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water mixtures using self-assembled polyelectrolyte multilayer membranes of high charge density
Materials Science and Engineering C 22 (2002) 459 – 463 www.elsevier.com/locate/msec
Pervaporation separation of alcohol/water mixtures using self-assembled polyelectrolyte multilayer membranes of high charge density Ali Toutianoush, Bernd Tieke * Institut fu¨r Physikalische Chemie der Universita¨t zu Ko¨ln, Luxemburgerstraße 116, D-50939, Cologne, Germany
Abstract The alcohol/water separation of polyelectrolyte multilayer membranes of high charge density prepared upon electrostatic layer-by-layer (LBL) adsorption of cationic and anionic polyelectrolytes is described. Polyvinylamine (PVA) was used as the cationic polyelectrolyte, and polyvinylsulfonate (PVSu), polyvinylsulfate (PVS) and polyacrylate (PAA) were used as anionic polyelectrolytes; the separation was studied under pervaporation conditions. At low water content in the feed (< 20 wt.%), the strongly hydrophilic PVA/PVSu membrane is best suited for separation, while at higher water content the less hydrophilic PVA/PAA membrane exhibits the best separation. Membranes prepared at pH 1.7 with no salt present in the polyelectrolyte solution exhibit a substantially worse separation capability than membranes prepared at pH 1.7 in the presence of NaCl, or at pH 6.8 in the absence of salt. Use of PAA of low molecular weight (m.w. 5000) leads to membranes of much lower total flux than use of PAA of molecular weight 250,000. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Polyelectrolyte membrane; Layer-by-layer adsorption; Pervaporation; Alcohol/water separation
1. Introduction Recent studies have shown that the alternating electrostatic layer-by-layer (LBL) adsorption of oppositely charged polyelectrolytes [1 –4] can be successfully applied to prepare separating membranes of very low, precisely controlled thickness [5 –17]. The membranes are suitable for the separation of gases [5 –7], small molecules [8– 12] and ions [13 – 17]. Very promising results were obtained for the separation of alcohol/water mixtures [8 –13] under pervaporation [18] conditions. The separation efficiency was found to depend on various preparative and operational parameters. Separation was found to be most efficient, if polyelectrolyte complexes of high charge density were used [10,11] such as polyethyleneimine/polyvinylsulfate [10], or polyvinylamine/polyvinylsulfate [19,20], but for polymers of high charge density a systematic structural variation has not been carried out so far. In the present paper, we report on the alcohol/water separation of polyelectrolyte multilayer membranes consisting of PVA as the cationic polyelectrolyte, and either poly* Corresponding author. Tel.: +49-221-470-2440; fax: +49-221-4707300. E-mail address: [email protected] (B. Tieke).
acrylate (PAA), polyvinylsulfate (PVS) or polyvinylsulfonate (PVSu) as the anionic polyelectrolyte. All membranes exhibit the same high charge density (if the term ‘‘charge density’’ is defined as the number of ion pairs divided by the number of carbon atoms per repeat unit of the polyelectrolyte complex), and therefore any differences in the separation behaviour can be related to the nature of the polar groups of the anionic polyelectrolytes. Besides the effect of the polar groups, other parameters were also studied: Molecular weight of PAA, pH and presence of salt in the polyelectrolyte solutions used for membrane preparation. The separation experiments were carried out under pervaporation [18] conditions. The structure – property relations established in this study allow to further improve the utility of polyelectrolyte LBL assemblies for the dehydration of organic solvents.
2. Experimental part 2.1. Supporting membrane A technical PAN/PET membrane hydrophilised in oxygen plasma (Sulzer Chemtech, Neunkirchen) was used as the support. It consists of a polyethyleneterephthalate (PET)
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fleece (thickness, 100 Am) coated with a porous polyacrylonitrile (PAN) layer of a thickness of 80 Am, the pore sizes being 20– 200 nm. 2.2. Polyelectrolytes Used in this study are poly(vinylamine) (PVA, Mw = 100,000; BASF, Ludwigshafen), poly(vinylsulfate potassium salt) (PVS, Mw = 350,000; Acros) poly(vinylsulfonate sodium salt) (PVSu, Mw = 7000; Polyscience), and polyacrylic acid (PAA(l), Mw = 5000; Acros; and PAA(h), Mw = 250,000; Aldrich). All compounds were used without further purification, the solutions were prepared with purified water (Milli-Q water). The alcohols were of analytical grade. 2.3. Preparation of the polyelectrolyte multilayer Polyelectrolytes were dissolved in aqueous medium in a concentration of 10 2 monomoles l 1 (monomole = mole of monomer units) and acidified to pH 1.7 (if not especially noted) using aqueous HCI. If not especially noted, sodium chloride was added to the polyelectrolyte solution in a concentration of 1 mol l 1. For adsorption of the individual layers, the supporting membrane was immersed (a) in the solution of the cationic polyelectrolyte, (b) in pure water, (c) in the solution of the anionic polyelectrolyte and (d) in water again. Steps (a) to (d) were repeated until 60 pairs of polycation/polyanion layers were adsorbed. Immersion time in the individual solutions was 30 min. Dipping of the membrane into the solution was carried out with a homebuilt, computerised dipping apparatus. The size of the membranes was 12 12 cm2. 2.4. Pervaporation Pervaporation measurements were carried out in a homemade apparatus. A 3 in. pervaporation cell was used. The feed was circulated under normal pressure, the pressure of the permeate side was 0.2 mbar. The permeate was collected in a cooling pipe and analysed by measuring the refractive index or by using gas chromatography. The separation factor a was calculated from the quotient of the weight ratio of water and alcohol in the permeate, YHOH/YROH, and in the feed, XHOH/XROH: YHOH =YROH : a¼ XHOH =XROH 3. Results and discussion In the first set of experiments, the optimum thickness of the membrane and the optimum temperature of the alcohol/ water pervaporation were determined. In order to find the optimum thickness, a series of PVA/PVS membranes with different number of deposited layers were prepared and
subjected to ethanol/water pervaporation using a feed solution containing 6.2 wt.% of water. As shown in Table 1, the total flux gradually decreases with increasing number of deposited layer pairs. Simultaneously, the water content in the permeate is raised until a nearly constant value is obtained for more than 50 layer pairs. The effect of the pervaporation temperature is shown in Table 2 for the pervaporation of a 1-propanol/water mixture containing 10.1 wt.% of water in the feed. Quite obviously a high pervaporation temperature favours a high flux, while the composition of the permeate remains nearly unaffected. A high separation efficiency is therefore obtained at the easiest, if membranes containing more than fifty polyelectrolyte layer pairs are used and the pervaporation is carried out at a high temperature. Thus, the following experiments were always carried out with membranes containing 60 polyelectrolyte layer pairs, and the pervaporation temperature was 58.5 jC, the highest value technically feasible with our homemade apparatus. 3.1. Influence of anionic groups In order to study effects of the ionic groups on the ethanol/water separation of polyelectrolyte multilayer membranes, three membranes consisting of PVA as cationic polyelectrolyte and polyvinylsulfonate (PVSu), polyvinylsulfate (PVS) or polyacrylate (PAA) as anionic polyelectrolyte were prepared. The anionic polyelectrolytes were of identical, high charge density so that any effect on the separation originating from the charge density was excluded. Moreover, the preparation conditions of the membranes were identical; all membranes were prepared from aqueous polyelectrolyte solutions of pH 6.8 with no salt added. pH 6.8 was chosen in order to make sure that all polar substituent groups of the polyelectrolytes are completely ionised. As shown in Fig. 1, the three membranes exhibit an excellent separation, if feed solutions containing more than 5 wt.% of water are used. Below 20 wt.% water in the feed solution, the PVA/PVSu membrane shows slightly better results than the other ones; above 20 wt.% water in the feed the PVA/PAA membrane is the best one. The three systems mainly differ in the acidity of the polar
Table 1 Ethanol/water pervaporation through PVA/PVS membranes containing different numbers of layer pairs (water content of feed solution: 6.2 wt.%; pervaporation temperature: 58.5 jC) Number of layer pairs
Total fluxa [g m 2 h 1]
Water content of permeatea [wt.%]
10 20 30 40 50 60
942 700 340 190 142 99
25.4 51.5 77.3 87.3 92.1 97.9
a
Mean value of two membranes.
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Table 2 1-Propanol/water pervaporation through PVA/PVS membranes (60 layer pairs) at different temperatures (water content of feed solution: 10.1 wt.%) Pervaporation temperature [jC]
Total fluxa [g m 2 h 1]
Water content of permeatea [wt.%]
24.6 F 0.1 35.9 48.3 59.5
106 436 779 1107
99.7 99.8 99.9 99.9
a
Mean value of two membranes.
groups, the sulfonic acid group being a much stronger acid than the carboxylic acid group. Thus the ion pairs formed in the PVA/PVSu membrane are more polar and attract the water molecules more strongly than the ion pairs of the other membranes; consequently a selective transport of water is already found at very low water concentration in the feed solution. However, at a high water concentration in the feed the strong affinity of the ion pairs to water leads to hydrolysis with the consequence that the pore size and the total flux are increased, but the selectivity is decreased.
Fig. 2. Dependence of total flux and of water content in permeate on water content in ethanol/water feed mixture for PVA/PVS membranes prepared from polyelectrolyte solutions of different pH, with or without addition of NaCl (membrane thickness: 60 layer pairs).
In order to study the effect of the pH and the presence of salt in the polyelectrolyte solutions on the separation behaviour, we prepared PVA/PVS membranes at pH 1.7 and 6.8, either in the presence of sodium chloride (concentration: 0.1 mol l 1) or without salt. In Fig. 2, the ethanol/water separation of the three membranes is represented. The worst separation was found for the membrane prepared from aqueous solution at pH 1.7 without sodium chloride added. The presence of salt, or the higher pH of 6.8 nearly caused the same positive effect on the separa-
tion capability, while the flux was only affected, if salt was present in the polyelectrolyte solution. In that case, a substantially lower total flux was observed. This may have the following origin: presence of salt in the polyelectrolyte solution reduces the size of the polyelectrolyte coils. Unfolding of the coils during polyelectrolyte adsorption is rendered more difficult so that the polymers are partially adsorbed as coils. Consequently, the thickness of the membrane increases [21], the separation increases as well but the flux becomes lower. If the membrane is prepared at pH 6.8, the polar groups of the two polyelectrolytes are completely ionized and the membrane consists of a regular network structure of high density and low thickness [22]. Consequently, a good separation and a high flux are obtained.
Fig. 1. Dependence of total flux and of water content in permeate on water content in ethanol/water feed mixture for PVA/PVSu, PVA/PVS and PVA/ PAA membranes (membrane thickness: 60 layer pairs; preparation conditions: pH 6.8, no salt added to the polyelectrolyte solutions). PAA(l) was used.
Fig. 3. Dependence of total flux and of water content in permeate on water content in ethanol/water feed mixture for PVA/PAA membranes containing PAA of molecular weight 5000 or 250,000 (membrane thickness: 60 layer pairs; preparation conditions: pH 6.8, no salt added to the polyelectrolyte solutions).
3.2. pH and salt effects
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3.3. Effects of molecular weight In order to study the effect of a variation of the molecular weight on the separation capability, two PVA/ PAA membranes with PAA of either low or high molecular weight (PAA(l): m.w. 5000; PAA(h): m.w. 250,000) were prepared under identical conditions and subjected to ethanol/water pervaporation. In Fig. 3, the separation diagrams of the two membranes are shown. A strong effect on the total flux is evident; the membrane with PAA(l) exhibits a flux being only a quarter of the value found for the membrane containing PAA(h). A reason could be that the short polymer chains are able to enter and stop the pores of the supporting membrane so that the permeation slows down and the flux is decreased. On the other hand, the long polymer chains of PAA(h) are preferentially adsorbed at the substrate surface and the pores of the support remain unaffected. Use of PAA(h) also leads to a better separation, especially if the water
content in the feed is only very low. It should be added that the PVA/PAA(h) membrane also exhibits a higher long-term stability than PVA/PAA(l), which might be caused by a gradual leaching out of PAA(l) from the membrane reducing its separation efficiency. 3.4. Separation of different alcohol/water mixtures The good results obtained with the PVA/PVSu membrane in ethanol/water separation suggested to also study the separation of other alcohol/water mixtures. In Fig. 4a and b, the separation diagrams of alcohol/water mixtures with the alcohols being t-butanol, 1- and 2-propanol or ethanol are compared. It turned out that the separation increased, when the hydrophilicity of the alcohols decreased. Two reasons are responsible for that: first, the strength of the hydrogen bonding decreases with increasing number of carbon atoms in the alcohol making the separation easier. Secondly, hydrophobic alcohol molecules are more strongly rejected by the hydrophilic pores of the membrane. A corresponding separation of alcohol/ water mixtures was recently reported for a PVA/PVS membrane [19,20].
4. Summary and conclusions
Fig. 4. Dependence of water content in permeate (a) and total flux (b) on water content in alcohol/water feed mixture for a PVA/PVSu membrane consisting of 60 layer pairs (preparation conditions: pH 1.7, NaCl added to the polyelectrolyte solutions).
Our studies indicate that polyelectrolyte multilayer membranes containing different anionic polyelectrolytes of high and equal charge density show slight differences in the alcohol/water separation depending on the acidity of the polar substituent groups. Among the three substituent groups, the sulfonate group is most hydrophilic, and thus becomes strongly hydrated even at a very low water content of the feed solution. Consequently, the PVA/PVSu membrane exhibits the highest flux and separation at low water concentrations of the feed solution. However, at water contents above 20 wt.%, the PVA/PVSu membrane is too hydrophilic. Strong swelling sets in, the flux becomes very high but the separation gets worse. For the water-rich mixtures, the less hydrophilic PVA/PAA membrane is clearly more suitable. However, most separation problems are based on the removal of small amounts of water from the organic solvent and here the PVA/PVSu membrane is clearly the best choice. This is confirmed by the excellent separation of various alcohol/ water mixtures obtained with this membrane. Our studies also indicate that effects originating from pH variation and addition of NaCl to the polyelectrolyte solution, or effects of the molecular weight might even be stronger than the effects caused by the variation of the polar groups. Increase of pH or salt addition has nearly the same positive effect on the separation; the use of low molecular weight polyelectrolytes causes problems when porous substrates are used. In that case the pores of the substrate may become closed by the polyelectrolyte.
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Acknowledgements The authors gratefully acknowledge Dr. A. Hu¨bner, Sulzer Chemtech, Neunkirchen, for generously supplying plasma-treated PAN/PET membranes and BASF, Ludwigshafen, for polyvinylamine. Fincancial support by the Deutsche Forschungsgemeinschaft (project Ti 219/3-4) is also gratefully acknowledged.
References [1] [2] [3] [4]
G. Decher, J.D. Hong, Ber. Bunsenges. Phys. Chem. 95 (1991) 1430. Y. Lvov, G. Decher, H. Mo¨hwald, Langmuir 9 (1993) 481. G. Decher, Science 277 (1997) 1232. P. Bertrand, A. Jonas, A. Laschewsky, R. Legras, Macromol. Rapid Commun. 21 (2000) 319. [5] P. Stroeve, V. Vasquez, M.A.N. Coelho, J.F. Rabolt, Thin Solid Films 284/285 (1996) 708. [6] J.-M. Leva¨salmi, T.J. McCarthy, Macromolecules 30 (1997) 1752. [7] F. van Ackern, L. Krasemann, B. Tieke, Thin Solid Films 329 (1998) 762.
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[8] L. Krasemann, B. Tieke, J. Membr. Sci. 150 (1998) 23. [9] J. Meier-Haack, T. Rieser, W. Lenk, D. Lehmann, S. Berwald, S. Schwarz, Chem. Eng. Technol. 71 (1999) 839. [10] L. Krasemann, B. Tieke, Chem. Eng. Technol. 23 (2000) 211. [11] L. Krasemann, A. Toutianoush, B. Tieke, J. Membr. Sci. 181 (2001) 221. [12] B. Tieke, L. Krasemann, A. Toutianoush, Macromol. Symp. 163 (2001) 97. [13] L. Krasemann, B. Tieke, Mater. Sci. Eng. C. 8/9 (1999) 513. [14] L. Krasemann, B. Tieke, Langmuir 16 (2000) 287. [15] J.J. Harris, J.L. Stair, M.L. Bruening, Chem. Mater. 12 (2000) 1941. [16] T.R. Farhat, J.B. Schlenoff, Langmuir 17 (2001) 1184. [17] J.L. Stair, J.J. Harris, M.L. Bruening, Chem. Mater. 13 (2001) 2641. [18] J. Neel, Introduction to pervaporation, in: R.Y.M. Huang (Ed.), Pervaporation Membrane Separation Processes, Elsevier, Amsterdam, 1991, p. 42. [19] B. Tieke, F. van Ackern, L. Krasemann, A. Toutianoush, Eur. Phys. J., E. 5 (2001) 29. [20] A. Toutianoush, L. Krasemann, B. Tieke, Colloids Surf., A. 198 – 200 (2002) 881. [21] G. Decher, J. Schmitt, Prog. Colloid & Polym. Sci. 89 (1992) 160. [22] S.S. Shiratori, M.F. Rubner, Macromolecules 33 (2000) 4213.
Pervaporation separation of alcohol/water mixtures using self-assembled polyelectrolyte multilayer membranes of high charge density Ali Toutianoush, Bernd Tieke * Institut fu¨r Physikalische Chemie der Universita¨t zu Ko¨ln, Luxemburgerstraße 116, D-50939, Cologne, Germany
Abstract The alcohol/water separation of polyelectrolyte multilayer membranes of high charge density prepared upon electrostatic layer-by-layer (LBL) adsorption of cationic and anionic polyelectrolytes is described. Polyvinylamine (PVA) was used as the cationic polyelectrolyte, and polyvinylsulfonate (PVSu), polyvinylsulfate (PVS) and polyacrylate (PAA) were used as anionic polyelectrolytes; the separation was studied under pervaporation conditions. At low water content in the feed (< 20 wt.%), the strongly hydrophilic PVA/PVSu membrane is best suited for separation, while at higher water content the less hydrophilic PVA/PAA membrane exhibits the best separation. Membranes prepared at pH 1.7 with no salt present in the polyelectrolyte solution exhibit a substantially worse separation capability than membranes prepared at pH 1.7 in the presence of NaCl, or at pH 6.8 in the absence of salt. Use of PAA of low molecular weight (m.w. 5000) leads to membranes of much lower total flux than use of PAA of molecular weight 250,000. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Polyelectrolyte membrane; Layer-by-layer adsorption; Pervaporation; Alcohol/water separation
1. Introduction Recent studies have shown that the alternating electrostatic layer-by-layer (LBL) adsorption of oppositely charged polyelectrolytes [1 –4] can be successfully applied to prepare separating membranes of very low, precisely controlled thickness [5 –17]. The membranes are suitable for the separation of gases [5 –7], small molecules [8– 12] and ions [13 – 17]. Very promising results were obtained for the separation of alcohol/water mixtures [8 –13] under pervaporation [18] conditions. The separation efficiency was found to depend on various preparative and operational parameters. Separation was found to be most efficient, if polyelectrolyte complexes of high charge density were used [10,11] such as polyethyleneimine/polyvinylsulfate [10], or polyvinylamine/polyvinylsulfate [19,20], but for polymers of high charge density a systematic structural variation has not been carried out so far. In the present paper, we report on the alcohol/water separation of polyelectrolyte multilayer membranes consisting of PVA as the cationic polyelectrolyte, and either poly* Corresponding author. Tel.: +49-221-470-2440; fax: +49-221-4707300. E-mail address: [email protected] (B. Tieke).
acrylate (PAA), polyvinylsulfate (PVS) or polyvinylsulfonate (PVSu) as the anionic polyelectrolyte. All membranes exhibit the same high charge density (if the term ‘‘charge density’’ is defined as the number of ion pairs divided by the number of carbon atoms per repeat unit of the polyelectrolyte complex), and therefore any differences in the separation behaviour can be related to the nature of the polar groups of the anionic polyelectrolytes. Besides the effect of the polar groups, other parameters were also studied: Molecular weight of PAA, pH and presence of salt in the polyelectrolyte solutions used for membrane preparation. The separation experiments were carried out under pervaporation [18] conditions. The structure – property relations established in this study allow to further improve the utility of polyelectrolyte LBL assemblies for the dehydration of organic solvents.
2. Experimental part 2.1. Supporting membrane A technical PAN/PET membrane hydrophilised in oxygen plasma (Sulzer Chemtech, Neunkirchen) was used as the support. It consists of a polyethyleneterephthalate (PET)
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fleece (thickness, 100 Am) coated with a porous polyacrylonitrile (PAN) layer of a thickness of 80 Am, the pore sizes being 20– 200 nm. 2.2. Polyelectrolytes Used in this study are poly(vinylamine) (PVA, Mw = 100,000; BASF, Ludwigshafen), poly(vinylsulfate potassium salt) (PVS, Mw = 350,000; Acros) poly(vinylsulfonate sodium salt) (PVSu, Mw = 7000; Polyscience), and polyacrylic acid (PAA(l), Mw = 5000; Acros; and PAA(h), Mw = 250,000; Aldrich). All compounds were used without further purification, the solutions were prepared with purified water (Milli-Q water). The alcohols were of analytical grade. 2.3. Preparation of the polyelectrolyte multilayer Polyelectrolytes were dissolved in aqueous medium in a concentration of 10 2 monomoles l 1 (monomole = mole of monomer units) and acidified to pH 1.7 (if not especially noted) using aqueous HCI. If not especially noted, sodium chloride was added to the polyelectrolyte solution in a concentration of 1 mol l 1. For adsorption of the individual layers, the supporting membrane was immersed (a) in the solution of the cationic polyelectrolyte, (b) in pure water, (c) in the solution of the anionic polyelectrolyte and (d) in water again. Steps (a) to (d) were repeated until 60 pairs of polycation/polyanion layers were adsorbed. Immersion time in the individual solutions was 30 min. Dipping of the membrane into the solution was carried out with a homebuilt, computerised dipping apparatus. The size of the membranes was 12 12 cm2. 2.4. Pervaporation Pervaporation measurements were carried out in a homemade apparatus. A 3 in. pervaporation cell was used. The feed was circulated under normal pressure, the pressure of the permeate side was 0.2 mbar. The permeate was collected in a cooling pipe and analysed by measuring the refractive index or by using gas chromatography. The separation factor a was calculated from the quotient of the weight ratio of water and alcohol in the permeate, YHOH/YROH, and in the feed, XHOH/XROH: YHOH =YROH : a¼ XHOH =XROH 3. Results and discussion In the first set of experiments, the optimum thickness of the membrane and the optimum temperature of the alcohol/ water pervaporation were determined. In order to find the optimum thickness, a series of PVA/PVS membranes with different number of deposited layers were prepared and
subjected to ethanol/water pervaporation using a feed solution containing 6.2 wt.% of water. As shown in Table 1, the total flux gradually decreases with increasing number of deposited layer pairs. Simultaneously, the water content in the permeate is raised until a nearly constant value is obtained for more than 50 layer pairs. The effect of the pervaporation temperature is shown in Table 2 for the pervaporation of a 1-propanol/water mixture containing 10.1 wt.% of water in the feed. Quite obviously a high pervaporation temperature favours a high flux, while the composition of the permeate remains nearly unaffected. A high separation efficiency is therefore obtained at the easiest, if membranes containing more than fifty polyelectrolyte layer pairs are used and the pervaporation is carried out at a high temperature. Thus, the following experiments were always carried out with membranes containing 60 polyelectrolyte layer pairs, and the pervaporation temperature was 58.5 jC, the highest value technically feasible with our homemade apparatus. 3.1. Influence of anionic groups In order to study effects of the ionic groups on the ethanol/water separation of polyelectrolyte multilayer membranes, three membranes consisting of PVA as cationic polyelectrolyte and polyvinylsulfonate (PVSu), polyvinylsulfate (PVS) or polyacrylate (PAA) as anionic polyelectrolyte were prepared. The anionic polyelectrolytes were of identical, high charge density so that any effect on the separation originating from the charge density was excluded. Moreover, the preparation conditions of the membranes were identical; all membranes were prepared from aqueous polyelectrolyte solutions of pH 6.8 with no salt added. pH 6.8 was chosen in order to make sure that all polar substituent groups of the polyelectrolytes are completely ionised. As shown in Fig. 1, the three membranes exhibit an excellent separation, if feed solutions containing more than 5 wt.% of water are used. Below 20 wt.% water in the feed solution, the PVA/PVSu membrane shows slightly better results than the other ones; above 20 wt.% water in the feed the PVA/PAA membrane is the best one. The three systems mainly differ in the acidity of the polar
Table 1 Ethanol/water pervaporation through PVA/PVS membranes containing different numbers of layer pairs (water content of feed solution: 6.2 wt.%; pervaporation temperature: 58.5 jC) Number of layer pairs
Total fluxa [g m 2 h 1]
Water content of permeatea [wt.%]
10 20 30 40 50 60
942 700 340 190 142 99
25.4 51.5 77.3 87.3 92.1 97.9
a
Mean value of two membranes.
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Table 2 1-Propanol/water pervaporation through PVA/PVS membranes (60 layer pairs) at different temperatures (water content of feed solution: 10.1 wt.%) Pervaporation temperature [jC]
Total fluxa [g m 2 h 1]
Water content of permeatea [wt.%]
24.6 F 0.1 35.9 48.3 59.5
106 436 779 1107
99.7 99.8 99.9 99.9
a
Mean value of two membranes.
groups, the sulfonic acid group being a much stronger acid than the carboxylic acid group. Thus the ion pairs formed in the PVA/PVSu membrane are more polar and attract the water molecules more strongly than the ion pairs of the other membranes; consequently a selective transport of water is already found at very low water concentration in the feed solution. However, at a high water concentration in the feed the strong affinity of the ion pairs to water leads to hydrolysis with the consequence that the pore size and the total flux are increased, but the selectivity is decreased.
Fig. 2. Dependence of total flux and of water content in permeate on water content in ethanol/water feed mixture for PVA/PVS membranes prepared from polyelectrolyte solutions of different pH, with or without addition of NaCl (membrane thickness: 60 layer pairs).
In order to study the effect of the pH and the presence of salt in the polyelectrolyte solutions on the separation behaviour, we prepared PVA/PVS membranes at pH 1.7 and 6.8, either in the presence of sodium chloride (concentration: 0.1 mol l 1) or without salt. In Fig. 2, the ethanol/water separation of the three membranes is represented. The worst separation was found for the membrane prepared from aqueous solution at pH 1.7 without sodium chloride added. The presence of salt, or the higher pH of 6.8 nearly caused the same positive effect on the separa-
tion capability, while the flux was only affected, if salt was present in the polyelectrolyte solution. In that case, a substantially lower total flux was observed. This may have the following origin: presence of salt in the polyelectrolyte solution reduces the size of the polyelectrolyte coils. Unfolding of the coils during polyelectrolyte adsorption is rendered more difficult so that the polymers are partially adsorbed as coils. Consequently, the thickness of the membrane increases [21], the separation increases as well but the flux becomes lower. If the membrane is prepared at pH 6.8, the polar groups of the two polyelectrolytes are completely ionized and the membrane consists of a regular network structure of high density and low thickness [22]. Consequently, a good separation and a high flux are obtained.
Fig. 1. Dependence of total flux and of water content in permeate on water content in ethanol/water feed mixture for PVA/PVSu, PVA/PVS and PVA/ PAA membranes (membrane thickness: 60 layer pairs; preparation conditions: pH 6.8, no salt added to the polyelectrolyte solutions). PAA(l) was used.
Fig. 3. Dependence of total flux and of water content in permeate on water content in ethanol/water feed mixture for PVA/PAA membranes containing PAA of molecular weight 5000 or 250,000 (membrane thickness: 60 layer pairs; preparation conditions: pH 6.8, no salt added to the polyelectrolyte solutions).
3.2. pH and salt effects
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3.3. Effects of molecular weight In order to study the effect of a variation of the molecular weight on the separation capability, two PVA/ PAA membranes with PAA of either low or high molecular weight (PAA(l): m.w. 5000; PAA(h): m.w. 250,000) were prepared under identical conditions and subjected to ethanol/water pervaporation. In Fig. 3, the separation diagrams of the two membranes are shown. A strong effect on the total flux is evident; the membrane with PAA(l) exhibits a flux being only a quarter of the value found for the membrane containing PAA(h). A reason could be that the short polymer chains are able to enter and stop the pores of the supporting membrane so that the permeation slows down and the flux is decreased. On the other hand, the long polymer chains of PAA(h) are preferentially adsorbed at the substrate surface and the pores of the support remain unaffected. Use of PAA(h) also leads to a better separation, especially if the water
content in the feed is only very low. It should be added that the PVA/PAA(h) membrane also exhibits a higher long-term stability than PVA/PAA(l), which might be caused by a gradual leaching out of PAA(l) from the membrane reducing its separation efficiency. 3.4. Separation of different alcohol/water mixtures The good results obtained with the PVA/PVSu membrane in ethanol/water separation suggested to also study the separation of other alcohol/water mixtures. In Fig. 4a and b, the separation diagrams of alcohol/water mixtures with the alcohols being t-butanol, 1- and 2-propanol or ethanol are compared. It turned out that the separation increased, when the hydrophilicity of the alcohols decreased. Two reasons are responsible for that: first, the strength of the hydrogen bonding decreases with increasing number of carbon atoms in the alcohol making the separation easier. Secondly, hydrophobic alcohol molecules are more strongly rejected by the hydrophilic pores of the membrane. A corresponding separation of alcohol/ water mixtures was recently reported for a PVA/PVS membrane [19,20].
4. Summary and conclusions
Fig. 4. Dependence of water content in permeate (a) and total flux (b) on water content in alcohol/water feed mixture for a PVA/PVSu membrane consisting of 60 layer pairs (preparation conditions: pH 1.7, NaCl added to the polyelectrolyte solutions).
Our studies indicate that polyelectrolyte multilayer membranes containing different anionic polyelectrolytes of high and equal charge density show slight differences in the alcohol/water separation depending on the acidity of the polar substituent groups. Among the three substituent groups, the sulfonate group is most hydrophilic, and thus becomes strongly hydrated even at a very low water content of the feed solution. Consequently, the PVA/PVSu membrane exhibits the highest flux and separation at low water concentrations of the feed solution. However, at water contents above 20 wt.%, the PVA/PVSu membrane is too hydrophilic. Strong swelling sets in, the flux becomes very high but the separation gets worse. For the water-rich mixtures, the less hydrophilic PVA/PAA membrane is clearly more suitable. However, most separation problems are based on the removal of small amounts of water from the organic solvent and here the PVA/PVSu membrane is clearly the best choice. This is confirmed by the excellent separation of various alcohol/ water mixtures obtained with this membrane. Our studies also indicate that effects originating from pH variation and addition of NaCl to the polyelectrolyte solution, or effects of the molecular weight might even be stronger than the effects caused by the variation of the polar groups. Increase of pH or salt addition has nearly the same positive effect on the separation; the use of low molecular weight polyelectrolytes causes problems when porous substrates are used. In that case the pores of the substrate may become closed by the polyelectrolyte.
A. Toutianoush, B. Tieke / Materials Science and Engineering C 22 (2002) 459–463
Acknowledgements The authors gratefully acknowledge Dr. A. Hu¨bner, Sulzer Chemtech, Neunkirchen, for generously supplying plasma-treated PAN/PET membranes and BASF, Ludwigshafen, for polyvinylamine. Fincancial support by the Deutsche Forschungsgemeinschaft (project Ti 219/3-4) is also gratefully acknowledged.
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