Influence of polyanion type and cationic counter ion on the SRNF performance of polyelectrolyte membranes

Journal of Membrane Science 403–404 (2012) 216–226

Contents lists available at SciVerse ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Influence of polyanion type and cationic counter ion on the SRNF performance of polyelectrolyte membranes Pejman Ahmadiannamini a , Xianfeng Li a,b,∗ , Ward Goyens a , Boudewijn Meesschaert a,c , Willem Vanderlinden d , Steven De Feyter d , Ivo F.J. Vankelecom a,∗∗ a Centre for Surface Chemistry and Catalysis, Faculty of Bioscience Engineering, Katholieke Universiteit Leuven (K.U. Leuven), Kasteelpark Arenberg 23 – P.O. Box 2461, 3001 Leuven, Belgium b R&D Center for Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China c Department of Industrial Science and Technology, Katholieke Hogeschool Brugge-Oostende (Associated to the Katholieke Universiteit Leuven as Faculty of Industrial Science), Zeedijk 101, 8400 Oostende, Belgium d Laboratory of Photochemistry and Spectroscopy, Division of Molecular and Nanomaterials, and INPAC – Institute for Nanoscale Physics and Chemistry, Katholieke Universiteit Leuven (K.U. Leuven), Celestijnenlaan 200 F, B-3001 Leuven, Belgium

a r t i c l e

i n f o

Article history: Received 3 November 2011 Received in revised form 10 January 2012 Accepted 24 February 2012 Available online 6 March 2012 Keywords: Membrane Polyelectrolyte complexes (PECs) SRNF Counterion Layer-by-layer method

a b s t r a c t Alternating deposition of oppositely charged polyelectrolytes (PEs) can create stable solvent resistant nanofiltration (SRNF) membranes with very high flux and selectivity. Combinations of poly(diallyldimethylammonium chloride) (PDDA) as polycation and poly(sodium styrene sulfonate) (PSS) or poly(vinyl sulfate) (PVS) as polyanions are reported from which supported membranes consisting of 5, 10, 15 and 20 bilayers are prepared via the layer-by-layer (LBL) method. The morphology of membranes prepared with polyanions in the Na- and H-form was studied by SEM and AFM. Membranes were applied in the filtration of organic solvents. Substitution of the counterion in the polyanion solutions influenced the interaction between the ions and the PE and consequently changed the PE conformation and the properties of the PEC films. The membranes prepared from polyanions in the H-form showed higher permeabilities and higher retentions than the ones prepared from the Na-form. All membranes showed very good retentions up to 99% for charged solutes in the pressure driven filtration of isopropanol (IPA) solutions and also showed excellent potential in the challenging polar aprotic solvents, such as N,N-dimethylformamide (DMF). © 2012 Elsevier B.V. All rights reserved.

1. Introduction The layer-by-layer (LBL) method is a simple and versatile method to prepare thin films for different applications, allowing easy control over thickness and surface properties [1,2]. Such alternative layers of positively and negatively charged polyelectrolytes (PEs) have unique properties due to their strong internal Coulomb interactions and the possibility to tune their electrostatic nature by the composition and the characteristics of the individual PE constituents. The LBL self-assembly technique has already been applied to a wide variety of materials, ranging from proteins to clay minerals and dyes [3–5]. The resulting PEC films have been explored in a

∗ Corresponding author at: Centre for Surface Chemistry and Catalysis, Faculty of Bioengineering Sciences, Katholieke Universiteit Leuven (K.U. Leuven), Kasteelpark Arenberg 23 – Box 2461, 3001 Leuven, Belgium. ∗∗ Corresponding author. E-mail addresses: [email protected] (X. Li), [email protected] (I.F.J. Vankelecom). 0376-7388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2012.02.052

wide variety of applications, such as sensing, selective patterning, nonlinear optics and membrane separations [6–9]. Supported PEC-based membranes have proven already to be promising materials for various membranes processes such as pervaporation [10,11], nanofiltration (NF) [12–16], reverse osmosis [17,18], forward osmosis [19,20] and fuel cells [21,22]. Large scale NF applications currently exist in water treatment [23–25]. A major challenge is to broaden the range of NF-applications to organic feeds in solvent-resistant NF (SRNF) [26–31]. A more widespread use requires solvent-resistant membranes that preserve their separation characteristics under more aggressive conditions of aprotic or strongly swelling solvents and elevated temperatures. We recently reported the first use of PEC membranes in SRNF [32,33]. The results showed that PEC membranes have a good potential for use in SRNF, especially in aprotic solvents, such as dimethylformamide (DMF) and tetrahydrofuran (THF), which are troublesome in SRNF applications. Sulfonated poly(ether ether ketone) (SPEEK) was used as the negatively charged polymer. The strong interaction between SPEEK and the positively

P. Ahmadiannamini et al. / Journal of Membrane Science 403–404 (2012) 216–226

charged poly(diallyldimethylammonium chloride) (PDDA) formed nanometer-thin, defect-free membranes. The charge density of PEs and the thickness of deposited PECs can be affected by a lot of factors, such as pH and salt concentration in the PE solutions. Besides the polymer charge density and the ionic strength of the solution, also the type of polyion, electrolyte, counterion, and coating solvent affect the structure of the PECs and their separation performance [34,35]. The interaction of counterions with macromolecules is referred to as the Hofmeister effect. Effects of varying ions are more pronounced for anions than for cations due to their larger differences in polarizability [35,36]. PEC films also have been demonstrated to be stimuli-responsive materials since the structure can rearrange upon external stimuli, such as ionic strength [37], pH [38], temperature [39], and solvents [40–42]. However, various PEC systems may behave differently upon exposure to external stimuli and therefore it is of a particular interest to study different PEC systems. In this paper, new types of PEC membranes will be investigated for SRNF applications using poly(vinylsulfonic acid sodium salt) (PVS) and the more rigid poly(sodium 4-styrenesulfonate) (PSS) as PEs with strong acidic groups. The specific effect of the cationic counterion on the construction and performance of the prepared PEC membranes will be discussed. The IPA and DMF permeabilities and the retentions for different probe molecules will be investigated in detail coupled to membrane structures, to determine the SRNF-potential of the resulting membranes. 2. Experimental 2.1. Materials Polyacrylonitrile (PAN) was purchased from Scientific Polymer Products. PDDA (Mw = 200,000–350,000) was obtained from Sigma–Aldrich as a 20 wt% aqueous solution. PSS was bought from Sigma–Aldrich (Mw = 70,000) in the sodium form as a 30% aqueous solution. PVS was bought from Sigma–Aldrich in the sodium form as a 25 wt% aqueous solution. IPA, THF and DMF were obtained from VWR, Chem Lab and Sigma–Aldrich, respectively and were used as solvents. Bengal rose sodium salt (RB) (dye content ∼ 95%), acid fuchsin (AF) (dye content ∼ 70%) and bromothymol blue (BTB) (dye content, 95%) for filtration and methylene blue (MB) (dye content ≥ 82%) for characterization were purchased from Sigma–Aldrich. The characteristics of the solutes used in this paper are listed in Table 1.

Fig. 1. Demonstration of the automated dip-coater (HTML, Belgium).

in the acidic form, which was obtained via ion exchange by adding 1 M HCl to the PVS (or PSS) solutions to adjust the pH to 2 and dialyzing against distilled water for 3 days to separate the produced salt (NaCl) from the polyanion solution by using a dialysis membrane (Mediacell International Ltd., Dialysis Tubing-visking, 12,000–14,000 Da). These solutions were referred to as PVS-H and PSS-H, respectively (Scheme 1). For adsorption of each bilayer, the PAN-H support was immersed in the solution of the cationic PE (PDDA), followed by rinsing with water, then immersed in the solution of the anionic PE (PSS or PVS) and rinsed again with water. The described procedure was repeated until a maximum of 20 polycation/polyanion bilayers were adsorbed (Scheme 2). Immersion time in the 2 coating and 2 rinsing solutions was each time 5 and 2 min, respectively. Drip time after each coating and rinsing step was 45 s. During all immersions, the membranes were moved up and down at a frequency of 0.5 Hz over an amplitude of 0.5 cm.

2.2. Preparation of membranes The PEC-based membranes were prepared by means of an automated dip-coater (HTML, Belgium) [43]. The equipment is comprised of four separate vessels alternatively containing a PE solution and a distilled water washing liquid (Fig. 1). Dipping time, drip time and number of cycles can be directed by a control device, as well as the oscillation frequency while immersed in the vessel to create liquid agitation while coating. Hydrolyzed polyacrylonitrile (PAN-H) was used as support. The PAN support was prepared via phase-inversion and converted to the PAN-H support by immersing it in an aqueous NaOH solution. The remaining NaOH was removed by washing with water. Finally, the PAN-H support was immersed in an aqueous HCl solution to convert the COONa groups into COOH groups [32]. More characterization information for the PAN-H support is available as supplementary information. PEs were dissolved in aqueous medium at a concentration of 0.2 wt%. Two types of PVS (or PSS) were used for the membrane preparation. One in the sodium form (PVS-Na or PSS-Na), the other

217

Scheme 1. Preparation of H-form polyanions via acidificaiton.

218

P. Ahmadiannamini et al. / Journal of Membrane Science 403–404 (2012) 216–226

Table 1 Some of the main solute properties. Component (MW, g/mol)

Charge

Molar volume (cm3 /mol)

Rose Bengale [RB] (1017)

−2

272.8

Acid Fuchsine [AF] (585.50)

−2

246.9

0

281.3

+1

241.9

Bromothymol Blue [BTB] (624.39)

Methylene blue [MB] (319.85)

2.3. Characterization 2.3.1. Fourier transform infrared spectroscopy (FTIR) For this analytical purpose, PEC films were cast on silicon wafers following the same deposition procedure as for the PANH supports. FTIR spectra were acquired using a Bruker IFS66V/S spectrophotometer in the transmission mode.

2.3.2. Scanning electron microscopy (SEM) SEM (Philips XL FEG30) was carried out to study the crosssection and surface structure of the membranes. The cross-section was obtained after breaking the membranes in liquid nitrogen. The SEM samples were first gold coated before analysis.

2.3.3. Atomic force microscope (AFM) AFM experiments were performed using a Multimode AFM with a Nanoscope IV controller (Veeco/Digital Instruments, Santa Barbara, USA). Samples were imaged in air in tapping mode with a drive frequency of 200–300 kHz. Silicon nitride oxide-sharpened tips (NCHR, Nanosensors, Germany) were used. The average

Structure

roughness (Ra ) and the root-mean-square value (RMS ) were calculated by Eq. (1).

  N N   1     1   Zi − Z 2 Zi − Z and RMS =  Ra = N

i=1

N

(1)

i=1

2.4. MB absorption studies The absorption of positively charged MB can give information on the charge of the most upper layer of multilayered PEC films, since the absorption of this rather large organic molecule is mostly confined to the surface layer with only a limited diffusion of the dye deeper into the bulk of the films. PEC films with different numbers of bilayers were immersed in 10−4 M MB aqueous solutions for 10 min. After immersion in the dye solution, the multilayer films were soaked in water for 1 min and dried with a mild air flow. The amount of MB absorbed on the film was determined by UV/vis spectroscopy.

P. Ahmadiannamini et al. / Journal of Membrane Science 403–404 (2012) 216–226

219

Scheme 2. A sketched diagram for fabricating LBL self-assembled membranes.

2.5. Filtrations

3. Results and discussion

NF-experiments were performed using a high-throughput apparatus (HTML, Belgium) containing 16 filtration cells with 4.53 cm2 membrane area each [43]. The system was pressurized with nitrogen to 40 × 105 Pa (40 bar). During filtration, the feed solution was stirred at 11.66 Hz (700 rpm) to minimize concentration polarization. The negatively charged RB and IPA were selected as solute and solvent for the filtration tests. Also, the performance of the prepared membranes was studied using smaller molecules (AF and BTB) in IPA. To study the SRNF performance of the PE membranes in the presence of electrostatic attraction, membranes with positive capping layers were prepared by dipping the final membranes in a PDDA solution. Prepared membranes were tested for the filtration of a RB/IPA solution. All the feed solutions were prepared in a 35 ␮M concentration. Permeate samples were collected in cooled flasks as a function of time, weighed and analyzed. The cells were filled up by about 40 ml of feed solution and about 5 ml of the total permeate was collected for each membrane after discarding the first 5 ml. The long-term stability tests of the PEC membranes were carried out using a SterlitechTM HP4750 stirred cell with an active area of 14.6 cm2 . The cell was filled with 300 ml of feed solution and was pressurized with nitrogen. During filtration, the feed solution was stirred at 16.66 Hz (1000 rpm) to minimize concentration polarization. The retention values were calculated from the permeate concentration and the concentration of the feed solution according to Eq. (2).

Multilayered PEC films were first prepared on a silicon wafer to fundamentally study the interaction between PDDA and the respective polyanion (PVS or PSS). By using the LBL method, 5, 10, 15 and 20 bilayers of PEC films were prepared. To clarify the influence of the counterion on the formation of the films, polyanions were used in the sodium or the acid form. FTIR spectra of the PEC films with different PDDA/PSS bilayer numbers are shown in Fig. 2. The peaks at 1034 and 1009 cm−1 are attributed to the sulfonated groups in PSS [44]. FTIR spectra of the PEC films with different PDDA/PVS bilayer numbers are shown in Fig. 3. The two peaks at 1058 and 1254 cm−1 can be assigned, respectively, to the symmetric and asymmetric stretching vibrations of the sulfonate groups of PVS [45]. The two distinct peaks at 2850 and 2919 cm−1 are due to the CH2 stretching modes of the polymer backbone [45]. With increasing bilayer number, the area of the peaks increases as well, which is a strong evidence for the LBL growth. Compared with the Na-form, the membranes prepared from the H-form show higher characteristic peak intensities indicating more PE deposition and hence, thicker films. Replacing Na+ by H+ decreases the electron density on the PE chain itself, since H+ is more electronegative than Na+ [46]. Indeed, the conformation of a charged PE chain is mainly governed by electrostatic repulsions. The internal electrostatic repulsions of a coated PE chain lead to an increased size of the chain in comparison with an uncharged chain. A decrease in the charge density of the PE chain reduces the repulsions [47]. As a consequence, the radius of gyration will be smaller, which results in chains with more coils leading to a lower surface area per chain upon adsorption, hence a thicker layer.

 R (%) =

1−

Cp Cf

 × 100

(2)

3.1. Cross section of multilayered PEC membranes deposited on PAN-H supports where Cp and Cf are the dye concentration in permeate and feed stream, respectively. All the measurements were based on at least three samples, and the average values were used.

The SEM cross-sections of the PAN-H support and of a multilayered PEC membrane with a 20 bilayers thick coating are shown

220

P. Ahmadiannamini et al. / Journal of Membrane Science 403–404 (2012) 216–226

Fig. 2. FITR spectra of multilayered PEC films based on PDDA/PSS: (a) PDDA/PSS-Na; (b) PDDA/PSS-H; (c) 20 bilayer of PEC films from PSS-Na and PSS-H.

in Fig. 4. The support shows a very open structure with finger-like pores. After introduction of PE solutions on the support, very thin dense layers are formed on top of the porous skin layer, which evidences the successful PEC deposition. As can be seen in the SEM images, the thickness of the PEC layers increased as Na+ was substituted by H+ , which is in agreement with the FTIR results.

The resulting PEC films were analyzed by AFM. Tables 2 and 3 give the surface roughness of the PEC films consisting of PSS and PVS, respectively. As can be seen, PEC films consisting of PSS in general show a lower surface roughness than those prepared from PVS. As the PSS applied here has a lower charge density than the PVS, a possible

explanation may be found in a better match of the distances between charged groups in the polycation and the polyanion leading to a flatter deposition with less loops [48]. In addition, the roughness of the films, measured on the 0.5 ␮m × 0.5 ␮m scans, increased as the bilayer number increased from 5 to 20. For the PVS-containing PECs, the films prepared from solutions in the H-form show a rougher surface as compared with the ones deposited from the Na-form solutions. As described earlier, adsorption of chains with lower charge densities and smaller intra-chain repulsion forces and hence more loopy-structured will take place in the H-form solutions and the surface will become rougher. In the case of the PSS-containing PECs, however, apart from the 5 bilayered films, this trend is reversed and the roughness values of the films prepared from Na-form solutions are higher as compared with the ones deposited from H-form

Table 2 The calculated surface roughness of PDDA/PSS films as obtained by AFM.

Table 3 The calculated surface roughness of PDDA/PVS films as obtained by AFM.

3.2. AFM results

5 bilayers

10 bilayers

15 bilayers

20 bilayers

5 bilayers

10 bilayers

15 bilayers

20 bilayers

PSS-Na

RMS (nm) Ra (nm)

0.89 0.52

2.25 1.44

2.70 1.58

4.55 3.49

PVS-Na

RMS (nm) Ra (nm)

5.18 4.07

5.69 4.27

5.49 4.39

6.38 5.01

PSS-H

RMS (nm) Ra (nm)

1.50 0.90

1.60 0.97

1.71 1.14

2.64 1.72

PVS-H

RMS (nm) Ra (nm)

5.31 4.26

7.54 5.78

8.96 7.17

9.89 7.93

P. Ahmadiannamini et al. / Journal of Membrane Science 403–404 (2012) 216–226

221

Fig. 3. FITR spectra of multilayered PEC films based on PDDA/PVS: (a) PDDA/PVS-Na; (b) PDDA/PVS-H; (c) 20 bilayer of PEC films from PVS-Na and PVS-H.

solutions. This different change of roughness upon counterion substitution in PSS and PVS is hard to explain at this moment.

place in zone II only, hence the surface charge of the PECs does not really change.

3.3. MB studies

3.4. Separation properties

The results of MB adsorption are presented in Fig. 5. The absorbance of MB was increased as Na+ was substituted by H+ , which is well in agreement with previous predictions: as Na+ was substituted by H+ the “loopy” PEs will provide more surface charge, resulting in more MB absorption. In addition, MB absorption was not significantly changed as more bilayers were formed. This can be explained by a model put forward by Ladam et al. [49]. Typically, PEC films can be subdivided into three zones. Zone I consists of the first few layers close to the substrate, which are mainly influenced by the substrate only, while Zone III is composed of a few layers close to the upper surface of the film. Zone II is the “bulk” film, which is affected by neither the interface nor the substrate and where the PE charge compensation is achieved via intrinsic charge compensation. The interfaces between the zones are rather diffuse. When the first few layers are deposited, zones I and III are formed. If the number of layers is increased, the growth in multilayer takes

Figs. 6 and 7 display the separation properties of the membranes with different number of bilayers prepared from PSS and PVS, respectively. As can be seen, the PVS-based membranes show higher permeability (∼10 times) and retention values than the PSS-based membranes. From the SEM images (Fig. 4), it is obvious that the thickness of PSS- and PVS-based membranes do not differ to such extend that this could explain the different permeability. All the membranes showed high IPA permeations and good RB retentions up to 99%. The membranes prepared from polyanions in the H-form showed higher permeabilities and higher retentions than the ones prepared from the Na-form. As there was only a very small variation in retention of this rather large molecule with the different membranes, filtrations with smaller solutes (AF and BTB) were added. All the membranes showed a very high retention on AF and a lower retention on BTB. Observed retentions prove that the presence of only 5 bilayers already creates defect free

222

P. Ahmadiannamini et al. / Journal of Membrane Science 403–404 (2012) 216–226

Fig. 4. The SEM cross sections of the PAN-H support at 2 different magnifications (a, b) and multilayered PEC membrane composed of 20 bilayers: (c) PDDA/PSS-Na; (d) PDDA/PSS-H; (e) PDDA/PVS-Na; (f) PDDA/PVS-H.

membranes. Obviously, the retention of the neutral BTB solute is much lower than that of the bi-charged RB and AF, since exclusion based on electrostatic repulsion (Donnan effect) plays a very important role.

3.4.1. Effect of the positively charged capping layer PEC membranes with positively charged capping layers were prepared to study their performance to reject the negatively charged RB from IPA solutions in more detail (Fig. 8). As can be seen, the membranes show high retentions despite the electrostatic attraction between solute and membranes. Possibly, RB molecules absorb on the surface of the membranes (as also visually observed at the end of the filtration), forming a negative layer on the surface. The negatively charged RB layer attached on the surface, in fact

Fig. 5. MB absorption of PEC films formed on glass slides measured at 665 nm.

P. Ahmadiannamini et al. / Journal of Membrane Science 403–404 (2012) 216–226

0.25

50

0.2

40

0.15

20

0.05

10 0

5

10

15

20

0 25

No. of bilayers

b 100

0.3

70 60

0.25

50

0.2

40

0.15

30

0.1

20

0.05

10 0

5

10

15

20

0 25

100

0.4

90

0.35

2.5

40

2

30

1.5

20

1

10

0.5

70 60

0.25

50

0.2

40

0.15

30

0.1

20

0.05

10 0

5

10

15

20

5

10

15

20

0 25

0 25

No. of bilayers Fig. 6. SRNF properties of multilayered PDDA/PSS PEC membranes for different charged solutes from IPA solutions: (a) RB; (b) AF; (c) BTB (solid and dotted lines are attributed to H- and Na-forms, respectively).

5 4.5

80

4

70

3.5

60

3

50

2.5

40

2

30

1.5

20

1

10

0.5 0

5

10

15

No. of bilayers

20

c 100

Retention (%)

0.3

0

90

0

Permeability (l.m-2.h-1.bar-1)

80

Retention (%)

3

50

b 100

No. of bilayers

0

60

No. of bilayers

0.35

80

c

3.5

0.4

90

0

4

70

0

Retention (%)

0

4.5

80

0 25

5

90

4.5

80

4

70

3.5

60

3

50

2.5

40

2

30

1.5

20

1

10

0.5

0

Permeability (l.m-2.h-1.bar-1)

0.1

5

90

Permeability (l.m-2.h-1.bar-1)

60

Retention (%)

0.3

70

Permeability (l.m-2.h-1.bar-1)

80

Permeability (l.m-2.h-1.bar-1)

Retention (%)

0.35

30

Retention (%)

a 100

0.4

90

0

5

10

15

20

Permeability (l.m-2.h-1.bar-1)

a 100

223

0 25

No. of bilayers

forming an extra layer that neutralizes the surface and can reject the RB from IPA due to Donnan exclusion. Due to the different conditions, the charge balances in these systems will be affected by different “intrinsic” and “extrinsic” charge compensation [50], bearing directly on the properties of the PE multilayers. For example, multilayers containing salt ions should be thicker, less interpenetrating, with individual chains having more mobility, yielding less stable structures [50]. 3.4.2. Effect of the ionic-cross linking Another important parameter which can control permeability of PECs is the charge density (ionic cross-linking) in the system. The charge density is expressed in terms of the number of ion pairs per number of carbon atoms in the repeat unit of the complex formed by the polycation and polyanion [51]. All this is expected to play a role in the SRNF applications. According to the manufacturer, both PSS and PVS were synthesized in a solvent via free radical polymerization of the sulfonated monomers and the degree of sulfonation for both is thus 100%. Given this, it can be concluded that PVS-based

Fig. 7. SRNF properties of multilayered PDDA/PVS PEC membranes for different charged solutes in IPA solutions: (a) RB; (b) AF; (c) BTB (solid and dotted lines are attributed to H- and Na-forms, respectively).

membranes have a higher density of cross-links than PSS-based membranes. As reported previously for aqueous NF applications, solution flux increases with decreasing ionic cross-linking in the system. The lower ionic cross-linking density in a system likely results in a more open film with higher flux and lower retention [51,52]. The conformation of PEs in the coating solution and even after deposition is highly dependent on the solvent quality. In the presence of a good solvent, the polymer chains will try to maximize the polymer/solvent contacts and swell. In the case of a poor solvent, the PE chains adopt a more compact conformation in order to reduce polymer/solvent interactions [42]. The PEC membranes with lower charge densities swell more in water due to the lower density of ionic cross-links and thus show higher permeabilities. As shown by Miller et al., films prepared from LBL deposition of

0.3

70

0.25

60

0.2

50 40

0.15

30

0.1

20

0

5

10

15

20

0.3

70

0.25

60

0.2

50 40

0.15

30

0.1

20

0.05

10

0.35

80

Retention (%)

Retention (%)

90

0.35

80

0.4

100

0.05

10 0

0 25

0

5

No. of bilayers

1

50 40

0.75

30

0.5

20

20

0.4 0.35 0.3

70

0.25

60 50

0.2

40

0.15

30

0.1

20

0.25

10 15

0 25

80

Retention (%)

1.25

60

10

20

90

Permeability (l.m-2.h-1.bar-1)

1.5

70

5

15

100

1.75

80

Retention (%)

b

2

90

0

10

No. of bilayers

b 100

0

Permeability (l.m-2.h-1.bar-1)

90

0

a

0.4

100

Permeability (l.m-2.h-1.bar-1)

a

P. Ahmadiannamini et al. / Journal of Membrane Science 403–404 (2012) 216–226

0.05

10

0 25

Permeability (l.m-2.h-1.bar-1)

224

0 0

5

No. of bilayers

10

15

20

0 25

No. of bilayers

Fig. 8. SRNF properties of PEC membranes with positively charged capping layer in RB/IPA solutions: (a) PDDA/PSS; (b) PDDA/PVS (solid and dotted lines are attributed to H- and Na-forms, respectively).

Fig. 9. SRNF properties of PEC membranes in DMF: (a) PDDA/PSS; (b) PDDA/PVS (solid and dotted lines are attributed to H- and Na-forms, respectively).

results. Streaming potential measurements might further prove this assumption. The combination of the high RB and AF retention and very high fluxes of the membranes with 5 bilayers makes the reported membranes promising for commercial SRNF. The membrane permeabilities only decreased slightly as more PE bilayers were deposited. As the number of bilayers increased beyond 5, the retention values of the membranes remained almost constant, confirming the formation of a defect-free coating with even less than 5 bilayers. This can be explained by the three zone model. When the first few layers are deposited, zones I and III are formed. If the number of layers is increased, the growth in multilayer takes place

different PE combinations exhibited stronger swelling in water than in ethanol [53]. The lower permeabilities for PEC membranes in solvents other than water can thus probably be ascribed to the lower swelling in organics. PEC membranes with a lower ionic cross-link density will reduce their swelling more drastically. This explains the lower permeabilities of the membranes consisting of PSS compared with those comprising PVS. Higher retentions for PDDA/PVS membranes than for PDDA/PSS membranes are presumably due to a higher negative surface charge on the former systems, which is well in consistence with the MB absorption Table 4 Comparison of performance between multilayered PEC and other SRNF membranes. Membrane

RB/IPA Permeability (l/m2 h bar)

Multilayered PECa Polypyrroleb PDMSc Poly(sulfone)d Poly(sulfone)/SPEEKd StarmemTM 120 StarmemTM 228 StarmemTM 240 MPF-50 n.m.: Not measured. –: Unstable in the solvent. a Present work. b Taken from Ref. [28]. c Taken from Ref. [55]. d Taken from Ref. [57].

0.12–1.57 1.42 0.086 0.14 0.47 0.49 0.087 0.29 0.72

BTB/IPA Retention (%) >99 98 91 77.5 95.6 100 99.6 97.9 96

RB/DMF

Permeability (l/m2 h bar)

Retention (%)

Permeability (l/m2 h bar)

Retention (%)

0.06–1.58 n.m. 0.79 0.17 3.70 n.m. n.m. n.m. n.m.

84–88.5 n.m. 93 26.3 10.6 n.m. n.m. n.m. n.m.

0.06–0.2 0.03 n.m. n.m. n.m. – – – –

94–98 90 n.m. n.m. n.m. – – – –

P. Ahmadiannamini et al. / Journal of Membrane Science 403–404 (2012) 216–226

a

100

20

Retention (%)

80

15

70 60 50

10

40 30

5

20 10 0

0

5

10

15

20

25

Permeability (l.m-2.h-1.bar-1)

90

0 30

Time (h)

b

100

0.5

Retention (%)

80

0.4

70 60

0.3

50 40

0.2

30 20

0.1

Permeability (l.m-2.h-1.bar-1)

90

10 0

0

5

10

15

20

25

0 30

Time (h) Fig. 10. The long-time SRNF properties of PEC membranes: (a) (PDDA/PSS)20 ; (b) (PDDA/PVS)20 (solid and dotted lines are attributed to H- and Na-forms, respectively).

225

showed rougher structures as the number of deposited layers increased. Upon substitution of H+ for Na+ in the polyanions, the reduced polymer chain repulsions lead to a smaller radius of gyration, resulting in a lower surface area per chain and hence thicker layers. PEC films swell less upon exposure to organic solvents than at water and their permeabilities are controlled by the ionic cross-linking density. The membranes prepared from polyanions in the H-form showed higher permeabilities and higher retentions, which stems from their loopier structures and their higher surface charges, respectively. PEC membranes with a lower density of ionic cross-links show lower permeabilities. Surface charge has a great influence on the retention of charged solutes. Due to Donnan exclusion, the membrane showed very high retentions for solutes with the same charge as the last layer on the surface of the multilayered PEC membranes. Also, positively charged capping membranes showed high retentions for negatively charged RB due to Donnan exclusion. The results suggest that prepared multilayered PEC membranes are very promising in SRNF applications. Acknowledgments Dr. Xianfeng Li acknowledges FWO and Katholieke Universiteit Leuven for a grant as a postdoctoral research fellow and the financial support from the National Basic Research Program of China (973 program no. 2010CB227202). This study is a part of the EDROR & ReNuD project which is financed by MIP-Vlaanderen. This research was done in the framework of an I.A.P.-P.A.I. grant (Grant IAP 6/27) on Supramolecular Catalysis sponsored by the Belgian Federal Government, a GOA grant from the Flemish Government, an FWO project, and long term structural Methusalem funding by the Flemish Government. Appendix A. Supplementary data

in zone II only. Transport through this zone is seemingly not the rate-determining step. As reported earlier, PEC based membranes are stable in a wide variety of organic solvents. To further know their performance in more aggressive solvents, RB was selected as solute and DMF as solvent. Compared with IPA, the DMF permeability is lower, due to the different physico-chemical properties of the solvents [24,54–57] or different interactions between the membrane and the solvent. A very complex interplay of different contributions caused by mutual PE-solvent–solute–membrane interactions has to be taken into account [58]. All the membranes show very high retentions (around 95%) for RB in DMF (Fig. 9), which confirms the good stability of the membranes in DMF, which is one of the most troublesome solvents in SRNF. For IPA/RB, the membrane performance is better than that of the different commercial membranes (Table 4), as very high retentions are combined with superior fluxes. Due to the low stability of these commercial membranes in DMF, the comparison in this solvent could not be established. Furthermore, somewhat longer-term stability of the (PDDA/PSS)20 and (PDDA/PVS)20 membranes were evaluated by filtration of RB solutions in THF and DMF, respectively for a period of 24 h. The results are depicted in Fig. 10, showing very promising stability of PEC membranes in SRNF application. 4. Conclusions A new combination of multilayered PEC membranes was successfully prepared by the LBL method from PDDA and two different polyanions (PSS or PVS), each either in Na- or H-form. All membranes prepared proved to be very useful for filtrations in organic solvents, including aprotic solvents such as DMF and THF, for which they showed excellent solvent stability. SEM and AFM results

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.memsci.2012.02.052. References [1] G. Decher, Fuzzy nanoassemblies: toward layered polymeric multicomposites, Science 277 (5330) (1997) 1232–1237. [2] X.Y. Shi, M.W. Shen, H. Mohwald, Polyelectrolyte multilayer nanoreactors toward the synthesis of diverse nanostructured materials, Prog. Polym. Sci. 29 (10) (2004) 987–1019. [3] G. Ladam, C. Gergeli, B. Seger, G. Decher, J.C. Voegel, P. Schaaf, F.J.G. Cuisinier, Protein interactions with polyelectrolyte multilayers: interactions between human serum albumin and polystyrene sulfonate/polyallylamine multilayers, Biomacromolecules 1 (4) (2000) 674–687. [4] M. Elzbieciak, D. Wodka, S. Zapotoczny, P. Nowak, P. Warszynski, Characteristics of model polyelectrolyte multilayer films containing laponite clay nanoparticles, Langmuir 26 (1) (2010) 277–283. [5] E.S. Dragan, S. Schwarz, K.J. Eichhorn, Specific effects of the counterion type and concentration on the construction and morphology of polycation/azo dye multilayers, Colloids Surf. A 372 (1–3) (2010) 210–216. [6] Y. Liu, T. Cui, Ion-sensitive field-effect transistor based pH sensors using nano self-assembled polyelectrolyte/nanoparticle multilayer films, Sens. Actuators B 123 (1) (2007) 148–152. [7] T.R. Hendricks, I. Lee, A versatile approach to selective and inexpensive copper patterns using polyelectrolyte multilayer coatings, Thin Solid Films 515 (4) (2006) 2347–2352. [8] Y. Yang, M. Hori, T. Hayakawa, M. Nogami, Self-assembled 3-dimensional arrays of Au@SiO2 core-shell nanoparticles for enhanced optical nonlinearities, Surf. Sci. 579 (2–3) (2005) 215–224. [9] Y. Ji, Q. An, J. Qian, H. Chen, C. Gao, Nanofiltration membranes prepared by layerby-layer self-assembly of polyelectrolyte, Prog. Chem. 22 (1) (2010) 261–267. [10] G. Zhang, W. Gu, S. Ji, Z. Liu, Y. Peng, Z. Wang, Preparation of polyelectrolyte multilayer membranes by dynamic layer-by-layer process for pervaporation separation of alcohol/water mixtures, J. Membr. Sci. 280 (1–2) (2006) 727–733. [11] Q. Zhao, J. Qian, Q. An, Z. Sun, Layer-by-layer self-assembly of polyelectrolyte complexes and their multilayer films for pervaporation dehydration of isopropanol, J. Membr. Sci. 346 (2010) 335–343. [12] X. Liu, M.L. Bruening, Size-selective transport of uncharged solutes through multilayer polyelectrolyte membranes, Chem. Mater. 16 (2) (2004) 351–357.

226

P. Ahmadiannamini et al. / Journal of Membrane Science 403–404 (2012) 216–226

[13] W. Shan, P. Bacchin, P. Aimar, M.L. Bruening, V.V. Tarabara, Polyelectrolyte multilayer films as backflushable nanofiltration membranes with tunable hydrophilicity and surface charge, J. Membr. Sci. 349 (1–2) (2010) 268–278. [14] R. Malaisamy, A. Talla-Nwafo, K.L. Jones, Polyelectrolyte modification of nanofiltration membrane for selective removal of monovalent anions, Sep. Purif. Technol. 77 (3) (2011) 367–374. [15] B.W. Stanton, J.J. Harris, M.D. Miller, M.L. Bruening, Ultrathin, multilayered polyelectrolyte films as nanofiltration membranes, Langmuir 19 (17) (2003) 7038–7042. [16] P. Ahmadiannmini, X.F. Li, W. Goyens, B. Meesschaert, I.F.J. Vankelecom, Multilayered PEC nanofiltration membranes based on SPEEK/PDDA for anion separation, J. Membr. Sci. 360 (1–2) (2010) 250–258. [17] W.Q Jin, A. Toutianoush, B. Tieke, Use of polyelectrolyte layer-by-layer assemblies as nanofiltration and reverse osmosis membranes, Langmuir 19 (2003) 2550–2553. [18] A. Toutianoush, W.Q. Jin, H. Deligoz, B. Tieke, Polyelectrolyte multilayer membranes for desalination of aqueous salt solutions and seawater under reverse osmosis conditions, Appl. Surf. Sci. 246 (4) (2005) 437–443. [19] C. Qiu, S. Qi, C.Y. Tang, Synthesis of high flux forward osmosis membranes by chemically crosslinked layer-by-layer polyelectrolytes, J. Membr. Sci. 381 (1–2) (2011) 74–80. [20] Q. Saren, C.Q. Qiu, C.Y. Tang, Synthesis and characterization of novel forward osmosis membranes based on layer-by-layer assembly, Environ. Sci. Technol. 45 (12) (2011) 5201–5208. [21] H. Deligöz, S. Yılmaztürk, M. Yılmazo˘glu, H. Damyan, The effect of selfassembled multilayer formation via LbL technique on thermomechanical and transport properties of Nafion® 112 based composite membranes for PEM fuel cells, J. Membr. Sci. 351 (1–2) (2010) 131–140. [22] J. Wang, C. Zhao, H. Lin, G. Zhang, Y. Zhang, J. Ni, W. Ma, H. Na, Design of a stable and methanol resistant membrane with cross-linked multilayered polyelectrolyte complexes for direct methanol fuel cells, J. Power Sources 196 (13) (2011) 5432–5437. [23] I.F.J. Vankelecom, K. De Smet, L. Gevers, P.A. Jacobs, Development of nanofiltration membranes, in: A.I. Schäfer, A.G. Fane, T.D. Waite (Eds.), Nanofiltration—Principles and Applications, Elsevier, Amsterdam, The Netherlands, 2003 (Chapter 2). [24] I.F.J. Vankelecom, L.E.M. Gevers, in: C.A.M. Afonso, J.G. Crespo (Eds.), Pressuredriven Membrane Processes Chapter in Green Separation Processes, WileyVCH, Weinheim, Germany, 2005. [25] P. Vandezande, L.E.M. Gevers, I.F.J. Vankelecom, Solvent resistant nanofiltration: separating on a molecular level, Chem. Soc. Rev. 37 (2) (2008) 365–405. [26] Y.H. See Toh, F.W. Lim, A.G. Livingston, Polymeric membranes for nanofiltration in polar aprotic solvents, J. Membr. Sci. 301 (1–2) (2007) 3–10. [27] X.F. Li, M. Basko, F. Du Prez, I.F.J. Vankelecom, Multifunctional membranes for solvent resistant nanofiltration and pervaporation applications based on segmented polymer networks, J. Phys. Chem. B 112 (51) (2008) 16539–16545. [28] X.F. Li, P. Vandezande, I.F.J. Vankelecom, Polypyrrole modified solvent resistant nanofiltration membranes, J. Membr. Sci. 320 (1–2) (2008) 143–150. [29] X.X. Loh, M. Sairam, J. Hans, G. Steinke, A.G. Livingston, A. Bismarck, K. Li, Polyaniline hollow fibres for organic solvent nanofiltration, Chem. Commun. 47 (2008) 6324–6326. [30] S. Basu, M. Maes, A.C. Odena, L. Alaerts, D.E.I.F.J. De Vos, Vankelecom, Solvent resistant nanofiltration (SRNF) membranes based on metal–organic frameworks, J. Membr. Sci. 344 (2009) 190–198. [31] A.V. Volkov, D.F. Stamatialis, V.S. Khotimsky, V.V. Volkov, M. Wessling, N.A. Platé, New membrane material for SRNF applications, Desalination 199 (1–3) (2006) 251–252. [32] X.F. Li, S. De Feyter, D. Chen, S. Aldea, P. Vandezande, F. Du Prez, I.F.J. Vankelecom, Solvent-resistant nanofiltration membranes based on multilayered polyelectrolyte complexes, Chem. Mater. 20 (12) (2008) 3876–3883. [33] X.F. Li, W. Goyens, P. Ahmadiannamini, W. Vanderlinden, S. De Feyter, I. Vankelecom, Morphology and performance of solvent-resistant nanofiltration membranes based on multilayered polyelectrolytes: Study of preparation conditions, J. Membr. Sci. 358 (1–2) (2010) 150–157. [34] R.v. Klitzing, J.E. Wong, W. Jaeger, R. Steitz, Short range interactions in polyelectrolyte multilayers, Curr. Opin. Colloid Interface Sci. 9 (1–2) (2004) 158–162. [35] R.v. Klitzing, Internal structure of polyelectrolyte multilayer assemblies, Phys. Chem. Chem. Phys. 8 (43) (2006) 5012–5033.

[36] Y. Zhang, P.S. Cremer, Interactions between macromolecules and ions: the Hofmeister series, Curr. Opin. Chem. Biol. 10 (6) (2006) 658–663. [37] Z. Gui, B. Du, J. Qian, Q. An, Q. Zhao, Construction and deconstruction of multilayer films containing polycarboxybetaine: effect of pH and ionic strength, J. Colloid Interface Sci. 353 (1) (2011) 98–106. [38] S.S. Ono, G. Decher, Preparation of ultrathin self-standing polyelectrolyte multilayer membranes at physiological conditions using pHresponsive film segments as sacrificial layers, Nano Lett. 6 (4) (2006) 592–598. [39] Z. Gui, J. Qian, Q. Zhao, Y. Ji, Y. Liu, T. Liu, Q. An, Controllable disintegration of temperature-responsive self-assembled multilayer film based on polybetaine, Colloids Surf., A 380 (1–3) (2011) 270–279. [40] W.F. Dong, S. Liu, L. Wan, G. Mao, D.G. Kurth, H. Möhwald, Controlled permeability in polyelectrolyte films via solvent treatment, Chem. Mater. 17 (20) (2005) 4992–4999. [41] B.S. Kim, O.V. Lebedeva, K. Koynov, H.F. Gong, G. Glasser, I. Lieberwith, O.I. Vinogradova, Effect of organic solvent on the permeability and stiffness of polyelectrolyte multilayer microcapsules, Macromolecules 38 (12) (2005) 5214–5222. [42] E. Poptoshev, B. Schoeler, F. Caruso, Influence of solvent quality on the growth of polyelectrolyte multilayers, Langmuir 20 (3) (2004) 829–834. [43] http://www.html-membrane.be. [44] R. Malaisamy, M.L. Bruening, High-flux nanofiltration membranes prepared by adsorption of multilayer polyelectrolyte membranes on polymeric supports, Langmuir 21 (23) (2005) 10587–10592. [45] D.K. Kim, S.W. Han, C.H. Kim, J.D. Hong, K. Kim, Morphology of multilayers assembled by electrostatic attraction of oppositely charged model polyelectrolytes, Thin Solid Films 350 (1–2) (1999) 153–160. [46] M.M. de Kok, M. Buechel, S.I.E. Vulto, P. van de Weijer, E.A. Meulenkamp, S.H.P.M. de Winter, A.J.G. Mank, H.J.M. Vorstenbosch, C.H.L. Weijtens, V. van Elsbergen, Modification of PEDOT:PSS as hole injection layer in polymer LEDs, Phys. Status Solidi A 201 (6) (2004) 1342–1359. [47] T.D. Stephan, J.B. Schlenoff, Mechanism of polyelectrolyte multilayer growth: charge overcompensation and distribution, Macromolecules 34 (3) (2001) 592–598. [48] A. Laschewsky, E. Wischerhoff, Polyelectrolyte multilayer assemblies containing nonlinear optical dyes, Langmuir 30 (26) (1997) 8304–8309. [49] G. Ladam, P. Schaad, J.C. Voegel, P. Schaaf, G. Decher, F. Cuisinier, In situ determination of the structural properties of initially deposited polyelectrolyte multilayers, Langmuir 16 (3) (2000) 1249–1255. [50] J.B. Schönhoff, H. Ly, M. Li, Charge and mass balance in polyelectrolyte multilayers, J. Am. Chem. Soc. 120 (1998) 7626. [51] L. Krasemann, B. Tieke, Selective ion transport across self-assembled alternating multilayers of cationic and anionic polyelectrolytes, Langmuir 16 (2) (2000) 287–290. [52] S.U. Hong, R. Malaisamy, M.L. Bruening, Optimization of flux and selectivity in Cl− /SO4 2− separations with multilayer polyelectrolyte membranes, J. Membr. Sci. 283 (1–2) (2006) 366–372. [53] M.D. Miller, M.L. Bruening, Correlation of the swelling and permeability of polyelectrolyte multilayer films, Chem. Mater. 17 (21) (2005) 5375–5381. [54] L.E.M. Gevers, G. Meyen, K. De Smet, P. Van De Veldeb, F. Du Prez, I.F.J. Vankelecom, P.A. Jacobs, Physico-chemical interpretation of the SRNF transport mechanism for solutes through dense silicone membranes, J. Membr. Sci. 274 (1–2) (2006) 173–182. [55] A. Dobrak-Van Berlo, I.F.J. Vankelecom, B. Van der Bruggen, Parameters determining transport mechanisms through unfilled and silicalite filled PDMS-based membranes and dense PI membranes in solvent resistant nanofiltration: comparison with pervaporation, J. Membr. Sci. 374 (1–2) (2011) 138–149. [56] D. Bhanushali, S. Kloos, C. Kurth, D. Bhattacharyya, Performance of solventresistant membranes for non-aqueous systems: solvent permeation results and modeling, J. Membr. Sci. 189 (1) (2001) 1–21. [57] X.F. Li, S. De Feyter, I.F.J. Vankelecom, Poly(sulfone)/sulfonated poly(ether ether ketone) blend membranes: morphology study and application in the filtration of alcohol based feeds, J. Membr. Sci. 324 (1–2) (2008) 67–75. [58] M. Dias, J. Hadgraft, M.E. Lane, Influence of membrane–solvent–solute interactions on solute permeation in model membranes, Int. J. Pharm. 336 (1) (2007) 108–114.