ultrathin self-assembled polyelectrolyte multilayer membranes

Eur. Phys. J. E 5, 29–39 (2001) THE EUROPEANPHYSICAL JOURNAL Ec©

EDP SciencesSocieta Italiana di FisicaSpringer-Verlag 2001

Ultrathin self-assembled polyelectrolyte multilayer membranes

B. Tiekea, F. van Ackern, L. Krasemann, and A. Toutianoush

Institut fur Physikalische Chemie, Universitat zu Koln, Luxemburgerstr. 116, 50939 Koln, Germany

Received 4 September 2000

Abstract. The paper is concerned with ultrathin membranes prepared upon alternating layer-by-layer ad-sorption of cationic and anionic polyelectrolytes on a porous substructure. The formation of the polyelec-trolyte multilayer membranes is characterised and the transport of gases, liquid mixtures and ions acrossthe membranes is studied. In particular, the use of the membranes for alcohol/water separation under per-vaporation conditions, and for the separation of mono- and divalent ions is described. It is demonstratedthat upon a suitable choice of polyelectrolytes and substructures, and a careful optimisation of preparationand operation conditions, membranes can be tailored exhibiting an excellent separation capability.

PACS. 68.47.Pe Langmuir-Blodgett films on solids, polymers on surfaces, biological molecules on surfaces– 81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layerson liquids) – 82.65.Fr Film and membrane processes: ion exchange, dialysis, osmosis, electroosmosis

1 Introduction

Good membranes are characterised by a high flux anda high separation capability. Either properties can becombined by the preparation of so-called composite mem-branes, which consist of a highly porous supporting mem-brane coated with a thin, homogeneous and dense separat-ing layer [1,2]. Usually the separating layer is cast fromsolution, but solution casting is limited to the prepara-tion of layers of more than a micrometer in thickness. Ifthinner separating layers are desired, more sophisticatedpreparation techniques have to be used, as for examplethe Langmuir-Blodgett (LB) technique [3–8] or methodsbased on molecular self-assembly such as physisorptionor chemisorption of organic compounds on a solid sup-port [4,9–17].

When this research project was planned in 1993, sev-eral studies on the separation capability of LB membraneswere already available [3], while only little was known onthe transport properties of ultrathin self-assembled films[18]. Decher and Hong [14] and Lvov, Decher and Mohwald[15] had just reported on a novel method for preparationof polyelectrolyte films with controlled thickness in the 10to 100 nm range. The method is based on the alternat-ing electrostatic adsorption of cationic and anionic poly-electrolytes and/or bolaamphiphiles on charged substratesand allows to prepare ultrathin polymer films in a simple,yet elegant and precise manner. However, studies on thetransport behaviour of the polyelectrolyte multilayers andtheir use as separating membranes were not yet reported.

a e-mail: [email protected]

Purpose of our project was therefore

(a) to use the layer-by-layer adsorption of polyelectrolytesreported by Decher et al. [14–16] for preparation ofnovel composite membranes with ultrathin separatinglayer, and

(b) to study the transport of gases, liquid molecules andions through these membranes.

The project is both of practical and scientific interest.Practical interest arises from the possibility to preparea new type of composite membrane with polyelectrolyteseparation layer suitable for materials separation, whilescientific interest is based on the study of the transportproperties of the polyelectrolyte multilayers and thus pro-vides important new information on the properties of solidpolyelectrolytes in general.

In a first step, suitable supporting membranes andpolyelectrolytes had to be chosen and suitable condi-tions for preparation of the composite membranes had tobe be determined. Subsequently, gas and liquid perme-ation should be investigated. Studies on liquid separationshould be carried out under pervaporation conditions, apromising technical separation method [19]. Finally, it wasplanned to investigate the permeation of ions and to studythe use of the polyelectrolyte membranes for ion separa-tion under reverse osmosis conditions.

In recent years, a number of studies on the trans-port properties of polyelectrolyte multilayer membranesalready appeared [20–30]. Purpose of this article is to com-prehensively describe our previous research activities inthis field.

30 The European Physical Journal E

Table 1. Supporting membranes and their characteristic properties.

Name Polymer Thickness Pore size Contact angle NotesCelgard 2400 Isotactic polypropylene 25 µm 40× 120 nm 110± 4◦ rough surface,(Kalle) hydrophobicPAN/PET Polyethyleneterephthalate PET: 100 µm 20–200 nm, 53± 7◦ weakly(Sulzer-Chemtech) (PET) fleece coated with PAN: 80 µm in diameter hydrophilic

polyacrylonitrile (PAN)Isopore Polycarbonate 10 µm 50 nm, 49.9± 2.5◦ etched ion track(Millipore) cylindric membrane,

pores hydrophilic,transparent

Trespaphan Atactic polypropylene 25 µm pore free 99± 3◦ smooth surface,hydrophilic,transparent

2 Formation and characterisationof the composite membranes

First step in the preparation of the composite membranesis the choice of a suitable supporting membrane. With oneexception, only porous substructures were used. The ad-vantage is that the transport across the composite mem-brane is only little affected by the substructure so thatthe separation achieved can be exclusively ascribed to theultrathin polyelectrolyte multilayer adsorbed at the sur-face. Supporting membranes used in our study are listedin Table 1.

In order to facilitate a homogeneous adsorption of thepolyelectrolytes, the supporting membranes were treatedwith oxygen plasma. Experimental details were previouslydescribed [23,30]. The effect of plasma treatment wasstudied using contact angle measurements, UV spectro-scopic studies of dye adsorption at the surface, and trans-mission electron microscopy (TEM). From these studies[30], the optimum time periods of plasma treatment couldbe determined to be 1 min for PAN/PET and Celgardmembranes, and 10 min for Trespaphan. The periods weresufficiently long to clearly hydrophilise the surface (con-tact angle ≤ 25◦), but not long enough to damage thesurface structure of the samples. Only the Isopore mem-brane was hydrophilic enough to be used without plasmatreatment.

Second step in the preparation of the composite mem-brane was the adsorption of the separation layer. For thispurpose, the dipping method first reported by Decheret al. [14–16] was used. In Figure 1, it is schematicallyrepresented. The plasma-treated support was dipped intoa solution of a cationic polyelectrolyte until a thin layerof this compound was adsorbed and the surface chargewas reverted. Subsequently, the support was washed anddipped into a solution of an anionic polyelectrolyte. Againa thin layer was adsorbed and the surface charge renderednegative again. Multiple repetition of the adsorption stepsleads to a polyelectrolyte multilayer film showing someadvantageous properties: since each adsorption step addsabout 0.5 nm to the total thickness of the film [31], an ul-trathin, homogenous coating of precisely controlled thick-ness is obtained. A wide variety of polyionic compounds

Fig. 1. Scheme of layer-by-layer adsorption of polyelectrolyteson plasma-treated porous supporting membrane. Multiple rep-etition of steps A and B leads to the ultrathin separating layer.Note that in reality the pore diameter is 20–200 nm, i.e. muchlarger than the size of the polyelectrolyte chains, the individualpolymer chains are less ordered and partially overlapping.

[32] can be adsorbed, which allows to tailor appropriatemembranes for various separation problems. In Table 2,the various compounds used for preparation of the sepa-ration layer are compiled.

In the third step, it was necessary to optimise theconditions for the polyelectrolyte adsorption. Usually theadsorption of the individual polyelectrolyte layers is de-tected using UV absorption spectroscopy or X-ray reflec-tivity measurements [14–16,31]. While these methods canbe easily applied to transparent supports, they are unfor-tunately not well suited for porous and opaque substruc-tures. Therefore only the transparent Isopore and Tres-paphan supports were suited for the UV spectroscopicmeasurements. In order to directly prove the adsorp-tion, the deeply red coloured, water-soluble 1,4-diketo-3,6-diphenyl-pyrrolo[3.4-c]pyrrol-4,4’-disulfonic acid (DPPS)[33] was used as a marker. Alternating multilayers of PAHand DPPS were built up on Isopore and Trespaphan sup-ports and the absorbance of the dye was evaluated as aquantitative measure of the amount of adsorbed material.

Typical UV spectra of an Isopore membrane coatedwith different numbers of PAH/DPPS layer pairs areshown in Figure 2. With increasing number of dippingcycles the maximum absorption of DPPS at λ = 488 nmincreases linearly. This indicates a steady adsorption asit is also found on glass supports. The quantitative eval-uation of the spectra proved to be a useful tool to opti-mise the conditions for polyelectrolyte adsorption on the

B. Tieke et al.: Ultrathin self-assembled polyelectrolyte multilayer membranes 31

Table 2. Polyionic compounds used for preparation of theseparating membrane.

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Fig. 2. UV-vis absorption spectra of PAH/DPPS alternateassemblies on finely porous, transparent Isopore (from [23]).

lowing results were obtained:

– polyelectrolyte multilayers can be adsorbed on allsupporting membranes provided they are hydrophilicenough. Hydrophilicity can be improved by a plasmatreatment of the substructure;

– the amount of material adsorbed per dipping cycle ona porous support is considerably larger than on a pore-free support due to the larger surface area;

– the amount of adsorbed material increases with thetime of plasma treatment of the support;

– the amount of adsorbed material depends on the pHof the polyelectrolyte solution. For example, the aminogroups of PAH dissolved at low pH are all protonatedso that the adsorption of PAH leads to a high concen-tration of positive charges at the membrane surface.Consequently, more DPPS molecules are adsorbed inthe subsequent dipping step than in case that a par-tially protonated PAH originating from a solution ofhigher pH was present at the surface;

– polyelectrolytes of high molecular weight are more eas-ily adsorbed on pore-free supports exhibiting a smoothsurface, while low molecular weight polymers are moreeasily adsorbed on porous membranes with roughersurface structure;

– in order to reach a saturation limit of adsorption adipping time of 30 min is required;

– time-consuming drying of an adsorbed polyelectrolytelayer prior to adsorption of the next layer is unneces-sary because it has no decisive influence on the adsorp-tion of the next layer.

Clearly the observations raise important questions onthe actual morphology and thickness of the separationlayer, and on the filling of the pores in the supportingmembrane by the adsorbed polyelectrolytes. To solve thesequestions, additional electron microscopic studies have tobe carried out. However, the previous studies were quitehelpful in defining optimum conditions for the preparationof the separating membranes. According to these studies itwas convenient to work with polyelectrolytes of relativelylow molecular weight, which are adsorbed onto a porous


Table 3. Absorbance at λmax = 488 nm of 30 layer pairs PAH/DPPS on Isopore and Trespaphan membranes adsorbed undervarious conditions.

Sample No. Conditions Support Absorbance

1 PAH (mw. 9800) dipping time 30 min, no drying after each adsorption step Isopore 0.3259

2 PAH (m.w. 57500), the other conditions as sample 1 Isopore 0.2495

3 Dipping time 10 min, the other conditions as sample 1 Isopore 0.2747

4 Drying after each dipping step, the other conditions as sample 1 Isopore 0.2759

5 As sample 1, no plasma treatment Trespaphan 0.0453

6 As sample 1, plasma treatment for 10 min Trespaphan 0.0665

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Fig. 3. Plot of absorbance at λmax = 488 nm vs. numberof layer pairs PAH/DPPS on (a) Isopore and (b) Trespa-phan membranes adsorbed under various preparation condi-tions (from [30]).

support by dipping the support in a solution of pH 1.7to 2.0 for about 30 min. After removal and careful wash-ing, the substrate was directly dipped into the next solu-tion without drying in between. If not especially quoted,the membranes used in our experiments were all preparedaccording to this recipe.

3 Gas permeation

Measurements of gas permeation were mainly carried outto demonstrate the gradual sealing of the pores in the sup-

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Fig. 4. Argon flow rates through PAN/PET and Isopore mem-branes coated with different numbers of PAH/PSS layer pairs(from [23]).

porting membrane while the polyelectrolyte layers wereadsorbed. Moreover, the permeation rates were used tocalculate ideal separation factors α, which are defined asthe ratio of the flow rates of the respective gases. Exper-imental details of the gas permeation measurements werepreviously reported [23]. In Figure 4, the flow rates of ar-gon are plotted versus the number of adsorbed layer pairsof PAH/PSS. Using PAN/PET as support, about 20 layerpairs are sufficient to get a flow reduction of 93%, while for60 layer pairs a flow reduction of 99.9% is obtained. WithIsopore having slightly larger pores on average, 30 layerpairs are necessary to get a flow reduction of 93%. If thebolaamphiphile DPPS is used instead of PSS, the flow re-duction after adsorption of 30 layer pairs is only 45%, andfor 60 layer pairs it is 92.4% (not shown in Fig. 4). Thisclearly shows that a bolaamphiphile is less suited for theconstruction of a separating membrane probably becauseof a higher defect concentration within the adsorbed layer.In a previous paper [23], it was shown that the flow ratesof argon, nitrogen and oxygen are nearly the same withinthe limits of error. Flow rates of CO2 were up to 2.4-timeshigher than for N2 or O2. The reason must be a higher sol-ubility of CO2 in the polar separation layer. The data alsoindicate that the PAN/PET and Isopore membranes withrather homogeneous surface structure are better suited assupports than the highly porous Celgard membrane.


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The gas flow measurements show that multiple adsorp-tion of polyelectrolyte pairs allows to seal the pores of thesupporting membrane so that the gas flow can be reducedby up to 99.9%. Therefore, it is unlikely that the poorseparation originates from defects in the separating mem-brane, but is a property of the membrane caused by onlysmall differences in diffusion and solubility of the indi-vidual gases. Poor gas separation was also found by an-other research group when gas permeation through poly-electrolyte multilayer membranes deposited on a poroussubstructure was studied [20].

4 Pervaporation

The polyelectrolyte multilayer membranes were also stud-ied on their use for separation of liquid mixtures. Thesemeasurements were carried out under pervaporation con-ditions for the following reasons. Firstly, solution-castmembranes of polyelectrolyte complexes had alreadly pre-viously been proven to be well-suited for separation ofethanol/water mixtures under pervaporation conditions[34–37]. Secondly, it has been shown in the previous chap-ter that the polyelectrolyte adsorption strongly reducesthe flow rates so that a sufficiently low flux required forpervaporation experiments can be easily achieved. Perva-poration was also chosen because it represents a promis-ing technique for the industrial separation of liquid mix-tures [19].

The experiments were carried out using the apparatusschematically represented in Figure 5. The alcohol/watermixture (“feed solution”) was circulated from a 2 l reser-voir to the pervaporation cell at a flow rate of 250 ml/min.The cell consists of a feed half cell and a permeate halfcell with the membrane mounted in between. It was keptat a constant temperature of 58.5 ◦C. The permeate halfcell was evacuated to a pressure ≤0.2 mbar. The perme-ate was collected in the first condenser cooled with liquid

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Fig. 6. Effect of the number of adsorbed layer pairs ofPAH/PSS on flux and water content in the permeate forsamples with and without thermal treatment at 120 ◦C for1 h. Support: PAN/PET; feed solution: ethanol/water mixturewith 6.2% (w/w) water (from [24,29]).

nitrogen. After pervaporation for 2 to 3 hours, the col-lected mass was weighed, and the flux was calculated fromthe mass/time data. The water content in the permeatewas analysed using gas chromatography or refractive indexmeasurements. From the result of the analysis the separa-tion factor α′ was calculated using the formula shown inFigure 5.

In Figure 6, flux and water content of the permeateare plotted as a function of the number of adsorbed layerpairs of polyallylamine/polystyrenesulfonate (PAH/PSS).Three main results were found: firstly, the water content inthe permeate was always higher than in the feed solution,which only contained 6.2% (w/w). This indicates prefer-ential transport of water across the membrane. Secondly,annealing of the membrane prior to pervaporation led toa decrease of the flux and an improvement of the separa-tion capability. Similar effects are known from other mem-branes and were discussed already previously [38]. Thirdly,the increase of the number of adsorbed layer pairs led toa decrease of the flux and an increase of the water contentin the permeate. Flux J and number n of deposited layerpairs showed the inverse relation J ∼ 1/(n − 15), whichindicates that the adsorption of the first fifteen layer pairsmainly serves for the sealing of the pores of the supportingmembrane [39]. The data also show that even after anneal-ing of the composite membrane, the water content in thepermeate did not exceed 70% and the separation factorα′ was not higher than 70. Therefore we tried to improvethe separation capability by varying the polyelectrolytestructure and by optimising the processing and operatingparameters.

Let us first concentrate on the effect of the polyelec-trolyte structure. In Figure 7, a very idealised model ofthe polyelectrolyte multilayer membrane is shown. Ac-cording to this model, the membrane exhibits a phys-ical network structure, the ion pairs representing thecross-linking units. From this model one can derive that


Table 4. Flux and water content in the permeate for PAH/PSS membranes prepared from polyelectrolyte solutions of differentpH. Feed solution: 6.2% (w/w) water; membrane thickness: 60 layer pairs.

pH H+ conc. [mol l−1] Flux [g m−2 h−1] Water content in permeate [%]

2.44 0.00362 3860 13.0

2.10 0.00794 822 38.2

1.79 0.0162 1270 28.5

Fig. 7. Idealised structure of the polyelectrolyte complexmembrane with the polyelectrolytes being completely ionised(from [29]). For simplification, the representation does not takeinto account the overlap of individual layers.

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Fig. 8. Dependence of flux and water content in perme-ate on the charge density ρc of the polyelectrolyte complex.Support: PAN/PET; feed solution: 6.2% (w/w) water; sep-arating membranes always consist of 60 layer pairs of poly-electrolyte complex. PAH: poly(allylamine); P4VP: poly(4-vinylpyridine); CHI: chitosan; PVAM: poly(vinylamine);PDADMAC: poly(diallyldimethyl ammonium bromide); PEI:branched poly(ethyleneimine); PSS: poly(styrenesulfonate);DEX: dextrane; PVS: poly(vinylsulfate) (from [28,29]).

cross-linking density and mash size are controlled by thecharge density ρc of the polyelectrolyte complex. ρc canbe expressed as the number ratio of ion pairs (= 1) andcarbon atoms per repeat unit of the polyelectrolyte com-plex.

In Figure 8, flux and separation behaviour of variouspolyelectrolyte multilayer membranes are plotted versus

the ρc parameter. The flux was found to be inversely pro-portional to ρc. The water content in the permeate in-creased with ρc. Obviously a high separation is obtainedat the easiest, if the charge density of the network ishigh. The clarity of this correlation is surprising, espe-cially because in the definition of ρc any contribution fromspecific structural elements such as the flexibility of themain chain and the side groups, presence of aromatic oraliphatic units, and different polar groups in the polyelec-trolytes has been neglected. The most probable reason isthat a high charge density of the polyelectrolytes favoursthe formation of small, highly polar mashes, which arewell permeable for the highly polar water molecules, butless permeable for the less polar, less hydrophilic ethanolmolecules.

Besides the chemical structure of the polyelectrolytes,the pH of the polyelectrolyte solutions used for membranepreparation was also varied. The pH determines the degreeof ionisation of the polar groups. For example, primaryamino proups are completely protonated at low pH val-ues. At moderate pH, they are only partially protonatedso that considerably more polyelectrolyte material has tobe adsorbed in order to neutralise the surface charges ofthe membrane. As a consequence, the thickness of thepolyelectrolyte multilayer is increased [31,40], while thecharge density within the multilayer is decreased. In Ta-ble 4, flux and water content in the permeate are listedfor PAH/PSS membranes prepared at different pH val-ues. It is shown that at pH 2.1 the water content in thepermeate becomes maximum and the flux becomes mini-mum. At that pH, most of the polar groups of PAH andPSS are present in an ionised state so that upon the saltformation a dense network structure is formed. At higherpH, the amino groups are increasingly deprotonated andthe charge density of the membrane is decreased. As aconsequence, the flux is increased and the separation getsworse. At very low pH, the sulfonate groups are proto-nated so that the charge density of the membrane is againdecreased and the separation gets worse, too. As recentlyshown, the most suitable pH for preparation of a densenetwork exhibiting maximum separation corresponds withthe mean of the pKa values of the two oppositely chargedpolyelectrolytes [28].

Besides the pH, the addition of salt to the polyelec-trolyte solution is another parameter influencing the sep-aration behaviour of the membrane. The addition of saltstrongly reduces the mutual electrostatic repulsion of thepolymer chains. Consequently, the polymer coils are be-coming denser and denser and are rather adsorbed as coilsthan in a flat conformation. As a consequence, the thick-ness of individual polyelectrolyte layers is increased and


Table 5. Effect of salt addition to the polyelectrolyte solution on flux and water content in permeate for PVAM/PVS andPAH/PSS separating membranes (thickness: 60 layer pairs). Concentration of NaCl in polyelectrolyte solution: 1 mol l−1, feedsolution: 6.2% (w/w) water, pervaporation temperature 58.5 ◦C.

Separation layer Flux [g m−2 h−1] Water content in permeate [% (w/w)]

without NaCl with NaCl Without NaCl with NaCl

PVAM/PVS 431 98.5 70.0 94.9

PAH/PSS 1270 790 28.5 35.0

Table 6. Influence of pervaporation temperature on flux andseparation factor α′. Samples: 60 layer pairs PAH/PSS onPAN/PET support, after annealing at 90 ◦C for 1 h. Feedsolution: ethanol/water mixture containing 6.2% (w/w) water.

T [◦C] Flux [g m−2 h−1] α′

27.5 33.3 4.8

37.5 59.3 15.6

48.2 134.6 42.2

58.5 229.1 65.5

the salt addition can be used to fine tune the overall mem-brane thickness [31]. The influence of salt addition on theseparation behaviour was studied for the two membraneslisted in Table 5. For membrane preparation, polyelec-trolyte solutions containing additional NaCl in a concen-tration of 1 mol l−1 were used. The resulting PAH/PSSmembranes exhibited a flux of 790 g m−2 h−1, i.e. about40% lower than membranes prepared from NaCl-free so-lution, while the flux of PVAM/PVS membranes wasstrongly reduced from 431 to 98.5 g m−2 h−1. The decreaseof the flux is presumably a consequence of the increase ofthe membrane thickness, especially because flux and mem-brane thickness are known to be inversely proportional.As also shown in Table 5, the water content in the perme-ate is only moderately increased for the two membranes.The reason might be that the adsorption as coils rendersthe interpenetration of the oppositely charged polyelec-trolytes and the neutralisation of the charges difficult.Consequently, the overall cross-linking density might belower than for a membrane prepared from salt-free solu-tion. The balance of the two opposing effects − increase ofthickness and decrease of cross-linking density − can leadto only weak effects on the separation capability, whichare difficult to predict.

We now describe influences of operating conditionson flux and separation capability of the composite mem-branes. Important parameters are pervaporation temper-ature and water content of the ethanol/water mixture. InTable 6, the effect of the pervaporation temperature isshown. If the pervaporation temperature is raised from27.5 to 58.5 ◦C, we do not only observe the expected in-crease of the flux, but also an increase of the separationfactor α′ by about a factor of 10. This is quite unusual andindicates that the flux increase is mainly due to a rapidlygrowing water permeation at elevated temperature. Thismust be a consequence of the fact that the interactions

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Fig. 9. Dependence of water content in permeate (a) andflux (b) on water content in feed solution for PVAM/PVS,PEI/PVS and PAH/PSS membranes. Samples: 60 layer pairson PAN/PET. PAH/PSS and PEI/PVS membranes were an-nealed at 90 ◦C for 1 h prior to pervaporation (from [29]).

of the polyelectrolyte complex with water increase morestrongly than with ethanol when the temperature is raised.

The influence of a variation of the water contentof the feed solution on flux and separation factor isshown in Figure 9. It can be seen that the sepa-ration capability of poly(vinylamine)/poly(vinylsulfate)(PVAM/PVS) and poly(ethyleneimine)/PVS (PEI/PVS)separating membranes is superior to the previously de-scribed PAH/PSS membrane [25]. The PVAM/PVS andPEI/PVS membranes are considerably more stable thanthe PAH/PSS membrane, which is already decomposed


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Fig. 10. Dependence of water content in permeate (a) andof flux (b) on water content in feed solution for various alco-hol/water mixtures. Samples: 60 layer pairs of PVAM/PVS onPAN/PET.

at a water concentration in the feed solution above 20%.The PVAM/PVS membrane is able to enrich water at anycomposition of the feed solution. For example, at 20% wa-ter in the feed, a separation factor of 700 is reached, whilethe flux is about 500 g m−2 h−1, a fairly good value. Athigher water content in the feed, still higher flux valuesare found. Similar properties are found for the PEI/PVSmembrane except that this membrane can only be used,if the water content in the feed is lower than 50%.

In Figure 10, pervaporation of various alcohol/watermixtures through the PVAM/PVS membrane is charac-terised. It is shown that the separation strongly increaseswith decreasing hydrophilicity of the alcohol. Mixturesof t-butanol/water are more efficiently separated thanpropanol/water or ethanol/water mixtures, because thetransport of the rather hydrophobic t-butanol through thehighly polar membrane is strongly hindered. Except forvery small water concentration in the feed, i.e. less than5%, the permeate always contains more than 99.9 % wa-ter. For example, the separation factor of the 97/3 mix-

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ture of t-butanol/water is about 3× 104. Simultaneously,the flux of ethanol/water mixtures is much lower thanof propanol/water or butanol/water mixtures. The reasonis that the hydrogen bonding between water and alcoholmolecules becomes progressively weaker so that the com-pounds can be separated more easily. For example, the80/20 mixture of 2-propanol/water exhibits a high fluxof 1.5 kg m−2 h−1, while for an ethanol/water mixture ofthe same composition the flux is only 0.5 kg m−2 h−1. Fluxand separation are similar [34] or clearly superior [35,36]to the values of composite membranes with solution-castpolyelectrolyte layers reported previously.

5 Ion separation

If two layers of oppositely charged polyelectrolytes arecast on top of each other, a so-called bipolar membraneis obtained. The ion permeation through bipolar polyelec-trolyte membranes cast from solution was already investi-gated several years ago [41,42]. It was found that divalentcations receive a much stronger repulsive force from thepositively charged layers than monovalent ones, and thatthe same holds for the divalent anions, which are rejectedby the negatively charged layers. Consequently, the bipo-lar membranes represent barriers for divalent ions in gen-eral and can be used for ion separation [41,42]. The self-assembled polyelectrolyte multilayers consist of cationicand anionic polyelectrolytes in alternating fashion andtherefore can be looked at as a multi-bipolar membrane ona molecular level, which likewise should be able to sepa-rate mono- and divalent ions. In Figure 11, a model of theion rejection by the self-assembled multilayer membraneis represented. The model implies that the ion separationbecomes progressively more effective, if the number of ad-sorbed polyelectrolyte layers is increased.

In order to study the ion permeation through the poly-electrolyte multilayers, the cell shown in Figure 12 wasused. The cell consists of two chambers separated by the


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Fig. 13. Effect of the thickness of the separating membraneon permeation rates PR of NaCl and MgCl2. Samples: differentnumbers of PAH/PSS layer pairs on PAN/PET (from [26,29]).

membrane. One of the chambers contains the aqueous saltsolution (concentration: 0.1 mol l−1) and the other onepure water. From the measurement of the initial increasein conductivity (∆Λ/∆t) in the water chamber, the per-meation rate PR was calculated using the equation shownin Figure 12. However, the PR values do not represent ab-solute values, because a possible contribution to (∆Λ/∆t)originating from the volume flow of water in the oppositedirection (osmotic flow) has not been corrected.

In Figure 13, the permeation rates of NaCl and MgCl2are plotted versus the thickness of the membrane. Withincreasing thickness, the permeation rates of the two saltsare decreased, but for MgCl2 the decrease is much strongerthan for NaCl. Therefore, the separation factor α′′ (de-fined as the ratio of the PR values of the respective salts)strongly increases with the membrane thickness. For ex-ample, a sample containing 60 layer pairs of PAH/PSSexhibits α′′ values of 15.1 for NaCl/MgCl2, and of 9.9 forNaCl/Na2SO4.

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Fig. 14. Effect of charge density ρc of polyelectrolyte mul-tilayer on permeation rate PR of (a) NaCl and (b) MgCl2.Samples: 60 layer pairs of polycation/polyanion on PAN/PET(from [26,29]).

The ion permeation is strongly affected by the struc-ture of the polyelectrolytes used for membrane prepara-tion. In Figure 14, the permeation rates of NaCl andMgCl2 are plotted as a function of the charge density ρc ofthe polyelectrolyte membrane. The higher the charge den-sity, the lower is the permeation rate, and the differencebetween the salts with mono- and divalent ions becomesprogressively larger with increasing ρc. The plots also in-dicate that membranes with PAH as the cationic polyelec-trolyte always exhibit lower permeation rates than mem-branes containing other polycations. The origin for theespecial behaviour of PAH is not yet clear.

Besides the polyelectrolyte structure, the processingparameters such as salt content and pH of the polyelec-trolyte solution have a strong influence on the ion perme-ation, too. The pH controls the ionisation of the polyelec-trolytes, which in its turn influences the charge density ofthe membrane, as discussed above. As recently shown, alow pH value favours the formation of a dense, less per-meable membrane of high charge density, which exhibits


Table 7. Permeation rates PR of NaCl, MgCl2 and Na2SO4 and selectivities α′′ for PAH/PSS and PVAM/PVS membranesprepared from salt-free and salt-containing polyelectrolyte solution (thickness: 60 layer pairs).

Membrane Addition of salt PR (NaCl) PR (MgCl2) PR (Na2SO4) α′′ (NaCl/MgCl2) α′′ (NaCl/Na2SO4)

[10−6 cm s−1] [10−6 cm s−1] [10−6 cm s−1]

PAH/PSS + 22.5 0.2 0.5 112.5 45.0

PAH/PSS − 44.4 2.9 4.5 15.1 9.9

PVAM/PVS + 3.1 0.07 0.3 44.3 10.3

PVAM/PVS − 7.3 0.8 3.8 9.1 1.9

an improved rejection of divalent ions. Upon salt addi-tion to the polyelectrolyte solution, the coil dimensionsof the polyelectrolytes are reduced. As a consequence, thepolyelectrolytes are no longer adsorbed in a flat conforma-tion, but as coils, so that the individual layers are muchthicker [31] and the permeation rates are decreased. Sinceespecially the PR values of the salts with divalent ionsare decreased, considerably higher separation factors areobtained (Tab. 7).

6 Summary and conclusions

Our study indicates that alternating electrostatic adsorp-tion of cationic and anionic polyelectrolytes on poroussupports is a versatile method to prepare compositemembranes with ultrathin, pore-free separation layer. Bycareful choice of the polyelectrolytes and the supportingmembranes, and by optimisation of the processing and op-erating parameters, composite membranes with excellentseparation capability for mixtures of polar liquids and ionsof different charge density can be tailored.

It is also shown that the transport of small moleculesand ions through the membrane is strongly a function ofthe density of the network formed upon the complex for-mation between the oppositely charged polyelectrolytes,a high charge density of the polyelectrolytes favouring ahigh separation capability and a low flux. The relation isfound to be so stringent that it can be used to predict thesuitability of any polyelectrolyte multilayer membrane forthe separating processes discussed above.

If highly charged polyelectrolytes are adsorbed, a closenetwork with a high density of ion pairs is formed, whichfavours the transport of highly polar liquids such as wa-ter, retards the transport of more hydrophobic alcohols,and rejects the divalent ions of high charge density morestrongly than the monovalent ions. The membranes arealso permeable for gases but a good separation could notbe observed. Improved separation might be found, if mix-tures containing non-polar gases such as N2 or O2 andhighly polar gases such as SO2 or NOx are investigated.In further experiments, the separation of organic liquidsof different hydrophilicity will be studied, as well as theion permeation under reverse osmosis conditions.

The authors are grateful to Dr. H. Scholz, Sulzer Chemtech,Neunkirchen, Dr. G. Ellinghorst, FhG Bremen, for providingthe PAN/PET membranes and for helpful discussions, and

to Dr. Mahr (BASF, Ludwigshafen) for providing polyviny-lamine. Financial support from the Deutsche Forschungsge-meinschaft (Ti 219/3-1, 3-2 and 3-3) is gratefully acknowl-edged.

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ultrathin self-assembled polyelectrolyte multilayer membranes

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