Organic–inorganic composite pervaporation membranes prepared by self-assembly of polyelectrolyte multilayers on macroporous ceramic supports

Organic–inorganic composite pervaporation membranes prepared by self-assembly of polyelectrolyte multilayers on macroporous ceramic supports

Journal of Membrane Science 302 (2007) 78–86 Organic–inorganic composite pervaporation membranes prepared by self-assembly of polyelectrolyte multila...

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Journal of Membrane Science 302 (2007) 78–86

Organic–inorganic composite pervaporation membranes prepared by self-assembly of polyelectrolyte multilayers on macroporous ceramic supports Yiwei Chen, Fenjuan Xiangli, Wanqin Jin ∗ , Nanping Xu Membrane Science and Technology Research Center, State Key Laboratory of Material-Oriented Chemical Engineering, Nanjing University of Technology, Xinmofan Road 5, Nanjing 210009, PR China Received 21 March 2007; received in revised form 10 June 2007; accepted 12 June 2007 Available online 15 June 2007

Abstract An organic–inorganic composite membrane was successfully prepared by means of electrostatic self-assembly of polyelectrolytes on silica sol–gel modified macroporous ceramic supports. The integrality of the as-prepared composite membrane was evaluated in pervaporation of ethanol/water mixtures. Membrane separation performance was optimized by studying the modification cycle, polyelectrolyte molecule structure, self-assemble conditions, thermal treatment conditions and operating temperatures. It was found that the polyelectrolyte molecule structure mainly affected the separation performance of the composite membrane after a two-cycle modification and raising pervaporation operating temperature was an efficient way to improve both membrane flux and water selectivity. The composite membrane deposited by 60 layer pairs of polyethylenimine/poly(vinyl sulfate) (PEI/PVS) showed a high flux of 18.4 kg m−2 h−1 and a water concentration enhancement from 6.2 to 35.3 wt.%. Our study demonstrates that the composite membrane prepared on macroporous ceramic support through our route is suitable for separation of liquid species at a molecular level. © 2007 Elsevier B.V. All rights reserved. Keywords: Composite membrane; Self-assembly; Polyelectrolyte multilayers; Macroporous ceramic support; Pervaporation

1. Introduction Vast majority of the studies on polyanion–polycation complex formation were performed in the bulk solutions until a new approach developed by Decher et al. [1,2] extended the experiments by allowing study the complexes of oppositely charged polyelectrolytes assembled onto solid substrates. Such polyelectrolyte complexes, also mentioned as polyelectrolyte multilayers (PEMs), show higher structural hierarchy that can be fully exploited in the design of macroscopic devices for nonlinear optics, catalysis, microelectronics, as well as biomedicine [3–5]. Due to their extremely low thickness, high homogeneity and nice tunability on molecular dimensions, PEMs are attractive materials as ultrathin selective layers assembled on porous supports to obtain composite membranes for separation



Corresponding author. Tel.: +86 25 83587211; fax: +86 25 83587211. E-mail address: [email protected] (W. Jin).

0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.06.019

of nanoscale species. One important technical factor encountered in electrostatic self-assembly on porous support is the choice of the support pore size. Though larger pore size means less flow resistance and higher flux, the difficulty in forming defect-free polyelectrolyte selective layer on such support is obvious. Since each adsorption step only adds about a few nanometers to the total thickness of the multilayers and the bridging or complexation of polyelectrolyte molecules is under nanoscale [2], the pore size of the support should be restrained within a feasible value to be well covered by PEMs [6,7]. Stroeve et al. [7] used macroporous polypropylene membranes (with 0.4 ␮m × 0.04 ␮m slit-like pores) as the substrate for carrying out self-assembly of PEMs from pair of polyallylamine/polystyrenesulfate (PAH/PSS). It was shown that the PEMs formed on the untreated macroporous support were nonselective for gas permeation even after 40 coatings of the polyelectrolytes. Porous supports such as polypropylene [7], poly(4-methyl1-pentene) (PMP) [8], polyamide [9], polyacrylonitrile (PAN) [6,10–16], poly(vinylpyrrolidone) (PVP) [17] and anodic alu-

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Table 1 Supports with different pore size levels for polyelectrolyte self-assembly to prepare PV membranes reported in literature Supports Organic

Inorganic

Carboxyl functionalized polyamide-6 (obtained by phase-inversion process) PAN, molecular weight cut-off of 20,000, pore size of about 3–5 nm PAN pore size of 20–200 nm Anodic alumina pore size of 20 nm

mina [18–20] was commonly used for self-assembly of PEMs. Some pervaporation membranes fabricated by successful selfassembly of polyelectrolytes on supports with different pore size levels were listed in Table 1, as well as their separation performances. The PEMs on the oxygen plasma treated PAN supports (pore size of 20–200 nm) were extensively studied by Tieke et al. [13–16] and the membranes showed a good selectivity to water in pervaporation while the flux were normally under 1 kg m−2 h−1 . PAN supports of more microporous level (molecular weight cut-off of 20,000 PEG, and the estimated pore size is about of 3–5 nm [21]) was selected by Feng et al. [6,10], and the PEMs deposited on the supports which have been hydrolysed in sodium hydroxide solution showed a better separation performance. Anodic alumina membrane with 20nm diameter surface pores was used by Bruening et al. [18] as supports and treated by UV/O3 cleaning before PEMs selfassembly. The as-prepared composite membrane was able to work in pervaporation, but the anodic alumina supports obtained by a special fabrication procedure had a limited potential for application. One could notice that most successful self-assembly of polyelectrolytes layers for pervaporation membranes are deposited on supports with microporous or mesoporous level. Ceramic microfiltration membranes with macropores as one kind of mature products have been commercially applied for years, the cost of which has been controlled much lower than those of the ultrafiltration or nanofiltration membranes. In our previous work [22], the poly-dimethyl siloxane (PDMS)/ceramic pervaporation membranes prepared by phase inversion of polymers on the macroporous ceramic supports (pore size, 0.2 ␮m) have showed exciting high flux (19.5 kg m−2 h−1 , 70 ◦ C) for ethanol removal in water/ethanol system. For organic–inorganic composite membrane, the organic top layer provides a good selectivity while the inorganic porous support favors the membrane a high flux because it restrains the deformation of organic layer, which might be considered as the effect of confined deformation. The organic–inorganic composite membrane prepared from macroporous supports will be considerably promising, so is the application of electrostatic self-assembly technique on fabricating composite membranes from macroporous supports. However, few studies have been researched on the self-assembly upon the ceramic macroporous supports because of the difficulty to obtain membranes for separation of species at a molecular level.

Membrane performance flux (kg m−2 h−1 )/selectivity

Reference

Water–ethanol (Cw = 10 wt.%), 0.02/1400, temperature:

50 ◦ C

[9]

Water–ethanol (Cw = 9 wt.%), 1.8/495, temperature: 70 ◦ C

[6,10]

Water–ethanol (Cw = 6.2 wt.%), 0.047/556, temperature: 60 ◦ C

[15]

Water–ethanol (Cw = 10 wt.%), 1.9/500, temperature:

50 ◦ C

[16]

In this paper, one new route was firstly proposed to prepare organic–inorganic composite membrane for pervaporation by means of electrostatic self-assembly of polyelectrolytes on pretreated macroporous ceramic supports. A silica sol–gel modification was applied to tune the pore size of the macroporous ceramic supports. Subsequently, a simple cleaning process was performed to make ceramic oxide surface fully hydroxylated. Fig. 1 illustrates the procedure and the necessity of the pretreatment process. As mentioned above, direct self-assembly of the polyelectrolytes on macroporous supports without pretreatment would be incomplete since the weak adsorption of the polyelectrolyte layers cannot cover the large pores on the membrane surface. With the tuning of the pore size of macroporous supports and wet cleaning of the oxide surface, the complete self-assembly could be achieved because the enhanced electrostatic adsorption of the polyelectrolyte layers could bridge the tuned pores. The integrality of the as-prepared composite membrane was studied in the separation of ethanol/water mixtures under pervaporation conditions. We observed that the composite membrane shows a high flux in pervaporation because of its composite structure of thin top layer and non-deformable porous support layer.

Fig. 1. Schematic procedure of the composite membrane fabrication and the influence of the support pretreatment on self-assembly of polyelectrolytes.

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2. Experimental 2.1. Materials As the macroporous support, a ZrO2 /Al2 O3 (top layer/support layer) tubular membrane (mean pore size of top layer, ∼0.2 ␮m) prepared by our Membrane Science & Technology Research Center (Nanjing, China) was used. Poly(allylamine hydrochloride), PAH, (Mw = 70,000), poly (styrenesulfonate), PSS, (Mw = 10,000), polyethylenimine, PEI, (Mw = 20,000), poly (vinyl sulfate), PVS, potassium salt, (Mw = 170,000) and poly (diallyldimethylammonium chloride), PDADMAC, (Mw = 200,000–350,000) was purchased from Aldrich. Tetraethyl orthosilicate (TEOS), N,N-dimethyl formamide (DMF), HNO3 , glycerol and the other reagents were of analytical grade and were used without further purification. All electrolyte solutions were prepared from pure water (conductivity, <5.00 ␮s cm−1 ). 2.2. Pretreatment of the porous supports Surface pretreatments of supports were of great necessity to get an appropriate surface for subsequent self-assembly. A SiO2 sol–gel modification was carried out on the macroporous supports to tune the pore size. Silica sol was prepared using acidic hydrolysis from TEOS. The molar ratio of the reagents was 1:6.4:0.085:3.8:0.8 of TEOS–water–HNO3 –ethanol–DMF. TEOS and ethanol were mixed by stirring in a flask. Then water, HNO3 as a catalyst and DMF as a binder was carefully added into the flask. The sol was stirred for 2 h at 70 ◦ C and then kept without stirring at 40 ◦ C for several days. After diluted with ethanol and added with glycerol (used as a plasticizer), the sol was stirred for 2 h at room temperature and kept for use. The pH of the sol was about 3.8. Sol–gel SiO2 coatings were obtained with a dipping procedure at room temperature. The sol was kept in the tube-side of support for a dipping time of 30 s. After drying in an oven at 50 ◦ C for 1 day, the deposited sol–gel films were sintered at 500 ◦ C for 2 h at a heating rate of 0.5 ◦ C/h. The dipping–coating–sintering procedure was repeated as required. SiO2 -modified supports were first treated in boiled water for 15 min and rinsed to get rid of any adhesive impurity. After soaked in acetone, the supports were transferred to the dilute Piranha solution (H2 SO4 :H2 O2 :H2 O, v:v:v = 1:1:3) at ∼60 ◦ C for 5–30 min. Then the supports were thoroughly rinsed with water and stored in water.

in water, (c) in the solution of the cationic polyelectrolyte and (d) in water again. Steps (a)–(d) were repeated until a concern numbers of deposition cycles of polyelectrolytes were absorbed and ultimately rinsed in water and dried by N2 . The dipping time for every polyelectrolyte solution varied from 5 to 30 min as required and the rest time in water was 5 min. 2.4. Characterization In our previous work [23], we have studied the growth of layer-by-layer (LBL) membranes self-assembled on porous ceramic supports by transmembrane streaming potential measurements. Here, to obtain the morphologies of membranes and confirm the deposition process, characterizations were performed by scanning electron microscopy (SEM). Both surface and cross-section morphologies of the supports and the self-assembled layers were characterized using a SEM (QUANTA-2000). Mean pore size and water permeability of the support before and after the sol–gel modification were measured to evaluate the appropriate cycle of sol–gel modification. Mean pore size of the supports were obtained using gas bubble pressure method (GBP), which were performed following the American Society for Testing and Materials (ASTM) Publication (F316-80). All samples were dipped into isobutyl alcohol (22.783 mN/m, 22 ◦ C) for 2 h under vacuum. During the process of the measurement, the flow rate and the transmembrane pressure of nitrogen were measured and calculated according to the standard. Water permeability was conducted using a cross-flow filtration apparatus under a pressure of 0.1 MPa and a temperature of 25 ◦ C. 2.5. Pervaporation experiments The integrality of the as-prepared composite membrane was evaluated by pervaporation of water/ethanol (H2 O, 6.2 wt.%) mixture. The home-made apparatus was presented schematically in Fig. 2. The effective area of membrane (A) is 17 cm2 . The temperature of the feed in the membrane module was controlled by thermocouple probe linked to a water-bath heater in the feed container. The water/ethanol mixture was circulated from the feed tank to the membrane module at a flow rate of 300 L h−1 . The

2.3. Preparation of the polyelectrolyte multilayers Polyelectrolytes were dissolved in water in a concentration of 2 × 10−2 monomoles l−1 (monomole = mole of monomer unit) and acidified to pH 2 for each pairs of electrolytes using aqueous HCl, if not otherwise stated. For enhancing the adsorption of the individual layers, the pretreated supports were first immersed in PEI solution for 30 min and then rinsed in water. Afterward the supports were immersed in sequence (a) in the solution of the anionic polyelectrolyte, (b)

Fig. 2. Experimental setup for pervaporation measurements.

Y. Chen et al. / Journal of Membrane Science 302 (2007) 78–86 Table 2 Mean pore size and water permeability of the supports after various modification cycles Number of silica sol–gel modification cycle

Mean pore size after modification (␮m)

Water permeabilitya (kg m−2 h−1 bar−1 )

0 1 2 3

0.23 0.15 0.09 0.08

1860 600 280 180

a

Conducted at 25 ◦ C.

permeate pressure was maintained 500 Pa. It took about 30 min to make the system stable. The permeate was first cooled by a condenser and then collected in a cryotrap which was immersed in liquid nitrogen. The permeation flux (J) was determined from the weight (W) of the collected permeant by using the following equation: W At where t is the experimental time interval for the pervaporation. The feed concentration and that of the permeate were determined by the gas chromatography (GC-8A, Shimadzu). The separation factor α was calculated from the quotient of the weight ratio of water and ethanol in the permeate YH2 O /YEtOH and in the feed, XH2 O /XEtOH : J=

α=

YH2 O /YEtOH XH2 O /XEtOH

3. Results and discussion 3.1. Silica sol–gel modification The silica sol–gel modification was applied to tune the pore size of the macroporous ceramic supports. Mean pore size and water permeability of the support with different modification cycles were characterized in search of the appropriate modification cycle preliminarily. Table 2 shows that the mean pore size of the support reduces with the increasing number of modification cycles as expected and becomes almost stable after two cycles. The water permeability of the modified support reduces sharply and still does

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not seem to be stable after three cycles, which causes a flux reduction of 90.3%. It is easy to understand that the sol–gel modification will form a thin layer on the macroporous support surface; the repeating of modification have the main function of self-repairing the possible defects of the previous sol–gel layer, rather than infinitely reduce the mean pore size of the membrane [24]. Moreover, the repeating of modification will cause, to some degree, increase in sol–gel layer thickness, which is also responsible for the decrease of flux. Here, we found that it is necessary to get a stable reduced mean pore size of the support when a two-cycle modification was carried out. Pervaporation performance of the composite membrane prepared by self-assembly of PAH/PSS on supports after various sol–gel modification cycles was measured to find the appropriate modification cycle. Table 3 shows that the composite membrane of 60 layer pairs of polyelectrolytes on the unmodified support proves not to be functional under pervaporation conditions. It means that the mending or sealing of the macropores of such scale fails by direct self-assembly. The performance improves distinctly by the increase of the modification cycle. For the support after a one-cycle modification, although 40 layer pairs of polyelectrolytes are still not enough to seal the support to adapt the pervaporation conditions, composite membrane with 60 layer pairs of polyelectrolytes exhibit a rather high flux and a hardly visible selectivity. It indicates that the self-assembly becomes dominant in support pore mending, and the tendency is more obvious for membranes which are selfassembled on the supports modified for two cycles. Under such conditions, the composite membranes with 40 or 60 layer pairs of polyelectrolytes present considerable water selectivities and remarkable high fluxes. In addition, the pervaporation performances of the membranes with 60 layer pairs of PAH/PSS are all superior to that with 40 pairs. It confirms that sol–gel modification could reduce the support pore size to the expected degree for polyelectrolytes self-assembly and make the process of selfassembly dominant in affecting the membrane pervaporation performance. By comparison of the pervaporation performance with different modification cycles, we found that a two-cycle modification is necessary to obtain the supports for pervaporation membrane fabrication since the performance improves little even after a three-cycle modification. It is consistent with the results of the

Table 3 Pervaporation of ethanol/water mixtures through composite membranes Number of silica sol–gel modification cycle

Number of self-assemble layer pairs

Pervaporation performance of composite membranea Permeate flux, J (kg m−2 h−1 )

c(H2 O) in permeate (wt.%)

α (H2 O/ethanol)

0

60







1

40 60

– 35.2

– 7.9

– 1.2

2

40 60

24.5 21.9

15.8 21.8

2.8 4.2

3

40 60

22.5 20.0

16.6 23.1

3.0 4.5

a

Measured at 65 ◦ C; the water content in feed is 6.2% (w/w); polyelectrolyte pairs, PAH/PSS.

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support mean pore size evolvement during modification repeating, which also shows pore size stay stable after a two-cycle modification. Consequently, we performed a two-cycle sol–gel modification for all samples in the following research. 3.2. SEM characterization Fig. 3(a) shows the surface images of the fresh supports obtained by SEM, which shows a somewhat rough and uneven

surface with big particles of ZrO2 . From the image of the support after a two-cycle sol–gel modification in Fig. 3(b), a more even plane is obtained. Fig. 3(c) shows the surface image of 40 layer pairs of PAH/PSS on the modified supports. Difference between the surface images of the fresh supports and the deposited polyelectrolytes could be observed. Some islands of polyelectrolytes appear at the surface have encountered each other to form and arrange clumpy layers. In our previous work [23], a dispersive X-ray spectroscopy (EDXS) analysis has been performed to

Fig. 3. SEM images of (a) the surface of fresh support, (b) the surface of modified support, (c) the surface of 40 layer pairs of PAH/PSS on modified support, (d) the cross-section of fresh support, (e) the cross-section of modified support and (f) the cross-section of 40 layer pairs of PAH/PSS on modified support.

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analysis the changes of membrane surface components which declared the successful polyelectrolytes deposition. Fig. 3(d)–(f) shows the cross-section images of the fresh support, modified support (modified for two cycles) and composite membrane (40 layer pairs of PAH/PSS). As shown in Fig. 3(d), the fresh support has a multilayer structure of ␣Al2 O3 /␣-Al2 O3 /ZrO2 (from left to right in the image). The silica layer, which is coated on the ZrO2 layer could be easily distinguished in Fig. 3(e) with the thickness of about several hundred nanometers and became vague after the self-assembly of PEMs in Fig. 3(f). Furthermore, it could also be observed that the cross-sectional morphology of ZrO2 layer varied to some extent, which may be mainly due to the self-assembly of PEMs both upon the support surface and inside the pores and leads to the indistinct interface between the PEMs and the support. 3.3. Influence of the molecular structure and the number of layer pairs of polyelectrolytes Charge density is the most representative character of the polyelectrolyte molecular structure and is the pronounced consequences for the mechanism of PEMs buildup because it determines the polyelectrolytes complexation and side-bridging, and consequently controls the architecture of PEMs such as thickness, density, compactness and completeness. The charge density ρc [14] was calculated by forming the ratio of the number of ion pairs (=1) and the number of carbon atoms per repeat unit of the cationic and anionic polyelectrolyte, e.g. for PEI/PSS ρc is 1/(2 + 8) = 0.1. In order to study the effect of the polyelectrolyte molecular structure and layer pairs number on the separation properties, a variety of membranes consisting of polyelectrolyte complexes with different layer pairs numbers and with different ρc values between 0.0625 (PDADMAC/PSS) and 0.25 (PEI/PVS) were prepared. The supports were modified by silica sol–gel twice and the composite membranes were tested in ethanol/water pervaporation using a feed solution with a water concentration of 6.2 wt.%. As shown in Fig. 4, the water content in the permeate was always higher than in the feed, which indicates the preferential transport of water across the membrane self-assembled from a series of polyelectrolytes. It is reported that the flux is high and the water concentration in the permeate is low when ρc is small because of the low cross-linking density [16]. We also obtained the similar results for the common used polyelectrolytes, i.e. for PDADMAC/PSS the flux is higher than 45 kg m−2 h−1 and water selectivity is small while the membrane deposited by PEI/PVS shows a flux of 18.4 kg m−2 h−1 and a water concentration in permeate of 35.3 wt.% (a separation factor of 8.0). It is generally ascribed to the difference of network densities of the top-layers formed by various polyelectrolytes. One should notice that the tendency of flux decrease with the increase of ρc is not so clear as that of the water selectivity. Although the water selectivity increases when ρc increases as expected, the flux remains surprising high for every couple of polyelectrolytes compared to the majority of reported

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Fig. 4. Dependence of pervaporation performance on the charge density ρc and number of the polyelectrolyte layer pairs: feed solution, 6.2 wt.% water; pervaporation at T = 65 ◦ C; dipping time of each adsorption, 30 min; not annealed.

pervaporation membranes. The high flux might be explained by the special construction of the composite membrane. The interface of the organic–inorganic membrane formed by electrostatic interaction between the OH groups located at the inorganic support surface and the charged chains of the deposited polyelectrolytes are rather different from that of the organic membrane. As mentioned above, the possible swelling or shrinkage effect of the organic layer was weakened by the conjunct inorganic support. Consequently, membrane collapse problem, which usually involved in organic membrane, would be mostly avoided. Furthermore, the composite membrane was prepared from the modified macroporous support, which had a high original flux. Thus, the combined performance of adoptable selectivity and high flux could be achieved as we expected. Similar high flux was also reported by Sullivan and Bruening [18] who prepared polyelectrolytes on anodic alumina membrane. However, such porous inorganic support prepared from high pure Al wafer with a limited membrane thickness and weak mechanical strength, which directly restricts its further industrial application. As mentioned above, after a two-cycle or three-cycle modification we found that the water selectivity improved and flux decreased with the increase of the number of self-assembled PAH/PSS layer pairs. As shown in Fig. 4, similar results were obtained for the composite membrane deposited by the other types of used polyelectrolytes complexes. It is mostly due to the increasing thickness of the polyelectrolyte multilayers when the number of polyelectrolyte pair increased. However, the selectivity improvement was still not very satisfied since the water concentration in the permeate was not high and did not exceed 40%. 3.4. Influence of self-assemble dipping time The self-assembly of polyelectrolytes were processed by means of layer-by-layer adsorption via electrostatic interaction. For dilute polyelectrolyte solutions, the polyelectrolytes need to diffuse from the bulk solution to the vicinity of the oppositely charged surface before they could be strongly influenced by the electrostatic field, which is formed by membrane sur-

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face and aqueous solution. It also takes time for polyelectrolytes constructing complexation on the charged surface. Solis [25] has studied the spontaneous equilibrium layering of a polyelectrolyte mixture close to a charged wall. Castelnovo and Joanny [26] propose scaling laws for the multilayer formation in the case of semiflexible polyelectrolytes. In this study, we restrict ourselves to the optimization of membrane fabrication. Then a study on the effects of dipping time of each adsorption on membrane separation performance will provide intuitionistic knowledge on polyelectrolyte adsorption equilibrium or saturation on ceramic porous supports. As shown in Fig. 5, after 10 min dipping, the pervaporation performance of the composite membrane deposited by 60 layer pairs of polyelectrolytes improves slowly with the increase of the dipping time. It seems that 10 min dipping for each adsorption may be enough for the polyelectrolyte complexation on porous supports while some other different dipping times was reported from several seconds to hours [8,9,27,28,11–16]. The work reported by Takeda et al. [29] indicates that the surface

OH group density is a major factor governing the adsorption of organic substances from the atmosphere onto the oxide surface. Ceramic oxide materials usually have the amount of surface hydroxyl groups of about (1–4) × 10−5 mol m−2 [30] (for example, SiO2 surface generally has about five –OH groups per square nanometer [31]), which may be different from that of other charged surfaces and the adsorption process differs as well [32]. Cabot et al. [33] carried out a detailed study on adsorption of poly(vinylimidazoles) onto silica surfaces, which showed that the polyelectrolytes solution condition, such as pH, viscosity, counter-ion and co-ion concentration, may also affect the adsorption equilibrium. The co-effects of these mentioned aspects may explain the difference between various results. Since no more studies on the kinetics of polyelectrolyte adsorption were attempted here, a dipping time of 30 min was adopted for self-assembly of the other samples for the sake of equilibrium. 3.5. Influence of annealing of the polyelectrolyte membrane Thermal treatment was commonly applied to organic materials to obtain a more compact structure and was found effective for improving membrane separation with organic supports. To study the thermal treatment effects on such composite membrane, we carried out the annealing of membranes at different temperatures. As shown in Fig. 6, the flux of membranes deposited by 60 layer pairs of PAH/PSS decrease when annealed at air at 75 ◦ C or higher, and the water concentration in the permeate increase only a little. The pervaporation performance has such a poor improvement after annealing in air, which differs much from the results for organic membranes. Tieke reported an obvious enhancement of water selectivity of the annealed polyelectrolytes/PAN/PET membrane [15]. The reason is probably that the annealing at the support PAN’s glass transition temperature cause a reduction of the PAN pore sizes. Moreover, some of the absorbed water in the polyelectrolyte layer is released, which leads to a

Fig. 5. Effect of dipping time of each adsorption on the ethanol/water mixture pervaporation performance (a) pervaporation flux and (b) water content in the permeate: feed solution, 6.2 wt.% water; pervaporation at T = 65 ◦ C; polyelectrolyte layer pairs, 60; not annealed.

Fig. 6. Effect of annealing conditions on the ethanol/water mixture pervaporation performance: feed solution, 6.2 wt.% water; pervaporation at T = 65 ◦ C; polyelectrolytes, 60 layer pairs of PAH/PSS; dipping time of each adsorption, 30 min; annealed for 2 h.

Y. Chen et al. / Journal of Membrane Science 302 (2007) 78–86

Fig. 7. Effect of operating temperature on the ethanol/water mixture pervaporation performance: feed solution, 6.2 wt.% water; polyelectrolytes, 60 layer pairs of PAH/PSS; dipping time of each adsorption, 30 min; not annealed.

decreased thickness and increased density. However, the inorganic supports we used are stable and have no shrinkage after the thermal treatment at such low temperatures. The possible structure change is some degree of dehydration or shrinkage of the polyelectrolyte layer, which is even not the same as that deposited on the organic supports since the inorganic supports will hinder the deformation of the former. 3.6. Influence of the pervaporation operating temperature Temperature is an important aspect in separation process wherever for organic or inorganic membranes since it influences a lot in membrane adaptability, separation efficiency, operating cost, etc. Then the dependence of the pervaporation performance on the operating temperature was studied and the composite membrane deposited by 60 layer pairs of PAH/PSS was used. As shown in Fig. 7, with the increase of operating temperature from 35 to 65 ◦ C, we are glad to observe not only the expected increase in flux but also an increase of water selectivity. The flux at 65 ◦ C was enhanced to more than 4 times of that at 35 ◦ C. Both increase of the flux and water selectivity indicates that higher temperature makes water more preferential to permeate through the membrane. In other words, the flux increase is mainly due to the rapidly growing water permeation when temperature elevates. The similar effect was also observed by other researches when treating with organic supports [15]. The reason may be due to the both increase of adsorption rate and diffusion rate. Thus, it provides an efficient way to enhance the whole pervaporation performance by a simple increase of operating temperature. 4. Conclusions Our study indicates that the self-assembly of polyelectrolytes on macroporous ceramic supports is efficient to prepare the organic–inorganic composite membrane suitable for ethanol/water separation under pervaporation conditions. The pretreatment on the supports we carried out by means of silica sol–gel modification and wet cleaning is efficient to make depo-

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sition of polyelectrolyte to form an ultrathin separation layer. SEM observations also confirm the successful polyelectrolytes deposition on the porous supports. The polyelectrolyte molecular structure was found the most important to control the membrane separation performance. The composite membrane deposited by 60 layer pairs of PEI/PVS shows a high flux of 18.4 kg m−2 h−1 and an enhancement of water concentration from 6.2 wt.% in the feed to 35.3 wt.% in the permeate (a separation factor of 8.0) at 65 ◦ C. Still this separation factor lies behind the good separation capability reported. However, it allow for about 10-fold larger water fluxes compared to the majority of pervaporation membranes in the literature. We ascribed it to the effect of confined deformation derived from the composite structure of organic–inorganic membranes. The further work for detailed study on the improvement of composite membrane separation performance will be investigated. Acknowledgements This work is sponsored by the National Basic Research Program of China (no. 2003CB615702), National Natural Science Foundation of China (NNSFC, no. 20446002, 20436030) and Natural Science Foundation of Jiangsu Province (BK2006722). References [1] G. Decher, J.D. Hong, Buildup of ultrathin multilayer films by a selfassembly process: I. consecutive adsorption of anionic and cationic bipolar amphiphiles, Macromol. Chem. Macromol. Symp. 46 (1991) 321. [2] G. Decher, Fuzzy nanoassemblies: toward layered polymeric multicomposites, Science 277 (1997) 1232. [3] P.T. Hammond, Recent explorations in electrostatic multilayer thin film assembly, Curr. Opin. Colliod Interf. Sci. 4 (2000) 430. [4] G. Decher, J.B. Schlenoff, Polyelectrolyte multilayers, an overview, in: G. Decher (Ed.), Multilayer Thin Films, Wiley-VCH, Weinheim, 2003, pp. 1–46. [5] M. Sch¨onhoff, Self-assembled polyelectrolyte multilayers, Curr. Opin. Colliod. Interf. Sci. 8 (2003) 86. [6] Z.Q. Zhu, X.S. Feng, A. Penlidis, Self-assembled nano-structured polyelectrolyte composite membranes for pervaporation, Mater. Sci. Eng., C 26 (2006) 1. [7] P. Stroeve, V. Vasquez, M. Coelho, J.F. Rabolt, Gas transfer in supported films made by molecular self-assembly of ionic polymers, Thin Solid Films 284/285 (1996) 708. [8] J.M. Levasalmi, T.J. McCarthy, Poly(4-methyl-1-pentene)-supported polyelectrolyte multilayer films: preparation and gas permeability, Macromolecules 30 (1997) 1752–1757. [9] J.M. Haack, W. Lenk, D. Lehmann, K. Lunkwitz, Pervaporation separation of water/alcohol mixtures using composite membranes based on polyelectrolyte multilayer assemblies, J. Membr. Sci. 184 (2001) 233. [10] Z.Q. Zhu, X.S. Feng, A. Penlidis, Layer-by-layer self-assembled polyelectrolyte membranes for solvent dehydration by pervaporation, Mater. Sci. Eng., C 27 (2007) 612. [11] W.Q. Jin, A. Toutianoush, B. Tieke, Size- and charge-selective transport of aromatic compounds across polyelectrolyte multilayer membranes, Appl. Surf. Sci. 246 (2005) 444. [12] W.Q. Jin, A. Toutianoush, B. Tieke, Use of polyelectrolyte layer-by-layer assemblies as nanofiltration and reverse osmosis membranes, Langmuir 19 (2003) 2550. [13] B. Tieke, F. van Ackern, L. Krasemann, A. Toutianoush, Ultrathin selfassembled polyelectrolyte multilayer membranes, Eur. Phys. J. 5 (2001) 29.

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