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Selective Ion Transport across Self-Assembled Alternating Multilayers of Cationic and Anionic Polyelectrolytes Lutz Krasemann and Bernd Tieke* Institut fu¨ r Physikalische Chemie der Universita¨ t zu Ko¨ ln, Luxemburgerstrasse 116, D-50939 Ko¨ ln, Germany Received September 17, 1999. In Final Form: November 5, 1999 Ultrathin membranes consisting of an alternating sequence of cationic and anionic polyelectrolytes were prepared by means of electrostatic layer-by-layer adsorption and investigated on their permeability for NaCl, Na2SO4, and MgCl2 in aqueous solution. It is demonstrated that the multi-bipolar structure of the polyelectrolyte membranes favors the separation of mono- and divalent ions by Donnan exclusion of the divalent ions. Various effects on the rate of ion permeation and the selectivity were investigated. Addition of salt to the polyelectrolyte solutions used for membrane preparation led to improved ion separation, while an increase of the pH had the opposite effect. Use of polyelectrolytes with high charge density also improved the ion separation. Especially good results were obtained if membranes containing polyallylamine (PAH) as the cationic polyelectrolyte were used. For 60 layer pairs of PAH/polystyrenesulfonate, for example, a separation factor R for Na+/Mg2+ up to 112.5 and for Cl-/SO42- up to 45.0 was found. The origins of the various effects are discussed in terms of different charge density and concentration of excess charges in the polyelectrolyte membrane.
1. Introduction The alternating electrostatic adsorption of cationic and anionic polyelectrolytes at charged surfaces has proven to be an easy method for preparation of ultrathin organized polymer films.1,2 While possible electronic3-6 applications were investigated in great detail, the use as membranes for materials separation has been studied only scarcely. Only few publications appeared which were concerned with gas separation,7-9 ethanol-water pervaporation,10-12 and a study of the diffusion of protons13 and Fe(CN)63ions.14. The preparation of ultrathin polyelectrolyte films starts with the immersion of, e.g., a positively charged substrate in an aqueous solution of an anionic polyelectrolyte so that a thin layer of this compound is adsorbed and the surface charge of the substrate is reverted. Subsequent dipping of this substrate into a solution of a cationic polyelectrolyte again leads to adsorption of a thin layer, and the surface charge is rendered positive again. Multiple repetition of the adsorption steps leads to a multilayer film with alternating positive and negative excess * To whom correspondence should be adressed. (1) Decher, G.; Hong, J.-D. Makromol. Chem. Macromol. Symp. 1991, 46, 321. (2) Decher, G. Science 1997, 277, 1232 (3) Fou, A. C.; Omitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl. Phys. 1996, 79, 7501. (4) Laschewsky, A.; Wischerhoff, E.; Kauranen, M.; Persoons, A. Macromolecules 1997, 30, 8304. (5) Wang, X. Balasubramanian, S.; Li, L.; Jiang, X.; Sandman, D. J.; Rubner, M. F.; Kumar, J.; Tripathy, S. K. Macromol. Rapid Commun. 1997, 18, 451. (6) Toutianoush, A.; Tieke, B. Macromol. Rapid Commun. 1998, 19, 591. (7) Stroeve, P.; Vasquez, V.; Coelho, M. A. N.; Rabolt, J. F. Thin Solid Films 1996, 284-285, 708. (8) Leva¨salmi, J.-M.; McCarthy, T. J. Macromolecules 1997, 30, 1752. (9) van Ackern, F.; Krasemann, L.; Tieke, B. Thin Solid Films 1998, 329, 762. (10) Krasemann, L.; Tieke, B. J. Membr. Sci. 1998, 150, 23. (11) Krasemann, L. Tieke, B. Mater. Sci. Eng. C 1999, 819, 523. (12) Krasemann, L. Tieke, B. Chem. Eng. Technol. in press. (13) Klitzing, R. v.; Mo¨hwald, H. Langmuir 1995, 11, 3554. (14) Bruening, M. L.; Harris, J. J.; DeRose, P. M. ACS Polym. Prepr. 1999, 40, 451.
Figure 1. Rejection model of multi-bipolar membrane (adapted from ref 15).
charges.1,2 Due to its multi-bipolar architecture, the film should be able to reject ions by electrostatic repulsive forces, also called Donnan exclusion. As already demonstrated for bipolar membranes, divalent ions receive a much stronger repulsive force from the positively charged layer than monovalent ones.15,16 Consequently, they are more strongly rejected and a good selectivity is obtained. The same is true for divalent anions which are rejected by the negatively charged layers. A corresponding rejection model for a multi-bipolar membrane is depicted in Figure 1. The model implies that the difference in the permeation of mono- and divalent ions becomes progressively more pronounced when the number of adsorbed layers is increased. The purpose of our study is to qualitatively demonstrate the validity of this model for the alternatingly adsorbed multilayers described above. For this purpose, permeation of aqueous NaCl, MgCl2, and Na2SO4 across the multilayers was investigated. Since the ion permeation will strongly be affected by the structure of the membrane which again is affected by the molecular structure of the polyelectrolytes and the pH conditions during membrane (15) Urairi, M.; Tsuru, T.; Nakao, S.; Kimura, S. J. Membr. Sci. 1992, 70, 153. (16) Tsuru, T.; Nakao, S.; Kimura, S. J. Membr. Sci. 1995, 108, 269.
10.1021/la991240z CCC: $19.00 © 2000 American Chemical Society Published on Web 12/16/1999
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Table 1. Permeation Rates of NaCl, MgCl2, and Na2SO4 for Separating Membranes of PAH/PSS Prepared from Polyelectrolyte Solutions (A) Containing 1 mol/L NaCl and (B) without NaCl
a
no. of adsorbed layers
PR (NaCl) [10-6 cm s-1]
5 10 60
62.6 39.2 22.5
5 10 60
121.7 75.6 44.4
PR (Na2SO4) [10-6 cm s-1]
R (Cl-/SO42-)
(a) with Addition of NaCl 2.0 31.3 1.1 35.6 0.2 112.5
3.7 1.5 0.5
16.9 26.1 45.0
(b) without Addition of NaCl 74.7 1.6 54.0 1.4 2.9 15.1
a a 4.5
a a 9.9
PR (MgCl2) [10-6 cm s-1]
R (Na+/Mg2+)
Not determined.
preparation, we also studied the influence of these parameters on the ion permeation. 2. Experimental Section Materials. Poly(allylamine hydrochloride) (PAH, molecular weight 9600), poly(diallyldimethylammonium chloride) (PDADMAC, molecular weight 250 000), and sodium poly(styrenesulfonate) (PSS, molecular weight 70 000) were purchased from Aldrich, poly(4-vinylpyridine) (P4VP, molecular weight 50 000) and poly(ethyleneimine) (PEI, molecular weight 70 000) from Polyscience, and dextransulfate (DEX, molecular weight 5000) and chitosan (CHI, molecular weight 100 000) from Fluka. All compounds were used without further purification. Milli-Q water (resistance g18 MΩ cm-1) was used as solvent. PAN/PET supporting membranes treated with oxygen plasma10 were provided by Sulzer Chemtech GmbH, Neunkirchen. Methods. Self-Assembly of Polyelectrolyte Multilayers. Polyelectrolytes were dissolved in water in a concentration of 10-2 monomol L-1 (monomole ) mole of monomer unit), the water being acidified with aqueous HCl. If not especially indicated, the pH value was1.7. For adsorption of the polyelectrolyte 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 up to 89 times. Immersion time for the individual steps was 30 min; the temperature was about 20 °C. For the dipping procedure, a home-built computerized apparatus was used. The size of the membranes was 12 × 12 cm2. Ion Permeation. Measurements of ion permeation were carried out using a homemade apparatus. The membrane of area A ) 4.52 cm2 was placed between two chambers with a volume V of 60 mL each. One of the chambers contained the electrolyte solution of concentration c ) 0.1 mol L-1 and the other one pure water. To determine the permeation rate PR of electrolytes through the membrane, the initial increase of conductivity ∆Λ in pure water per unit time ∆t was measured under constant stirring in all experiments. Effects of stirring on PR were not observed. PR was obtained using the equation
PR ) (∆Λ/∆t) Λm-1 V(Ac)-1 with Λm being the molar conductivity of the corresponding salt solution. The diffusion coefficient D was obtained by multiplying PR with the estimated thickness of the separation layer. The separation factor R is simply the ratio of the permeation rates of NaCl and MgCl2 or NaCl and Na2SO4.
3. Results and Discussion The polyelectrolyte multilayers were adsorbed on a plasma-treated PAN/PET supporting membrane with pore sizes of 20-200 nm. Details are described in the experimental part and in previous publications.9-12 By use of standard conditions (pH 1.7, no added salt in the polyelectrolyte solution), each adsorbed polyelectrolyte layer has a thickness of about 0.5 nm,17 i.e., the adsorption of an individual polycation/ polyanion layer pair in a so(17) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992, 89, 160.
Figure 2. Effect of the thickness of the separating membrane on permeation rates PR of NaCl and MgCl2. Separating membrane: PAH/PSS.
called dipping cycle adds about 1 nm to the total thickness of the polyelectrolyte multilayer. For measuring the ion permeation, the polyelectrolyte membrane was placed between two chambers, one of them filled with pure water and the other one with a salt solution of the same volume (0.1 mol/L). The ion permeation across the membrane was determined by measuring the increase of ion conductivity ∆Λ per unit time ∆t and calculating the permeation rate PR as outlined in the experimental part. In Figure 2, the permeation rates of NaCl and MgCl2 are plotted versus the number of adsorbed layer pairs of PAH/PSS. With an increase in the number of adsorbed layers, the polyelectrolyte membrane becomes thicker and thicker and, consequently, less permeable. For both salts an inverse proportionality is found, but for MgCl2 the decrease in PR is more pronounced than that for NaCl. With increasing thickness, the multi-bipolar membrane rejects the Mg2+ ions more, in good agreement with the behavior expected from the model in Figure 1. If the membrane consists of 60 layer pairs, the ratio R of the PR values of NaCl and MgCl2 is 15.1. The permeation rates are strongly dependent on the preparation conditions of the membrane. Addition of salt to the polyelectrolyte solution strongly reduces the mutual electrostatic repulsion of the polymer chains, the polymer coils becoming denser and denser so that they are rather adsorbed as coil than in a flat conformation. As a consequence, the thickness of the individual adsorbed layers increases and the salt addition can be used to fine tune the overall membrane thickness.15 The increase of thickness also affects the ion permeation. In Table 1, the PR values of NaCl, MgCl2, and Na2SO4 are shown for membranes consisting of different number of layer pairs prepared from NaCl-containing solution. For NaCl and MgCl2, comparable data from salt-free polyelectrolyte
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Langmuir, Vol. 16, No. 2, 2000 289 Table 2. PR and r (Na+/Mg2+) Values for Membranes (60 Layer Pairs) Prepared with Different Polyelectrolytes polycation
polyanion
PR (NaCl)
PR (MgCl2)
R (Na+/Mg2+)
PEI PAH PEI PAH PAH CHI P4VP PDADMAC
PVS PVS PSS DEX PSS PSS PSS PSS
21.6 15.9 60.8 36.2 44.4 85.8 98.4 93.4
6.7 1.3 55.1 2.5 2.9 62.4 110.4 99.5
3.2 12.2 1.1 14.5 15.1 1.4 0.9 0.9
Figure 4. Effect of the pH of the polyelectrolyte solutions used for preparation of the separation layer on permeation rates PR of NaCl and MgCl2 . Separation layer: 60 layer pairs PAH/ PSS. Figure 3. Effect of charge density Fc of polyelectrolyte complex on permeation rate PR of (a) NaCl and (b) MgCl2. Separating membranes always consist of 60 layer pairs of polycation/ polyanion.
solution are also shown. As expected, the PR values decrease with the number of adsorbed layers, but for the membranes from the salt-containing solution, much smaller PR values are observed than from the salt-free one. Also, the PR values of MgCl2 are much smaller than for NaCl so that a much higher separation factor results. A PAH/PSS membrane consisting of five layer pairs exhibits an R value of 31.3 for NaCl/MgCl2 permeation, while for 60 layer pairs an R value of 112.5 is found. The main reason for the reduction of the PR values is the larger thickness of the individual polyelectrolyte layers. Another reason might be that the larger thickness prevents the oppositely charged polyelectrolyte chains from complete interpenetration and neutralization of their charges. In that case more excess charges are present in the membrane, which more effectively repel the permeating ions and thus contribute to the observed effect. The charge density of the polyelectrolytes might be another important factor determining the PR values. A high charge density should be more effective in ion repulsion, and a higher separation factor should result. The effect of charge density was studied by measuring the permeation rates of NaCl and MgCl2 across membranes made of different polyelectrolytes of variable charge density. In Figure 3a, the results of NaCl permeation are shown. The charge density Fc 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 -anion. For example, the PEI/PVS complex exhibits a Fc value of 1/(2 + 2) ) 0.25. As expected, the permeation rate
PR is inversely proportional to Fc; i.e., the permeation rate is high, when the charge density is low and vice versa. It is also noticed that membranes containing PAH always exhibit a lower permeation rate than membranes with other polycations. For permeation of MgCl2, similar results are found, but with increasing charge density the decrease of PR is even more pronounced (Figure 3b). Again, if PAH is used as the cationic polyelectrolyte, much lower PR values are found. The corresponding R values are listed in Table 2. At present we can only speculate on the origin of the exceptional behavior of PAH. PAH differs from the other cationic polyelectrolytes used in this study by the -CH2NH3+ substituent groups, which are highly flexible and perhaps more able to effectively repel the permeating Mg2+ ions. The charge density of the polyelectrolytes depends not only on the molecular structure but also on the degree of ionization of the polar groups. The ionization is again dependent on the pH of the aqueous solution from which the polyelectrolyte is adsorbed. The effect of pH on the permeation rate was studied for NaCl and MgCl2. As shown in Figure 4, the permeation rates of both salts decrease with the pH, but the effect is more pronounced for MgCl2. The main reason is that upon increasing pH the ammonium groups of PAH are more and more deprotonated, so that more PAH has to be adsorbed to neutralize the negatively charged substrate surface. Because of the partial deprotonation, the adsorbed PAH chains attain a loopy conformation so that the thickness of individual PAH layers progressively increases with the pH.18 At first glance this seems to resemble the effect of (18) Yoo, D.; Shiratori, S. S.; Rubner; M. F. Macromolecules 1998, 31, 4309.
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NaCl addition to the polyelectrolyte solution, which also causes an increase of the layer thickness as discussed above. However, there is a pronounced difference: In case of NaCl addition the increase of layer thickness is due to incorporation of charged chain segments; i.e., the concentration of excess charges in the membrane is increased and the Mg2+ ions are more effectively rejected. In the case of a pH increase, the larger thickness per layer originates from incorporation of nonionized chain segments so that the concentration of excess charges is rather decreased than increased. Since the nonionized parts are able to swell in water, the free volume is increased, the permeation rates of all ions become higher, and the selectivity is decreased. 4. Summary and Conclusions Our study shows that the layer-by-layer technique is a suitable method to prepare multi-bipolar membranes capable of separating mono- and divalent ions. The ion transport can be well described by the rejection model shown in Figure 1; i.e., the divalent ions are rejected by Donnan exclusion and the rejection becomes even more pronounced, if the number of adsorbed layers is increased. It is also shown that the ion permeation depends on the concentration of excess charges in the individual poly(19) Toutianoush, A.; Tieke, B. To be submitted for publication.
Letters
electrolyte layers, which can be varied upon different conditions of membrane preparation, e.g. by addition of salt to the polyelectrolyte solution or by increasing the pH value. In both cases, the thickness of the adsorbed polyelectrolyte layers increases. While salt addition leads to additional incorporation of charged chain segment and thus improves the ion separation, the increase of the pH value may lead to deprotonation of the cationic polyelectrolyte so that noncharged chain segments are incorporated, which deteriorate the ion separation. Another important factor influencing the ion transport is the molecular structure of the polyelectrolyte. A high charge density favors a dense, less permeable membrane exhibiting improved rejection of divalent ions. The rejection of divalent cations is even more effective, if PAH with highly flexible -CH2NH3+ groups is present in the membrane. Presently we systematically study the transport of small molecules and ions of different size as well as the transport of ions under reverse osmosis conditions.19 Acknowledgment. The authors are grateful to Dr. H. Scholz, Sulzer Chemtech GmbH, Neunkirchen, for providing the PAN/PET supporting membrane. Financial support from the Deutsche Forschungsgemeinschaft (Project No. Ti 219/3-3) is also gratefully acknowledged. LA991240Z
