Layer-by-layer self-assembled polyelectrolyte membranes for solvent dehydration by pervaporation

Layer-by-layer self-assembled polyelectrolyte membranes for solvent dehydration by pervaporation

Materials Science and Engineering C 27 (2007) 612 – 619 www.elsevier.com/locate/msec Layer-by-layer self-assembled polyelectrolyte membranes for solv...

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Materials Science and Engineering C 27 (2007) 612 – 619 www.elsevier.com/locate/msec

Layer-by-layer self-assembled polyelectrolyte membranes for solvent dehydration by pervaporation Zhaoqi Zhu, Xianshe Feng ⁎, Alexander Penlidis Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 Received 7 November 2005; accepted 3 December 2005 Available online 27 June 2006

Abstract Polyelectrolyte composite membranes were prepared by the electrostatic layer-by-layer self-assembly of oppositely charged polyelectrolytes, and the membranes were used for dehydration of isopropanol by pervaporation. The effects of membrane preparation conditions on the separation performance of the resulting membranes were investigated. It was found that a high charge density of the polyelectrolyte was favorable to the formation of permselective membranes and that the polyelectrolyte molecules should be sufficiently larger than the pore size of the microporous substrate in order to reduce the number of polyelectrolyte depositions required to form a defect-free membrane. It has been demonstrated that using the appropriate substrate and under suitable conditions for polyelectrolyte deposition, a permselective membrane can be formed with as few as two polyelectrolyte bilayers, which is much less than those commonly used in the literature (e.g., 60–90 bilayers). At 70 °C, the polyelectrolyte composite membrane with two bilayers exhibited a flux of 1.8 kg/(m2 h) and a permeate water concentration of over 98 wt.% for the dehydration of isopropanol containing 9 wt.% water, which corresponds to a separation factor of greater than 495. © 2006 Elsevier B.V. All rights reserved. Keywords: Self-assembly; Polyelectrolytes; Pervaporation; Composite membrane; Isopropanol dehydration

1. Introduction Pervaporation is an energy-efficient alternative to distillation for the dehydration of aqueous organic mixtures, especially for azeotropic and close-boiling liquid mixtures. However, the economic benefit potential of using pervaporation for the dehydration of organic solvents has not been fully worked out in industry because of the lack of availability of high performance commercial pervaporation membranes. Extensive efforts have been made to develop advanced membranes by exploiting improved membrane materials and/or engineering approaches of controlling membrane structures and morphologies [1–3]. Polyelectrolytes are promising membrane materials for solvent dehydration by pervaporation because of their excellent hydrophilicity. Electrostatic layer-by-layer assembly of oppositely charged polyelectrolytes is a demonstrated technique for making nanostructured polymer composites from polyelectrolytes [4]. Each polycation or polyanion single layer can be as thin as 0.5–3 nm, ⁎ Corresponding author. Fax: +1 519 746 4979. E-mail address: [email protected] (X. Feng). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.12.002

depending on ionic strength of the polyelectrolyte as well as concentration, temperature and pH of the polyelectrolyte solution during electrostatic deposition. This technique has been used to make composite polyelectrolyte membranes with a self-assembled separating layer for pervaporation separations [5–13]. Because the thin separating layer can be tailor-made from a wide variety of polyelectrolytes that are currently available, the nano-assembly of polyelectrolyte bilayers is expected to become a promising technique to fabricate pervaporation membranes for solvent dehydration. However, the present literature data show that a significantly large number of depositions of anionic/cationic polyelectrolytes are often needed in order to achieve a high membrane selectivity, especially when the substrate membrane is microporous [14]. For instance, van Ackem et al. [15] found that after 60 cycles of dip coatings with poly(allylamine hydrochloride) and various anionic polymers, the selectivities of the resulting membranes for ethanol dehydration were still rather low. It is believed that in spite of the large number of dip coatings with the polyelectrolyte, the deposition layer was still very thin (ca. 60 nm) and not all the pores on the substrate were plugged or bridged. This is supported by the

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fact that the oxygen/nitrogen permeance ratio was essentially the same as for the microporous substrate itself [15]. If open pores are present on the membrane surface, the membrane will be defective and nonselective. In addition, considering that each cycle of polyelectrolyte deposition takes more than 1 h, from a membrane manufacturing point of view the time required for membrane formation would be too long to be practically acceptable if a large number of deposition cycles are required. It is thus highly desired to reduce the number of polyelectrolyte depositions substantially while still retaining a good permselectivity for possible commercial use. This motivates us to closely look at the properties of the substrate and the deposition solutions as well as the deposition procedure, and a modified single-sided deposition technique has thus been developed [14]. It has been demonstrated that compared with the polyelectrolyte membranes reported in the literature, a 10-fold reduction in the cycles of polyelectrolyte deposition can be achieved while still retaining a very good selectivity by using the modified deposition technique. The separation performance of a polyelectrolyte membrane is primarily determined by the chemical properties of the polyelectrolytes and the structure and morphology of the membrane. The parameters involved in the membrane formation procedure affect the membrane structure. In our previous study [14], the effectiveness of the single-sided deposition method was demonstrated with the membrane fabrication conditions being pre-selected empirically, and the effects of membrane preparation conditions on the separation performance of the resulting membranes were not investigated. The present study addresses the dehydration of isopropanol by pervaporation using polyethylenimine/poly(acrylic acid) and chitosan/poly(acrylic acid) composite membranes and the influence of membrane formation conditions on the separation performance. Hydrolyzed polyacrylonitrile ultrafiltration membrane was used as the substrate, and the effects of hydrolysis conditions were also investigated. 2. Experimental

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2.2. Membrane preparation The PAN membrane was hydrolyzed in 1 N sodium hydroxide solution. The hydrolysis will render the membrane surface negatively charged. After thorough rinsing with de-ionized water, the hydrolyzed PAN membrane was used as a substrate for the initial cycle of polycation deposition. The contact angle of the substrate membrane was measured with a Cam-plus contact angle meter. In order to accomplish polyelectrolyte deposition onto only one side of the substrate surface, a single-sided deposition technique [14] developed recently was used. Basically, the PAN substrate was first mounted in the sealing assembly of a wide-mouth bottle containing the polyelectrolyte solution, and then the bottle was set upside down so that the polyelectrolyte would contact the surface of the PAN membrane for 30 min. Excessive polyions that were adsorbed on the substrate surface were washed away by rinsing with water, followed by subsequent deposition of oppositely charged polyions. As such, a pair of electrostatic self-assembled PEI/PAAc polyelectrolyte layers was formed. These steps, i.e., i) polycation depotion, ii) rinsing with water to remove excess polycations on membrane surface, iii) polyanion deposition, and iv) rinsing with water to remove excess polyanions on membrane surface, were repeated to form multiple polyelectrolyte bilayers based on the electrostatic layer-by-layer self-assembly. Polyelectrolyte solutions at a concentration of 0.02 monomol/ L (monomol = mole of monomer unit) were used for layer-bylayer buildup. They were prepared by dissolving the polyelectrolytes in either deionized water (for PAAc and PEI) or a dilute aqueous solution of hydrochloric acid (for chitosan). The solution pH was measured to be 5.5–6 for PAAc and 5 for chitosan, and the pH for PEI was adjusted to 5 using hydrochloric acid. All the membrane samples used in this work have less than 10 bilayers. The water-wet polyelectrolyte composite membranes were post-treated by drying in an oven with forced air circulation at 50 °C overnight to facilitate the conformation change of the polyelectrolytes in the membrane. Table 1 shows the empirically pre-selected “base-line” conditions for membrane preparation.

2.1. Materials 2.3. Pervaporation test Polyacrylonitrile (PAN) ultrafiltration membranes with a nominal molecular weight cutoff of 20,000 were supplied by Sepro Membranes. The PAN membranes were hydrolyzed under controlled conditions before being used as the substrate for the layer-bylayer deposition of polyelectrolytes. Polyethylenimine (PEI, Mw 750,000) and poly(acrylic acid) (PAAc) (Mw 1,000,000) were used as the polycation and polyanion, respectively. The PEI, which had a short branched structure, was supplied by Aldrich Chemicals in the form of an aqueous solution containing 50 wt.% PEI. PAAc with different molecular weights was obtained from Aldrich (35 wt.% in water, Mw 250,000) and Polysciences (fine powders, Mw 1,000,000). The high molecular weight grade of chitosan (degree of deacetylation 75–85%) was supplied by Aldrich. Sodium hydroxide and isopropyl alcohol were obtained from EM Science, and hydrochloric acid (37 wt.% aqueous solution) was purchased from Aldrich Chemicals. De-ionized water was used as the solvent in preparing the polyelectrolyte solutions for sequential layer-bylayer deposition of the oppositely charged polyelectrolytes.

The performance of the composite polyelectrolyte membranes for dehydration of isopropanol by pervaporation was evaluated. The experimental setup and procedure have been described elsewhere [16]. The feed temperature was controlled with a thermal Table 1 Pre-selected “base-line” conditions for membrane fabrication Hydrolysis of polyacrylonitrile substrate

1 N NaOH solution at 75 °C for 20 min

Polyelectrolytes

PAAc, Mw = 1,000,000 PEI, Mw = 250,000 0.02 monomol/L

Deposition conditions

Post-treatment

Polyanion Polycation Solution concentration Temperature Time Temperature Time

50 °C 30 min 50 °C 12 h

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bath. The liquid solution was pumped to the permeation cell that housed the membrane, and vacuum was applied to the downstream side of the membrane. During the experiments, the permeate pressure was maintained below 5 mmHg. The effective area of the membrane for permeation was 13.9 cm2. The permeate vapor was condensed and collected in cold traps immersed in liquid nitrogen. The compositions of the feed and permeate were analyzed with a Hewlett Packard gas chromatograph (HP Series II 5890) equipped with a thermal conductivity detector. An aqueous isopropanol (IPA) solution containing 9 wt.% of water was used as the feed for pervaporation experiment. Throughout the experiments, the quantity of permeate removed by the membrane during each run was below 0.1% of the initial feed loaded in the feed tank, and consequently the variation in the feed composition during a pervaporation run was negligible. The membrane permeability was characterized by the permeation flux determined by J =Q / (At), where Q is the amount of permeate collected over a given period of time t, and A is the membrane area, whereas the membrane selectivity was measured by the permeate concentration for a given feed concentration (i.e., 9 wt.% water). 3. Results and discussion 3.1. Effect of PAN hydrolysis time and temperature It has been reported that the hydrolysis of microporous PAN ultrafiltration membrane would change the pore size on the membrane surface [17 18]. The number of electrostatic interaction “sites” on a hydrolyzed PAN substrate membrane is affected by the degree of hydrolysis. Both the pore size and the degree of hydrolysis of the PAN substrate directly influence the deposition of polyelectrolytes on the substrate. It is therefore anticipated that the hydrolysis conditions of the PAN substrate membranes will affect the separation performance of the resulting composite polyelectrolyte membranes. Fig. 1 shows the permeation flux and permeate water concentration as a function of temperature for membranes prepared with PAN substrates hydrolyzed for 20 min at different temperatures. These membranes are comprised of 4 selfassembled PEI/PAAc bilayers. In general, the polyelectrolyte membranes are very permselective to water. At a feed concentration of 9 wt.% water, a permeate water concentration of as high as 99.9% has been achieved. It is shown that an increase in the hydrolysis temperature of the PAN substrate tends to increase the membrane selectivity while the permeation flux decreases. The membrane selectivity appears to be affected by the hydrolysis temperature significantly. While increasing the hydrolysis temperature from 70 to 75 °C resulted in a considerable increase in the water concentration in the permeate, a further increase in the hydrolysis temperature from 75 to 80 °C did not lead to notable improvement in the membrane selectivity. The purpose of hydrolyzing PAN substrate is mainly to change the surface properties. Hydrolysis of PAN in a sodium hydroxide solution will transform the surface –CN groups into carboxylic groups, and therefore the substrate surface becomes negatively charged. This will enhance the deposition of cationic polyelectrolyte on the substrate membrane. The higher the temperature,

Fig. 1. Permeation flux and permeate water concentration as a function of temperature for membranes prepared with PAN substrates hydrolyzed for 20 min at different temperatures: (Δ) 70 °C, (□) 75 °C, (⋄) 80 °C. (Feed water content 9 wt.%, number of polyelectrolyte bilayers 4).

the faster the hydrolysis, and for a given period of time an increase in the hydrolysis temperature will increase the degree of hydrolysis. The contact angle can be used as a measure of the degree of hydrolysis. When the –CN groups were transformed into the more hydrophilic carboxylic groups, the contact angle of the hydrolyzed PAN membrane decreased. This can be seen from Table 2 which shows the contact angle of PAN membranes hydrolyzed at different temperatures. It may also be pointed out that hydrolysis will decrease the pore size of the wet PAN membranes [18], presumably due to the increased hydrophilicity that intensifies membrane swelling by water. The mass transfer resistance of the substrate will thus increase. It is therefore reasonable that while the hydrolysis of PAN substrate is expected to enhance the selectivity of the resulting polyelectrolyte membranes, the permeation flux will be compromised. Fig. 2 shows the separation performance of the membranes prepared with PAN substrates hydrolyzed at 75 °C for different periods of time, and the corresponding contact angles of the PAN substrate are presented in Table 3. At a hydrolysis time of 10 min, the contact angle of hydrolyzed PAN membrane was 72°, which is relatively high. This means only a small fraction of –CN groups are converted into the more hydrophilic carboxyl groups. Therefore, the amount of polyelectrolyte that could be adsorbed onto the charged PAN substrate surface is limited, which results in a low selectivity and a high flux. When the hydrolysis time was increased to 20 min, a substantial increase in the membrane selectivity was observed. However, a further increase in the hydrolysis time did not further improve the membrane selectivity. Note that excessive hydrolysis would affect the mechanical strength and integrity of the substrate membrane. It is shown that the hydrolysis of PAN substrate

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Table 2 Contact angle of polyacrylonitrile substrate membranes hydrolyzed at 75 °C for different periods of time

Table 3 Contact angle of polyacrylonitrile substrate membranes hydrolyzed for 20 min at different temperatures

Hydrolysis time (min) Contact angle (°)

Hydrolysis temperature (°C) Contact angle (°)

30 60

20 68

10 72

membranes in 1 N NaOH solution at 75 °C for a period of 20 min was appropriate. 3.2. Effect of polyelectrolyte properties As the polyelectrolyte membrane formation is mainly driven by the electrostatic layer-by-layer assembly, the charge density and molecular weight of the polyelectrolytes are important to the separation performance of the membranes. The electrostatically assembled polyelectrolyte membranes may be considered to have a physical network structure with ionic crosslinking of the oppositely charged ion pairs derived from the cationic and anionic polyelectrolytes. The crosslinking density of the polyelectrolyte network will be affected by the charge density of the polyelectrolytes. A higher charge density means easy access of the crosslinking sites for the formation of the polyelectrolyte complex. Chitosan is a weak, low charge density cationic polyelectrolyte. Different pervaporation membranes made from chitosan and chitosan derivatives have been reported for the dehydration of organic solvents [16,19]. More recently, chitosan has also attracted attention as a polyelectrolyte to construct layer-by-layer coatings endowed with biospecific properties [20]. PEI is also a weak polyelectrolyte but with a higher charge density than chitosan. It is of interest to see how the membranes prepared using chitosan as the cationic polyelectrolyte perform for pervaporation separation of liquid mixtures. The se-

Fig. 2. Permeation flux and permeate water concentration for membranes prepared with PAN substrates hydrolyzed at 75 °C for different periods of time: (□) 10 min, (Δ) 20 min, (⋄) 30 min. (Feed water content 9 wt.%, number of polyelectrolyte bilayers 4).

80 38

75 68

70 80

paration performance of the polyelectrolyte membranes prepared from PEI and PAAc was compared with that of the membranes made from chitosan and PAAc under the same conditions, and the results are shown in Fig. 3. Fig. 3 shows that the charge density of the polyelectrolytes significantly influences the membrane permselectivity of the membranes. The membranes fabricated from PAAc and high charge density PEI are more selective to water permeation for the dehydration of isopropanol than the membranes prepared from PAAc and low charge density chitosan, but the former membrane is less permeable. This agrees with physical reasoning that a high charge density favors the formation of “tight” polyelectrolyte complex networks with densely packed cross linkages between the oppositely charged polyelectrolytes. As a result, the membrane will be more discriminating to the permeation of water from mixtures containing less polar and more hydrophobic molecules. Similar results have also been observed with membranes made from anionic poly(styrene sulfonate) and cationic PEI and chitosan, respectively, for the separation of water from ethanol [21]. The importance of polyelectrolyte charge density can also be explained from the standpoint of the polyelectrolyte mutilayer formation process. During the deposition step, there is a competition between the adsorption of the polyelectrolyte from the deposition solution by electrostatic attraction and the desorption of the adsorbed polyelectrolyte from the substrate surface. Adsorption is favored by the high charge density of the

Fig. 3. A comparison of pervaporation performance of polyethylenimine/poly (acrylic acid) membranes (Δ) and chitosan/poly(acrylic acid) membranes (□). (Feed water content 9 wt.%, number of polyelectrolyte bilayers 2).

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polyelectrolyte used. In addition, the surface charge should be overcompensated to lead to a charge reversal, allowing for further deposition of oppositely charged polyelectrolyte. If the adsorbed polyelectrolyte cannot revere the surface charge of the previously adsorbed polyelectrolyte layer, the subsequent further deposition with the oppositely charged polyelectrolyte will be difficult to accomplish due to electrostatic repulsion. Some experimental investigations have revealed that there exists a charge density threshold below which the layer-by-layer self-assembly of oppositely charged polyelectrolytes cannot occur successively to form mutilayer films under electrostatic interactions [22–24]. Another factor influencing the membrane permselectivity is the molecular size of the polyelectrolyte. In order to have a high permeation flux, a microporous substrate can be used as a sublayer for the membrane formation by the layer-by-layer deposition. Depending on the pore size of the microporous substrate relative to the sizes of the polyelectrolytes used for making the membrane, three cases can be distinguished in a simplistic view. If the sizes of the polyelectrolyte “coils” in the deposition solution are smaller than the pore size of the substrate, the polyelectrolyte will enter the pores or be adsorbed on the wall surface of the pores. A thin and defect-free polyelectrolyte layer cannot be formed until the pore sizes become small enough so that at least some of the polyelectrolyte molecules can form a self-assembled layer to bridge the pores. A large number of depositions may be needed to completely seal all pores. The membranes so obtained are usually not highly permeable because the pores in the substrate are filled with the polyelectrolytes. Only when the pore sizes are sufficiently small as compared to the polyelectrolyte coils, will the polyelectrolytes be likely to bridge the pores and form a thin and uniform selfassembled polyelectrolyte layer, which would be highly permselective. Considering that in reality during the initial stages of polyelectrolyte deposition there is no warranty that the surface will be fully covered with the polyelectrolyte islets to form an interconnected domain, the relative sizes of the polyelectrolytes and the pores on the substrate are especially critical for the membrane formation. Fig. 4 illustrates the effect of the molecular weight of PAAc on the separation performance of the polyelectrolyte membranes consisting of 4 bilayers of PEI and PAAc. It is shown that the membrane prepared from the high molecular weight PAAc (Mw = 1,000,000) has better separation performance in terms of membrane selectivity (over 99 wt.% of water was enriched in the permeate). It is not surprising that the permeation flux is lower because the higher the molecular weight of the polyelectrolytes, the thicker the membrane for a given number of deposited bilayers. On the other hand, the membrane that was made from the lower molecular weight PAAc (Mw = 250,000) under the same conditions had a much lower water concentration in the permeate. This could be due to incomplete sealing of the substrate pores by the polyelectrolyte or incomplete coverage of integral polyelectrolyte bilayers. As one may expect, the selectivity of the latter membrane could be increased by increasing the number of the self-assembled polyelectrolyte bilayers; however, when the membrane selectivity became eventually comparable to that of the membrane with the high molecular weight PAAc, the permeation flux was found to be lower primarily due to the increased

Fig. 4. Pervaporation performance of the polyethylenimine/poly(acrylic acid) membranes fabricated with polyanions of different molecular weights: (□) 1,000,000, and (Δ) 250,000. (Feed water content 9 wt.%, number of polyelectrolyte bilayers 4).

membrane thickness. For many polyanion/polycation systems, the thickness of a self-assembled mutilayer film tends to grow either exponentially or linearly with the number of polyelectrolyte depositions [25]. The experimental data in Fig. 4 suggest that high molecular weights of polyelectrolytes are preferred in order to form self-assembled membranes with a limited number of polyelectrolyte depositions while still achieving good permselectivity and integrity. 3.3. Effect of polyelectrolyte deposition conditions The adsorption of polyelectrolytes onto an oppositely charged non-porous surface is reported to be a two-step process, i.e., fast adsorption to the surface, and slower relaxation due to a change in polyelectrolyte conformation at the surface [26]. This mechanism may also apply to the formation of a self-assembled polyelectrolyte membrane over a microporous substrate if the size of polyelectrolyte is much larger than the pores on the substrate. It should be pointed out, however, that while sorption is generally a fast process, for dilute polyelectrolyte solutions, the polyelectrolyte needs to diffuse from the bulk solution to the vicinity of the oppositely charged surface formed previously before the polyelectrolyte can be adsorbed to the surface under electrostatic interactions. The rate of diffusion is influenced by such factors as molecular weight, temperature and concentration, and the diffusivity of polyelectrolyte is expected to be orders of magnitude lower than the liquid diffusivities of small molecules (which are in the order of 10–5 cm2/s). The sorption uptake of polyelectrolyte on the surface will increase with time until an equilibrium or saturated sorption is reached. Polyelectrolytes can undergo a conformational change to yield a more organized structure, which is helpful to the formation of defect-free membranes. Note that in

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a multilayer system, polyelectrolyte molecules in the interior bilayers are restricted and it is difficult for them to change their conformation. As a result, rearrangement of polyelectrolyte molecules on the surface of multilayers is much easier than that in the interior. Therefore, a sufficiently long time should be allowed for diffusion, adsorption and relaxation of the polyelectrolyte molecules before proceeding with the next deposition of the oppositely charged polyelectrolyte for the layer-by-layer buildup. A relatively long time (usually 30 min) for each polyelectrolyte deposition has been used in the literature. Considering the additional time required for the water rinsing step between the sequential depositions of oppositely charged polyelectrolytes, the time needed to fabricate a membrane would be too long to be acceptable from an industrial point of view. A decrease in the deposition time would significantly reduce the overall time required for membrane manufacturing. In Fig. 5, the possibility of using a shorter period of polyelectrolyte deposition time was explored. When the deposition time was reduced from 30 to 20 min, the membrane selectivity was lowered slightly as shown by the decrease in the water permeate concentration, but there was essentially no change in the overall permeation flux. When the diffusion of polyelectrolyte molecules from the bulk solution to the vicinity of a charged surface is the rate controlling step for polyelectrolyte adsorption, a longer deposition time would favor the formation of polyelectrolyte layers with fewer defects because adsorbed polyelectrolytes can change their conformation to self-cure the defects in a separating layer. Although detailed studies on kinetics of polyelectrolyte adsorption were not attempted here, it appears that 30 min of deposition time (which was used as a pre-selected parameter in earlier studies) is appropriate for making polyelec-

Fig. 5. Performance of membranes prepared with different periods of polyelectrolyte deposition time: (Δ) 20 min, (□) 30 min. (Feed water content 9 wt.%, number of polyelectrolyte bilayers 2).

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Fig. 6. Performance of membranes prepared at different temperatures of polyelectrolyte deposition: (⋄) 25 °C, (Δ) 50 °C, (□) 80 °C. (Feed water content 9 wt.%, number of polyelectrolyte bilayers 2).

trolyte membranes investigated here. At 70 °C, the polyelectrolyte composite membrane with only two bilayers exhibited a flux of 1.8 kg/(m2 h) and a permeate water concentration of over 98 wt.% for the dehydration of isopropanol containing 9 wt.% water, which corresponds to a separation factor of greater than 495. This further shows that using appropriate substrate and under suitable

Fig. 7. Performance of membranes post-treated at 50 °C for 1 h (Δ) and 12 h (□). (Feed water content 9 wt.%, number of polyelectrolyte bilayers 2).

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One is to dry the membrane, and the other is to further change the conformation of deposited polyelectrolytes to improve the separation performance of the membrane. Figs. 7 and 8 show the effects of the post treatment time and temperature on the separation performance of the polyelectrolyte membranes. It is shown that the heat treatment will increase the membrane selectivity, but this is accompanied with a decrease in the membrane permeability. This is understandable because this process will cause membrane shrinkage and thus the membrane structure will be become denser. It should be noted that moderate temperatures were used to treat the membranes; it was found that if the temperature was too high, tiny cracks would be developed on the membrane surface because of the excessive stress applied to the thin membrane layer due to fast evaporation of water from the membrane surface. Thermal treatment of polymeric membranes to induce structural change has been widely used in the development of reverse osmosis membranes. 4. Conclusions

Fig. 8. Performance of membranes post-treated at 23 °C (□) and 50 °C (⋄) for a period of 12 h. (Feed water content 9 wt.%, number of polyelectrolyte bilayers 3).

conditions for polyelectrolyte deposition, permselective membranes can be fabricated with as few as two bilayers, which is much less than those commonly used in the literature work (e.g., 60–90 bilayers). An increase in the temperature of deposition solution will not only increase the diffusion rate of polyelectrolytes in a deposition solution but also promote the conformational change of adsorbed polyelectrolytes during the deposition process, which favors the formation of high performance membranes. To investigate the effect of the polyelectrolyte temperature on the membrane performance, three different deposition temperatures were used, and the results are shown in Fig. 6. It can be seen that as deposition temperature increased, the water concentration in the permeate increased, but the permeation flux did not follow a monotonic change. A possible explanation can be found based on the polyelectrolyte layer growth and the likely presence of defects. For a given period of time, fewer polyelectrolyte molecules can be adsorbed at a lower temperature, and there is a greater chance of forming cavity defects in the thin self-assembled polyelectrolyte layer, which would make the membrane more permeable but less selective. When the temperature is high enough that the resulting membrane would become defect-free, a further increase in the temperature would enhance the bilayer growth, resulting in a thicker membrane with thus a lower permeability, but the membrane selectivity would not be affected significantly. 3.4. Post treatment After a self-assembled polyelectrolyte multilayer film is formed on the microporous substrate, the water-wet membrane assembly may be subjected to post treatment before being used for liquid separation by pervaporation. Heat treatment is a commonly used method for conditioning membranes and it serves two purposes.

Polyelectrolyte membranes have been prepared by sequential deposition of oppositely charged polyelectrolyte using the electrostatic self-assembly technique, and the membranes are found to be very selective for dehydration of isopropanol by pervaporation. The effects of some parameters involved in the procedure of membrane formation on the separation performance of the resultant membranes were investigated. It was shown that polyelectrolytes with a high charge density favor the formation of permselective membranes by the electrostatic self-assembly, and the sizes of the polyelectrolyte molecules should be sufficiently large as compared to the pore size of the microporous substrate in order to reduce the number of polyelectrolyte depositions during membrane formation. As such, high molecular weight polyelectrolytes are preferred for making the composite polyelectrolyte membranes. Using hydrolyzed polyacrylonitrile membrane as a substrate, the polyelectrolyte membranes prepared under empirically pre-selected “base-line” conditions showed good performance for isopropanol dehydration. By properly controlling the membrane formation conditions, a permselective membrane can be formed with as few as 2 polyelectrolyte bilayers, which was much less than those commonly used in the literature (e.g., 60–90 bilayers). Acknowledgements Research support from the Natural Sciences and Engineering Research Council (NSERC) of Canada is gratefully acknowledged. References [1] R.Y.M. Huang (Ed.), Pervaporation Membrane Separation Processes, Elsevier, 1991. [2] X. Feng, R.Y.M. Huang, Ind. Eng. Chem. Res. 36 (1997) 1048. [3] A.W. Verkerk, P. van Male, M.A.G. Vorstman, J.T.F. Keurentjes, J. Membr. Sci. 193 (2001) 227. [4] G. Decher, Science 277 (1997) 1232. [5] J. Meier-Haack, W. Lenk, D. Lehmann, K. Lunkwitz, J. Membr. Sci. 184 (2001) 233. [6] W. Lenk, J. Meier-Haack, Desalination 148 (2002) 11.

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