentration range. Permeability coefficients in PBM and SCM have minimum values around C0 Å 0.1 mol/liter. An increase in P at low concentration represents the effect of membrane swelling for PBM and SCM because the osmotic pressure in the external solution decreases with a decrease in concentration. The calculated results for SCM / PAM are very similar to those of PBM. The permeability
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F
r
1/ q
S D 2Cd QX
r
2
0
1/
S D G
1 / (2Cd /QX ) 2 / W 0 W lnq 1 / (2C0 /QX ) 2 / W
3.3. Permeability Coefficient In Figs. 7a and 7b, the permeability coefficients of HCl and KCl for PBM, SCM, PAM, and SAM are shown as a function of the concentration of the external solutions. In Figs. 7 SCM / PAM means the calculated results of the permeability coefficient using
v/ v0 RT Q( v/ / v0 ){(Cd /QX ) 0 (C0 /QX )}
2C0 QX
2
,
[4]
we can obtain the following value for A: AÅ
v/v0 . v/ / v0
[5]
From Eqs. [2] and [5], we can obtain the ionic mobilities of the proton, potassium, and chloride ions and they are listed in Table 3 with the mobility ratio of H / to K / , v(H / / K / ). The mobility ratio of a proton to a potassium ion v(H / /K / ) in water is 4.758. v(H / /K / ) in SCM is smaller than that in water. On the other hand, v(H / /K / ) values in PAM and SAM are larger than that in water. This implies that proton mobility in the anion exchange membrane is larger than those of metal ions because proton mobilities in water are usually the largest among the cations. If we comTABLE 3 Anion and Cation Mobilities (11016 m2 mol J01 s01) v/(H/)/v/(K/) in Water is 4.758 SCM v/ (H/) v0 (Cl0) v/ (K/) v0 (Cl0) v (H//K/)
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v/ v0 v/ v0
Å 0.43 Å 0.066 Å 0.38 Å 0.38 1.1
PAM v/ v0 v/ v0
Å 1.1 Å 0.19 Å 0.076 Å 0.088 14
SAM v/ v0 v/ v0
Å 88 Å 8.8 Å 6.2 Å 6.2 14
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istics of poly ( sulfone ) involve preparing a thin membrane of less than 100 mm because it is impossible to prepare a poly ( styrene ) type ion exchange membrane with a thickness of less than 100 mm. ACKNOWLEDGMENT We are most grateful to Mr. Hirofumi Horie of Asahi Glass Co., Ltd., for providing us with the samples.
REFERENCES FIG. 8. Schematic diagram of proton penetration model in composite membrane (PBM).
pare PAM and SAM, v(H / /K / ) values are the same but v/ across the PAM is 1/10th of that across SAM. Therefore, we cannot say that PAM has a specificity for proton transport. This result suggests that the anion-exchange layer of poly(sulfone) modified with a quaternary amino group in PBM does not have tremendously high permselectivity for the proton. 3.4. Proton Transport across Composite Membrane As cited above, it is very difficult to consider that the composite membrane ( PBM ) has a specificity for proton transport ( 7 ) . However, this composite membrane has the characteristics of a bipolar membrane. If a reverse bias voltage is applied to this membrane, a proton penetrates the anion exchange layer from the anion-exchange-layerside compartment, and at the same time in the intermediate interface of the membrane, water is dissociated into H / and OH 0 which exit from the cation exchange layer and anion exchange layer, respectively. H / from the external solution and OH 0 from the intermediate surface are recombined in the anion exchange layer to produce water as shown in Fig. 8. This is the reason that the polysulfone is resistant to alkaline environment. The special character-
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1. Sata, T., Izuo, R., Mizutani, Y., and Yamane, R., J. Colloid Interface Sci. 40, 317 (1972). 2. Sata, T., J. Colloid Interface Sci. 44, 393 (1973). 3. Mizutani, Y., J. Membrane Sci. 49, 121 (1990). 4. Horie, H., Bull. Soc. Sea Water Sci. Jpn. 51 (1997). 5. Bassignana, I. C., and Reiss, H., J. Membr. Sci. 15, 27 (1983). 6. Mauro, A., Biophys. J. 2, 179 (1962). 7. Coster, H. G. L., Biophys. J. 5, 669 (1965). 8. Ramı´rez, P., Manzanares, J. A., and Mafe´, S., Ber. Bunsenges. Phys. Chem. 95, 499 (1991). 9. Mafe´, S., Manzanares, J. A., and Ramı´rez, P., Phys. Rev. A 42, 6245 (1990). 10. Ramirez, P., Rapp, H. J., Reichle, S., Strathmann, H., and Mafe´, S., J. Appl. Phys. 72, 259 (1992). 11. Ramı´rez, P., Rapp, H.-J., Mafe´, S., and Bauer, B., J. Electroanal. Chem. 375, 101 (1994). 12. Simons, R., Desalination 28, 41 (1979). 13. Simons, R., Nature (London) 280, 824 (1979). 14. Simons, R., and Khanarian, G., J. Membr. Biol. 38, 11 (1978). 15. Manzanares, J. A., Kontturi, K., Mafe´, S., Aguilella, V. M., and Pellicer, J., Acta Chem. Scand. A 45, 115 (1991). 16. Ramı´rez, P., Rapp, H. J., Reichle, S., Strathmann, H., and Mafe´, S., J. Appl. Phys. 72, 259 (1992). 17. Aritomi, T., van den Boomgaard, Th., and Strathmann, H., Desalination 104, 13 (1996). 18. Tanioka, A., Shimizu, K., Miyasaka, K., Zimmer, H. J., and Minoura, N., Polymer 37, 1883 (1996). 19. Kawaguchi, M., Murata, T., and Tanioka, A., J. Chem. Soc. Faraday Trans. 93, 1351 (1997). 20. Shimizu, K., and Tanioka, A., Polymer 38, 1997, in press. 21. Lakshminarayanaiah, N., ‘‘Transport Phenomena in Membrane.’’ Academic Press, New York, 1969.
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Characteristic Proton Transport through Composite Membrane Composed of Quaternary-Amino Poly(sulfone) and Sulfonated Co-poly(styrene and divinylbenzene) Rie Eto and Akihiko Tanioka 1 Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama Meguro-ku, Tokyo 152, Japan Received June 26, 1997; accepted November 24, 1997
the Donnan exclusion. However, the proton selectivity is considered to be very high in a mixed solution of trivalent ions such as Al 3/ and protons if the anion exchange layer is composed of poly(sulfone) with a quaternary amino group and the thickness is about 1/10th that of the cation exchange layer which is 100 mm (4). The thickness of an anionexchange layer of 10 mm is large enough to exclude the cations. Therefore, it is very difficult to consider that the anion exchange layer has high proton selectivity. The bipolar membrane is a layered structure involving a cation-selective membrane joined to an anion-selective membrane. It is well-known that rectification is observed in the current and voltage properties and that water splitting occurs at the boundary between the anion and cation layers (5–7). The water splitting mechanism in a bipolar membrane has been discussed for a long time. Mafe´ et al. explained the water splitting using the Donnan effect and the Nernst–Planck equation; water molecules dissociate into cations (H / ) and anions (OH 0 ) in accordance with the second Wien effect (8, 9). The efficiency of the water splitting is explained as a function of the dielectric constant at the boundary surface. On the other hand, Simons suggested that the water splitting was caused by the chemical reaction of binary or ternary amino groups with water molecules (10– 14). Such discussion tells us that the intermediate boundary region is responsible for water splitting. In this study, the proton transport mechanism through the composite membrane composed of quaternary amino poly(sulfone) sulfonated copoly(styrene and divinylbenzene) was examined based on the measurements of membrane potential, current–voltage curves, and ionic permeability. The relationship existing between water splitting and proton transport through the anion exchange layer from the mixed solution will be discussed.
Proton transport phenomena across a composite membrane (PBM), where an anion exchange layer composed of poly(sulfone) modified with quaternary amino groups was pasted on a cation exchange layer composed of a sulfonated copolymer of poly(styrene) and poly(divinylbenzene), were investigated. This composite membrane shows the characteristics of a bipolar membrane based on current–voltage measurements. If the voltage is applied from the anion-exchange layer, water is dissociated into H / and OH 0 , and then OH 0 is transported to the anode through the poly(sulfone) layer, which suggests the possibility of polymer decomposition of the anion-exchange layer by the base. For the separation of H / and Al 3/ from their mixed solution by the PBM, the voltage is always applied from the anion exchange layer to the cation exchange layer to cause water splitting. However, there is no evidence of poly(sulfone) decomposition during H / and Al 3/ separation though the reverse bias voltage is applied to this membrane. H / is transported from the external solution and combined with dissociated OH 0 from the intermediate surface in the anion exchange layer to produce water. The dissociated H / in the intermediate surface is transported to the cation exchange layer side. Therefore, the poly(sulfone) layer is always in a neutral state, which implies that PBM has resistance to the alkaline atmosphere. q 1998 Academic Press
Key Words: bipolar membrane; water splitting; rectification effect; poly(sulfone); proton transport.
1. INTRODUCTION
It is well-known that an ion-exchange membrane which has a thin oppositely charged layer of less than several tens of nanometers on the surface has a high selectivity for monovalent counterions vs bivalent counterions (1–3). In this case, the bivalent counterion is repulsed on the surface layer though the monovalent counterion can penetrate it. If the thickness of the thin layer is increased, the selectivity is reduced because the monovalent ion is also excluded by
2. EXPERIMENTAL
2.1. Samples Anion exchange membranes (PAM) composed of poly(sulfone) with quaternary amino groups, anion exchange
1 To whom correspondence should be addressed. Fax: /81-3-5734-2876. E-mail: [email protected].
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TABLE 1 Characteristics of the Various Monopolar Charged Membranes
QX (mol/liter) (by HCl aq. sol.) QX (mol/liter) (by KCl aq. sol.) X (mol/liter) Electrical resistance (Vrcm2) Water content (%) Thickness (mm) v (Cl0)/v (H/) v (Cl0)/v (K/)
SCM
PAM
SAM
6.7 1 1001 4.2 1 1001 8.5 2.0 15 130 0.15 1.0
7.7 1 1002 9.4 1 1002 6.4 0.4 20 40 0.18 1.1
5.7 1 1001 5.7 1 1001 4.0 3.6 26 220 0.10 1.0
membranes (SAM) composed of poly(butadiene-costyrene) rubber with introduced positively charged groups of quaternary amine, cation exchange membranes (SCM) composed of sulfonated copolymers of poly(styrene) and poly(divinylbenzene), and composite membranes (PBM), where a solution of poly(sulfone) modified with quaternary amino groups is pasted on the SCM, were prepared for the measurements. Poly(sulfone) modified with a quaternary amino group is considered to be very weak in an alkaline atmosphere. The thicknesses of PAM, SAM, SCM, and PBM were 40, 220, 130, and 120 mm, respectively. Charge densities (X) from titlation, electrical resistances, water contents, and thicknesses of monopolar emembranes (SCM, PAM, and SAM) are shown in Table 1.
ride. I–V curves were measured at KCl concentrations of 0.025, 0.05, 0.075, and 0.1 mol/liter and at temperatures of 17, 23, and 327C. 2.3. Membrane Potential Measurements (18) The apparatus for membrane potential measurement is shown in Fig. 3. Charged membranes (PAM, SCM, and SAM) were installed in the center of the measuring cell, which had two containers on both sides of the membrane. Electrolyte solutions of different concentration were poured into these containers; the left one was varied from 0.001 to 2.0 mol/liter and the right one was kept constant at 0.001 mol/liter. An electrometer connected with Ag–AgCl glass electrodes (HS-205C, TOA) was used for the measurement of electrical potential. The measurements were performed while stirring the solutions at 237C after 10 min of membrane installation. Two Ag–AgCl glass electrodes were placed in saturated KCl solutions, and each solution was connected to each container via a salt bridge. KCl and HCl were used to determine the effective charge density and ionic mobility ratio. 2.4. Permeability Coefficient Measurement (19)
2.2. Measurement of Current–Voltage Curve under Reverse Bias Condition
The permeabilities of HCl, KCl, MgCl2 , and AlCl3 at various concentrations were measured for comparison with the results of the membrane potential at 237C. The apparatus for the permeability measurement is shown in Fig. 4. Charged membranes were installed in the center of the measuring cell, which had two containers on both
The current–voltage curves under reverse bias conditions across the membrane were measured with the Luggin capillary method as shown in Fig. 1 (15–17). The membrane (cross-sectional area A Å 0.79 cm2 ) was placed between two electrodialytic half cells in an aqueous solution of potassium chloride, two Ag/AgCl electrode plates (cross-sectional area A Å 4.0 cm2 ) were laid on both sides of the cell, and two Luggin capillaries were placed between an electrode and the membrane. The current was introduced through the two Ag/ AgCl electrode plates. The applied voltage (V ) was measured by a voltmeter (PM-16A, TOA) connected with the Luggin capillaries and the current (I) by a picoammeter (AM-271A, TOA). The voltage was measured as a function of distance between the membrane and a Luggin capillary containing agar gel mixed with 3 mol/liter KCl aqueous solution as shown in Fig. 2. The electric potential difference across the membrane was determined by extrapolating to d Å 0. Thus, I–V curves were obtained between the measuring current and the applied voltage across the membrane. Because the plate is made of Ag/AgCl, we limited the applied current to the maximum 2.5 mA/cm2 in order to prevent breakage of the electrode. Two cells were maintained by circulation of the same aqueous solution of potassium chlo-
FIG. 1. Schematic diagram of current–voltage measurement apparatus.
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FIG. 2. Measured voltage between two membrane surfaces as a function of the distance between the membrane and a Luggin capillary, where the applied current density is 1.562 (mA/cm2 ), the temperature is 177C, and the concentration of KCl solution in the cell is 0.075 mol/liter.
sides of the membrane. The electrolyte solution was poured into one side of the container and the deionized water in the opposite side. A conductivity cell ( CG-511B, TOA ) connected to a conductivity meter ( CM-20S, TOA ) was placed in the deionized water side to measure the
FIG. 3. Schematic diagram of membrane potential measurement apparatus.
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conductivity of the permeated ion as a function of time. Before measurement, the relationship between the conductivity and the concentration was determined. For the PBM, the permeability coefficients were measured in two different ways; in one the anion exchange layer faced the deionized water side, and in the other the cation exchange layer faced the deionized water side. However, the results of both measurements were quite similar, so we will discuss the former case.
FIG. 4. Schematic diagram of ionic permability measurement apparatus.
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3. RESULTS AND DISCUSSION
3.1. Current–Voltage Characteristics Figure 5 shows the current–voltage curves of the PBM at 237C, where the KCl concentrations in the measurement cell are 0.025, 0.05, 0.075, and 0.1 mol/liter. For 0.025 and 0.05 mol/liter, we can observe a clear rectification effect between 0750 and /200 mV. On the other hand, the I–V curves become ohmic for 0.075 and 0.1 mol/liter because the concentration increase in the external solution reduces the Donnan effect in the membrane. For 0.025 and 0.05 mol/ liter, a large current is again observed at more than 0750 mV of applied voltage, which indicates that water splitting is occurring in the membrane. These results show that the PBM has the characteristics of a true bipolar membrane (18, 19). Figure 6 shows the current–voltage curves of PBM at 0.05 mol/liter, where the temperatures in the measurement cell are 17, 23, and 327C. A large current density is observed at the high measurement temperature, which implies that water splitting is accelerated with the temperature increase. It is concluded that PBM has the characteristics of a bipolar membrane because the rectification effect and water splitting are observed. Usually, it is considered that PBM is not strong in alkaline environment because poly(sulfone)
FIG. 6. Current–voltage characteristics of PBM at 0.05 mol/liter, where the temperatures in the measurement cell are 177C ( h ), 237C ( L ), and 327C ( n ).
modified with quaternary amino groups is very weak in alkaline solution. If the voltage is applied from the anion-exchange layer, the water is dissociated into H / and OH 0 , and OH 0 is transported to the anode through the anionexchange layer of poly(sulfone), which suggests the decomposition of the anion-exchange layer by the base. During the separation of H / and Al 3/ from their mixed solution using the PBM, the voltage is always applied from the anion exchange layer to the cation exchange layer, which causes the water splitting (7). However, this membrane is very stable in the alkaline atmosphere in spite of the production of OH 0 . 3.2. Membrane Potential According to the Teorell–Meyer–Siever theory, the membrane potential Dfm is given by q
RT Cdq1 / (2C0 /QX ) 2 / 1 Dfm Å 0 ln F C0 1 / (2Cd /QX ) 2 / 1 q
FIG. 5. Current–voltage characteristics of PBM at 237C, where the KCl concentrations in the measurement cell are 0.025 mol/liter ( h ), 0.05 mol/ liter ( L ), 0.075 mol/liter ( n ), and 0.1 mol/liter ( 1 ).
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RT 1 / (2Cd /QX ) 2 / W 0 , W ln q F 1 / (2C0 /QX ) 2 / W
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[1]
CHARACTERISTIC PROTON TRANSPORTED
63
FIG. 7. Permability coefficients of HCl (a) and KCl (b) as a function of the external solution in the high concentration side.
where C0 Å 1 1 10 03 mol/liter, Cd was varied from 0.001 to 2.0 mol/liter, QX the effective charge density, Q the parameter which indicates charge effectiveness, R the gas constant, F the Faraday constant, and T the absolute temperature. W is given by
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WÅ
v/ 0 v0 1 0 ( v0 / v/ ) Å , v/ / v0 1 / ( v0 / v/ )
[2]
where v/ and v0 are the cation and anion mobilities, respectively. If W is obtained, v0 / v/ can be calculated. After
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TABLE 2 Permability Coefficients (cm2/s) of HCl and KCl through Various Membranes at C0 Å 0.05 mol/liter
SCM PAM SAM
HCl
KCl
1.1 1 1009 1.8 1 1007 1.8 1 1007
6.0 1 1009 5.4 1 1009 2.2 1 1008
measurements of the membrane potential as a function of the external solution concentration, Eq. [1] was applied to the experimental results and nonlinear regression was carried out. The parameters W and QX were then obtained, and finally from Eq. [2] the ionic mobility ratio v0 / v/ was calculated. QX, which were measured in HCl and KCl aqueous solutions, and the ionic mobility ratios of Cl 0 to H / and Cl 0 to K / for SCM, PAM, and SAM are listed in Table 1. The charge density of SCM and PAM are similar and that of SAM is 10 times larger. v0 (Cl 0 )/ v/ (H / ) for all membranes is less than one. Even if the membrane is an anion exchange membrane, the proton mobility is about six times that of chloride ion. On the other hand, the potassium mobility is nearly equal to that of chloride for all membranes.
coefficients of KCl are different from those of HCl. They have minimum values around C0 Å 0.1 mol/liter. An increase in P at low concentration indicates the effect of membrane swelling as cited above for all membranes. These permeability coefficients are close to each other between C0 Å 0.1 and 2 mol/liter. However, the differences are slightly larger between C0 Å 0.01 and 0.1 mol/liter because of the membrane swelling. In this case, P through SAM is larger than that through PAM. In Table 2 the permeability coefficients P of HCl and KCl for SCM, PAM, and SAM at C0 Å 0.05 mol/liter are given. If we apply the following equation for a theoretical permeability coefficient based on the Nernst–Planck equation and Donnan equilibrium to the experimental data for HCl and KCl with Q Å 0.047 for SCM, Q Å 0.011 for PAM, and Q Å 0.18 for SAM which were determined from membrane potential and titration measurements (21), PÅ
1
d d/ d0 Å / , P P/ P0
[3]
where P is the permeability coefficient of SCM / PAM if SCM and PAM are superimposed, P/ the permeability coefficient of PAM, P0 that of SCM, d the membrane thickness of SCM / PAM, d/ that of PAM, and d0 that of SCM. The permeability coefficients of HCl through PAM increase with an increase in the concentration of the external solution, and those through SAM have a minimum value around C0 Å 0.02 mol/liter. A decrease of P is caused by the membrane swelling effect and an increase of P is based on the reduction of the Donnan effect in the membrane. For both membranes, the permeability coefficients are similar through this concentration range. Permeability coefficients in PBM and SCM have minimum values around C0 Å 0.1 mol/liter. An increase in P at low concentration represents the effect of membrane swelling for PBM and SCM because the osmotic pressure in the external solution decreases with a decrease in concentration. The calculated results for SCM / PAM are very similar to those of PBM. The permeability
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F
r
1/ q
S D 2Cd QX
r
2
0
1/
S D G
1 / (2Cd /QX ) 2 / W 0 W lnq 1 / (2C0 /QX ) 2 / W
3.3. Permeability Coefficient In Figs. 7a and 7b, the permeability coefficients of HCl and KCl for PBM, SCM, PAM, and SAM are shown as a function of the concentration of the external solutions. In Figs. 7 SCM / PAM means the calculated results of the permeability coefficient using
v/ v0 RT Q( v/ / v0 ){(Cd /QX ) 0 (C0 /QX )}
2C0 QX
2
,
[4]
we can obtain the following value for A: AÅ
v/v0 . v/ / v0
[5]
From Eqs. [2] and [5], we can obtain the ionic mobilities of the proton, potassium, and chloride ions and they are listed in Table 3 with the mobility ratio of H / to K / , v(H / / K / ). The mobility ratio of a proton to a potassium ion v(H / /K / ) in water is 4.758. v(H / /K / ) in SCM is smaller than that in water. On the other hand, v(H / /K / ) values in PAM and SAM are larger than that in water. This implies that proton mobility in the anion exchange membrane is larger than those of metal ions because proton mobilities in water are usually the largest among the cations. If we comTABLE 3 Anion and Cation Mobilities (11016 m2 mol J01 s01) v/(H/)/v/(K/) in Water is 4.758 SCM v/ (H/) v0 (Cl0) v/ (K/) v0 (Cl0) v (H//K/)
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v/ v0 v/ v0
Å 0.43 Å 0.066 Å 0.38 Å 0.38 1.1
PAM v/ v0 v/ v0
Å 1.1 Å 0.19 Å 0.076 Å 0.088 14
SAM v/ v0 v/ v0
Å 88 Å 8.8 Å 6.2 Å 6.2 14
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istics of poly ( sulfone ) involve preparing a thin membrane of less than 100 mm because it is impossible to prepare a poly ( styrene ) type ion exchange membrane with a thickness of less than 100 mm. ACKNOWLEDGMENT We are most grateful to Mr. Hirofumi Horie of Asahi Glass Co., Ltd., for providing us with the samples.
REFERENCES FIG. 8. Schematic diagram of proton penetration model in composite membrane (PBM).
pare PAM and SAM, v(H / /K / ) values are the same but v/ across the PAM is 1/10th of that across SAM. Therefore, we cannot say that PAM has a specificity for proton transport. This result suggests that the anion-exchange layer of poly(sulfone) modified with a quaternary amino group in PBM does not have tremendously high permselectivity for the proton. 3.4. Proton Transport across Composite Membrane As cited above, it is very difficult to consider that the composite membrane ( PBM ) has a specificity for proton transport ( 7 ) . However, this composite membrane has the characteristics of a bipolar membrane. If a reverse bias voltage is applied to this membrane, a proton penetrates the anion exchange layer from the anion-exchange-layerside compartment, and at the same time in the intermediate interface of the membrane, water is dissociated into H / and OH 0 which exit from the cation exchange layer and anion exchange layer, respectively. H / from the external solution and OH 0 from the intermediate surface are recombined in the anion exchange layer to produce water as shown in Fig. 8. This is the reason that the polysulfone is resistant to alkaline environment. The special character-
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