Ind. Eng. Chem. Res. 2000, 39, 2491-2495
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A Novel Separation System Using Porous Thermosensitive Membranes Yong-Jin Choi, Takeo Yamaguchi,* and Shin-ichi Nakao Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
A novel separation system consisting of thermosensitive membranes was proposed for macromolecular separation, and the system was evaluated in this study. The pore surface of a porous substrate was covered with a thermosensitive N-isopropylacrylamide (NIPAM) grafted polymer, which enabled the hydrophobicity of the pore surface to be varied drastically by a slight temperature change. A solution containing hydrophobic and hydrophilic solutes was continuously supplied to the feed side, and the membrane temperature was changed stepwise between above and below the lower critical solution temperature (LCST) of NIPAM. Above the LCST, the hydrophobic solutes adsorb onto the hydrophobic pore surfaces; hence, only the hydrophilic solutes can pass through the membrane. Lowering the temperature below the LCST caused the pore surface to become hydrophilic and the adsorbed solutes to desorb from the pore surface. In addition, the hydrophobic solutes condensed in the permeate side. Nonionic surfactant NP-10 was used as a model solute for the proposed system, and the validity of the proposed system was demonstrated using the membrane prepared. Introduction Thermosensitive gel has two kinds of properties. One is a surface hydrophobicity change, and the other is a volume change in response to slight changes above and below the lower critical solution temperature (LCST). Based on these characteristics, some applications for bioseparation using the gel have been reported.1-3 One application is water absorption from solution and the reduction of the solution volume.4,5 The other application is adsorptive separation using the reversible change in the surface property of the gel. Specifically, the gel can reversibly adsorb and desorb hydrophobic solutes, and its properties can be controlled by a slight temperature change of approximately 10 K.6-9 This property can be used for an adsorption column method.10,11 In this study, we used the change in the surface property of the gel and combined this characteristic with a membrane separation process. The concept of the proposed system is illustrated schematically in Figure 1. Consider a hydrophobic interaction between membranes and solutes. A solution containing hydrophobic and hydrophilic solutes is continuously supplied to the feed side, and the membrane temperature is changed stepwise below and above the LCST. Above the LCST, only hydrophilic solutes will permeate through the pores of thermosensitive membranes, and hydrophobic solutes will adsorb on the pore surface of the membranes because of a hydrophobic interaction. Changing the temperature to a value below the LCST results in the pore surface becoming hydrophilic, and the adsorbed hydrophobic solutes desorb and concentrate in the permeate side. With a stepwise temperature change below and above the LCST, only specific solutes will be concentrated and purified stepwise in the permeate side. The predicted results for this separation system are shown in Figure 2. We can collect * To whom correspondence should be addressed. Phone: +81-3-5841-7345. Fax: +81-3-5841-7227. E-mail: yamag@ chemsys.t.u-tokyo.ac.jp.
Figure 1. Separation system using a thermosensitive gel membrane.
Figure 2. Predicted solute concentration in the permeate (feed: hydrophobic and hydrophilic solute mixture).
hydrophobic and hydrophilic solutes separately in the permeate side. Potential application of this system is
10.1021/ie9907627 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/02/2000
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purification of bioproducts such as protein or water treatment. The system can separate macromolecules by adsorption-desorption phenomena on the pore surface, and large particles can be rejected by the membrane pores. The proposed system will have several advantages compared with conventional separation processes. The adsorption selectivity used in the system is unlike ultrafiltration so that a mixture of similar molecular sizes can be separated. The system can operate with external stimuli swings without an elution liquid, unlike chromatographic separation.12,13 The process has feed and permeate streams, and the operator can allow the feed stream to flow continuously and can collect the permeate with a temperature swing operation. A thin thermosensitive membrane will enhance heat transfer compared with an adsorption column, and the time lag for a temperature change will be ignored for a temperature swing operation. The performance does not seriously depend on the membrane adsorption capacity if the time interval between adsorption and desorption is controlled. To obtain the present system, we needed to make a porous membrane with a fast response thermosensitive polymer on its pore surface. There have been several reports of stimuli responsive membranes.14-18 Most cross-linked hydrogels respond slowly to environmental changes because of poor water diffusivity in the gel. A linear grafted polymer fixed on a porous substrate can improve the response so that a linear grafted membrane will show a fast response.16 Thus, we employed graft polymerization in this study. To initiate the graft polymerization for a porous substrate, several treatment techniques can be used. Plasma is a well-known technique for modifying the surface of materials and can maintain the bulk properties of the substrate including mechanical strength. However, we found that a grafted polymer can be formed in the pores of porous substrates by using plasma-induced graft polymerization.19,20 The graft polymer formation profile in the substrate will be controlled, and the pores of a porous membrane covered with a thermosensitive linear polymer will be prepared for the system. The grafted chain covalently bonds to the pore surface, and the membrane will show durable properties. In this study, N-isopropylacrylamide (NIPAM) with an LCST of 32 °C was used as the grafted polymer. To understand the performance and potential of the system, we first investigated a simple system using nonionic surfactant solutes. The system was evaluated using a single-component solution. Experimental Section Materials. Porous polypropylene (PP) film was used as the porous substrate. The PP substrate, with a thickness of 25 µm and a pore size of 0.25 × 0.075 µm, was supplied by Celgard Co. Ltd. NIPAM was used as the grafted monomer. Poly(oxyethylenenonylphenyl ether) (NP-10), supplied by Nikko Chemical Co. Ltd., was used as a model solute for the proposed system. NP10 is a nonionic surfactant consisting of 10 ethylene oxide units and nonylphenyl groups and a molecular weight of approximately 700 g/mol. The NIPAM grafted membrane was made by a plasma-graft-filling polymerization technique, which has been described elsewhere.19,20
Figure 3. Relationship between the grafting time and grafted amount (plasma power, 10 W; grafting temp, 60 °C): b, 3 wt % NIPAM in water/MeOH (1/1); O, 1 wt % NIPAM in water.
Morphological Analysis. Morphological details of PP-g-NIPAM membranes were analyzed by microscopic Fourier transform infrared (FT-IR) spectroscopy. The profile of grafted polymer formation was obtained by measuring the ratio of the FT-IR amide II peak (1550 cm-1) to the methylene peak (1450 cm-1), and the spectrum was scanned at 3 µm steps in the crosssectional direction. The aperture size of the measurement was 10 × 50 µm. The surface areas of the grafted membranes and substrates were measured by Brunauer-Emmett-Teller (BET) nitrogen adsorption. Temperature Swing Membrane Separation. A single membrane prepared does not have enough internal surface area for adsorption, and a sandwiched membrane consisting of 10 sheets of PP-g-NIPAM membrane was used for the system operation using NP-10. Ten sheets of membranes were placed on a membrane cell, and those membranes were fixed by an O-ring without further treatment. The effective membrane area was 10 cm2. The grafting amount of PP-gNIPAM membranes was between 0.05 and 0.17 mg/cm2. Two feed reservoirs containing approximately 200 ppm NP-10 were prepared, and the temperature of one reservoir was kept below the LCST of NIPAM, while the other reservoir temperature was kept above the LCST. These two liquids were alternately supplied to the membrane cell. The membrane temperature was changed stepwise, and each step was maintained for 20 min. The concentration of NP-10 in the permeate side was measured by UV-visible spectroscopy. The applied pressure at the feed side was kept between 0.25 and 0.65 kgf/cm2. Results and Discussion Membrane Preparation. Thin porous PP-g-NIPAM membranes with a grafted amount of 0.03-0.45 mg/cm2 were obtained. The relationship between the grafted amount and time is shown in Figure 3. The grafted amount was both proportional to and controlled by the reaction time. The figure also shows the effect of monomer solvent on the grafting rate. The monomer solvent was a pure water or water/methanol mixture. The grafting rate with a methanol aqueous solution was lower than that with a water solvent. Water played an important role in obtaining a high grafting rate for this reaction.20 We could control the grafting rate by changing the monomer solvent composition. Morphological Analysis of the Grafted Membrane. FT-IR spectra of a PP-g-NIPAM membrane
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Figure 4. Grafted layer formation profile in a PP-g-NIPAM membrane: (O) monomer solution, 1 wt % NIPAM in water; amount of grafting, 0.35 mg/cm2; (b) monomer solution, 3 wt % NIPAM in water/MeOH (1/1); amount of grafting, 0.17 mg/cm2.
showed NIPAM characteristic peaks of NH at 31253570 cm-1, CdO at 1650 cm-1, and amide II at 15151670 cm-1. To evaluate the grafting profile of the membrane, the FT-IR peak ratio of amide II (derived from NIPAM) at 1550 cm-1 to the methylene group (from PP) at 1450 cm-1 was obtained across the membrane cross section. Figure 4 shows NIPAM grafted polymer formation in the PP substrate with a water solvent and a methanol/water solvent. The grafted membrane was 25-30 µm in thickness, and the figures show the cross-sectional view of the membranes. Using a methanol/water mixture as the solvent, a homogeneous grafting formation throughout the thickness of the membrane was obtained. On the other hand, the membrane made with a water solvent had an asymmetric grafting formation, and the grafted amount at the surface was larger than that in the center. The grafting process included two steps: the diffusion of
monomer through the pores, and then the reaction of grafted polymer from the radicals. The balance between the two steps determined the location of the grafted polymer in the substrate. Increasing the methanol concentration led to a decreasing reactivity relative to the monomer diffusivity. This resulted in a homogeneous grafted polymer formation throughout the substrate. Changing the solvent controlled the membrane morphology. As shown in this study, homogeneous membranes were obtained by using methanol aqueous solvents. These were used as the adsorptive thermosensitive membranes for evaluating the system. Parts a and b of Figure 5 show surface morphological changes of the porous substrate and PP-g-NIPAM membrane, observed by scanning electron microscopy (SEM). Although the grafting enlarged the pore size, the effect was not serious and we could observe the substrate matrix and pores in the dry state grafted membrane. Figure 6 shows the dependence of the grafting amount on the surface area of the dry membranes. The total surface area of the membrane decreased with an increase in the grafting amount because the substrate matrix was covered with grafted polymer and the pore size gradually decreased as shown in SEM photographs. A decrease in the surface area led to a decrease in the adsorption amount on the pore surface; the reduction was just below 0.1 mg/cm2. Adsorptive membranes with 0.1 mg/cm2 grafting were employed and are described in the following section. Temperature Swing Membrane Separation. Figure 7 shows the NP-10 concentration in permeate and solution flux behavior through the sandwiched PP-gNIPAM membrane in response to a stepwise temperature change between 14 and 39 °C. As expected, water flux at 14 °C decreased by a factor of 6 compared with
Figure 5. Scanning electron microphotographs of (a) a porous substrate and (b) the PP-g-NIPAM membrane.
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Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 Table 1. Adsorption Amount, Concentrated Amount, and Recovery Ratio of the Separation System for a Temperature Swing Operation adsorption amount [mg/m2] temp of the lower temp period (°C) 1st cycle 2nd cycle 3rd cycle average
Figure 6. Dependence of the grafting amount on the surface area of the dry membrane (plasma power is 10 W and the monomer solution is 3 wt % NIPAM in water/MeOH (1/1)).
Figure 7. NP-10 concentration and flux behavior through the PPg-NIPAM membrane system in response to a stepwise temperature change between 14 and 39 °C.
Figure 8. NP-10 concentration and flux behavior through the PPg-NIPAM membrane system in response to a stepwise temperature change between 24 and 38 °C.
the flux at 39 °C. The change was quick and almost completed within 60 s. NP-10 was adsorbed on the pore surface at 39 °C and desorbed at 14 °C in accordance with the concept. The pore surface of membrane reproducibly changed between hydrophilic and hydrophobic in response to a stepwise temperature change. Desorbed NP-10 had a concentration between 250 and 800 ppm in the permeate when the feed was switched to a lower temperature solution. The effect of temperature was also investigated. Figure 8 shows the solution flux and concentration change in the permeate for a lower operating temperature fixed at 24 °C. The flux changed from 11 kg m-2 h-1 at 38 °C to 5 kg m-2 h-1 at 24 °C, although the flux at 14 °C was 2 kg m-2 h-1. Grafted chains did not completely swell in the pore at 24 °C. However, desorbed NP-10 had concentrations of 200-900 ppm in the permeate when a lower temperature solution was supplied. NP-10 in the permeate had 4.5 times the concen-
14
24
0.28 0.26 0.27 0.27
0.27 0.25 0.25 0.26
conc amount [mg/m2] 14 0.19 0.17 0.18 0.18
recovery ratio [%]
24
14
24
0.23 0.20 0.20 0.21
57 65 68 67
84 79 79 80
tration of NP-10 in the feed solution. The results show that desorption and concentration in the permeate took place more efficiently for the 24 °C case in comparison with the 14 °C case. The adsorption and desorption processes were applied several times, and the adsorption and concentration cycles were reproducible. To interpret the results, the adsorption and concentration were estimated from the permeation results shown in Figures 7 and 8. The amount adsorbed on a pore surface can be obtained from the breakthrough curve of the solute concentration in the permeate. Each desorption peak indicates the desorbed amount that permeated through the membrane and in this study is defined as the concentrated amount. The total desorption amount consisted of the desorbed amount that returned to the feed side and the concentrated amount that passed through the membrane. Assuming that the adsorption amount is equal to the desorption amount for one cycle, because similar results can be obtained for repeated cycles, the adsorption amount minus the concentrated amount corresponds to the desorbed amount that returned to the feed side. Although there should be permanent adsorption, this was not taken into account in the estimation because this portion was not available during continuous operation. Mass balances for the adsorbed and concentrated amounts were calculated for each cycle by equations given below, and the results are shown in Table 1. The adsorbed amount, Wad, and concentrated amount, Wperm., can be obtained as follows:
Wad )
∫
Wperm )
JA(Cf - Cp) dt 100As
∫
JA(Cp - Cf) dt 100As
(1)
(2)
where Cf and Cp [wt %] are solute concentrations in the feed and permeate, respectively, J [kg m-2 h-1] is the flux, t [hours] is time, A is the effective membrane area [m2], and As [m2] is the specific surface area of the dry membrane estimated by BET adsorption. The recovery ratio, R, is defined by the following equation:
R ) Wperm/Wad
(3)
For both lower temperature cases, the adsorption amounts did not differ significantly. Both the concentrated amount and the recovery ratio at 24 °C were greater than those at 14 °C. The pore surface at 14 °C must be more hydrophilic than the surface at 24 °C, and the desorption amount at 14 °C must be greater than that at 24 °C. The discrepancy in the desorption could be derived from the mass transfer of solutes in the pores.
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The permeability at 24 °C was 2.5 times higher than that at 14 °C. Permeability generally depends on the pore diameter and solution viscosity. The viscosity ratio between 14 and 24 °C is about 1.3, and the temperature dependence of viscosity might not be important in the system. However, the elevated temperature must lead to the enlargement of the pore size as well as a high solute diffusivity in membrane pores. As the results show, solutes at 24 °C condensed more efficiently than solutes at 14 °C in the permeate. These results demonstrated that the pore size below the LCST should be affected by condensing in the permeate side due to mass transfer in the pores. The proposed system has the potential to apply bioproduct purification or water treatment technology. However, the system needs to enhance the surface area of a single membrane and optimize mass-transfer properties and adsorption-desorption phenomena. Then, the system must be compared with conventional separation systems. Conclusions A novel separation system was proposed using porous thermosensitive membranes. The pore surfaces of a porous substrate were covered with NIPAM thermosensitive grafted polymer, which enabled the hydrophobicity of the pore surface to be dramatically varied by a slight change in temperature. The membrane was made by a plasma-graft-filling polymerization technique. NIPAM grafted polymer formation in the porous substrate was controlled by changing the monomer solvent. Below 0.1 mg/cm2 grafting, the substrate matrix was covered with grafted polymer and still retained the porous structure and surface area of the dry state. NP-10 was adsorbed on the pore surface at 38 °C and desorbed at 14 or 24 °C in accordance with the proposed concept. Desorbed NP-10 was concentrated in the permeate side. The pore size below the LCST should be affected by the solute concentration in the permeate side due to mass transfer. The validity of the proposed system was demonstrated. The system enabled us to operate continuous concentration and/or purification cycles with a temperature swing operation. Acknowledgment We thank Asahi Chemical Co. Ltd. for supplying the porous substrate, Kozin Co. for supplying N-isopropylacrylamide, and Nikko Chemical Co. for supplying NP-10. Literature Cited (1) Eremeev, N. L.; Sigolaeva, L. V.; Kast, O. A.; Kazanskaya, N. F. Concentrating protein solutions with the use of temperaturesensitive gels. Biotekhonologiya 1994, 8, 25. (2) Park, T. G. Stabilization of enzyme immobilized in temperature-sensitive hydrogels. Biotechnol. Lett. 1993, 15, 57.
(3) Kayaman, N.; Kazan, D.; Erarslan, A.; Okay, O.; Baysal, B. M. Structure and protein separation efficiency of poly(Nisopropylacrylamide) gels: Effect of synthesis conditions. J. Appl. Polym. Sci. 1998, 67, 805. (4) Freitas, R. F. S.; Cussler, E. L. Temperature-sensitive gels as extraction solvents. Chem. Eng. Sci. 1987, 42, 97. (5) Huang, X.; Unno, H.; Akehata, T.; Hirasa, O. Analysis of kinetic behavior of temperature-sensitive water-absorbing hydrogel. J. Chem. Eng. Jpn. 1987, 20, 123. (6) Yamagiwa, K.; Sasaki, T.; Ohkawa, A.; Hirasa, O. Adsorption of hydrophobic nonionic surfactant on poly(vinylmethyl ether) gel. J. Chem. Eng. Jpn. 1993, 26, 747. (7) Yamagiwa, K.; Sasaki, T.; Takesono, S.; Ohkawa, A.; Hirasa, O. Adsorption of Triton X-100, Tryptophan and BSA on temperature-sensitive poly(vinylmethyl ether) gel. J. Chem. Eng. Jpn. 1995, 28, 697. (8) Kanazawa, H.; Yamamoto, K.; Matsushima, Y.; Takai, N.; Kikuchi, A.; Sakurai, Y.; Okano, T. Temperature-responsive chromatography using poly(N-isopropylacylamide)-modified silica. Anal. Chem. 1996, 68, 100. (9) Sugiyama, K.; Misuno, S.; Shiraishi, K. Adsorption of protein on the surface of thermosensitive poly(methyl methacrylate) microspheres modified with the N-(2-hydroxylpropyl)methacrylamide and 2-(methacryloyloxy)ethylphosphorylcholine moieties. J. Polym. Sci., Polym. Chem. Ed. 1997, 35, 3349. (10) Seida, Y.; Nakano, Y.; Ichida, H. Surface Properties of Temperature-Sensitive N-Isopropylacrylamide-Copolymer Gels. Kagaku Kogaku Ronbunshu 1992, 18, 346-352. (11) Seida, Y.; Nakano, Y. Adsorption and Desorption Properties of Thermosensitive Polymer Hydrogel under Temperature Swing. Kagaku Kogaku Ronbunshu 1999, 25 (6), 1024-1026. (12) Kim, M.; Saito, K.; Furusaki, S.; Sato, T.; Sugo, T.; Ishigaki, I. Adsorption and elution of bovine gamma globulin by affinity membrane containing hydrophobic amino acid as a ligand. J. Chromatogr. 1991, 585, 45. (13) Kim, M.; Saito, K.; Furusaki, S.; Sugo, T.; Ishigaki, I. Protein adsortion capacity of porous affinity membrane based on polyethylene matrix. J. Chromatogr. 1991, 586, 27. (14) Iwata, H.; Oodate, M.; Uyama, Y.; Amemiya, H.; Ikada, Y. Preparation of temperature-sensitive membranes by graft polymerization onto a porous membrane. J. Membr. Sci. 1991, 55, 119. (15) Ito, Y.; Inaba, M.; Chung, D. J.; Imanishi, Y. Control of water permeation by pH and ionic strength through a porous membrane having poly(carboxylic acid) surface-grafted. Macromolecules 1992, 25, 7313. (16) Lee, Y. M.; Shim, J. K. Preparation of pH/temperature responsive polymer membrane by plasma polymerization and its riboflavin permeation. Polymer 1997, 38, 1227. (17) Mika, A. M.; Childs, R. F.; Dickson, J. M.; McCarry, B. E.; Gagnon, D. R. J. Membr. Sci. 1995, 108, 37. (18) Yamaguchi, T.; Ito, T.; Sato, T.; Shinbo, T.; Nakao, S. Development of a Fast Response Molecular Recognition Ion Gating Membrane. J. Am. Chem. Soc. 1999, 121, 4078. (19) Yamaguchi, T.; Nakao, S.; Kimura, S. Plasma-graft filling polymerization: Preparation of a new type of pervaporation membrane for organic liquid mixtures. Macromolecules 1991, 24, 5522. (20) Yamaguchi, T.; Nakao, S.; Kimura, S. Evidence and mechanisms of filling polymerization by Plasma-Induced graft polymerization. J. Polym. Sci., Polym. Chem. Ed. 1996, 34, 1203.
Received for review October 21, 1999 Revised manuscript received February 15, 2000 Accepted February 19, 2000 IE9907627
