INTRODUCTION
Polyelectrolyte multilayers prepared upon alternate electrostatic adsorption of positively and negatively charged polymers have recently shown promise as membranes and catalysts. Recent progress in the preparation of membranes for gas separation, separation of liquid mixtures under pervaporation conditions, ion separation under dialysis conditions, salt transport under nanofiltration (NF) and reverse osmosis conditions, separation of enantio-mers, and others have been reported. The purpose of this article is to summarize recent activities in the field of membrane[30-50] and catalyst applications1-51-78-1 of polyelectrolyte multilayer assemblies. It describes the use of polyelectrolyte multilayers as carriers for catalysts and biocatalysts, and then covers examples of catalyzed reactions such as enzyme reactions, photocatalytic reactions, and metal-catalyzed processes.
BACKGROUND OF MULTILAYER FILMS
About a decade ago, Decher and coworkers1-1-4-1 reported on a new method to prepare ultrathin polymer films, based on the alternate electrostatic adsorption of cationic and anionic polyelectrolytes on solid substructures. Meanwhile, the layer-by-layer deposition technique developed into a standard method for preparation of polymer films with controlled structure and thickness in the nanometer range. Numerous studies appeared, which were either concerned with fundamental aspects of film growth, structure, and morphology,[5-22] or with the preparation and characterization of functional polyelectrolyte multilayers with photo- and electroactive properties applicable in micro- and optoelectronic devices. The state of the art has recently been reviewed in books[23,24] and articles.[25-29]
Probably one of the most interesting properties of polyelectrolyte multilayer assemblies is their selectivity in the transport of gases,[30-34] solvent mixtures,[35-42] ions,[43-47] and enantiomers.[48] In several studies, the use of polyelectrolyte multilayers as membranes was demonstrated, e.g., for dehydration of organic solvents under pervaporation conditions,1-35-42-1 for water softening and desalination under nanofiltration[49,50] and reverse osmo-sis[49] conditions, respectively. Furthermore, the incorporation of biocatalysts[51-68] and inorganic catalysts[69-78] opened a new area for the formation of membrane reactors and catalytic membranes. Early studies demonstrate the utility of the polyelectrolyte multilayers as reactors and catalysts.
PREPARATION OF MEMBRANES
Soon after the method of layer-by-layer assembly of polyelectrolytes was reported, ultrathin polyelectrolyte multilayers were studied as permselective membranes. In a typical process of membrane preparation, a negatively charged porous or nonporous supporting membrane is dipped into a dilute aqueous solution of a positively charged polyelectrolyte so that the polymer is adsorbed at the substrate surface as a molecularly thin film and the surface charge is reverted (Fig. 1). After careful washing, the coated substrate is dipped into the aqueous solution of a negatively charged polyelectrolyte so that this polymer is adsorbed on top of the previous one and the surface charge is reverted again. By repeating the adsorption steps several times, a polyelectrolyte multilayer is obtained, whose thickness is adjustable between a few nanometers and about half a micrometer by varying the number of dipping cycles. Scanning electron micrographs[44,79] indicate that the polyelectrolytes do not enter and block the pores of the support, but a smooth coating of the substructure with a dense and defect-free polyelectrolyte skin layer is obtained. Previous studies of materials transport across polyelectrolyte multilayer membranes have recently been reviewed.[79-87] Here an updated summary of the research activities is presented.
Fig. 1 Scheme of layer-by-layer assembly of polyelectrolytes on activated porous supporting membrane. The separating membrane is obtained upon multiple repetition of steps A and B. In reality, pore diameters are 20-200 nm, while polymer chains are less ordered and partially overlapping.
STUDIES OF MATERIALS TRANSPORT ACROSS POLYELECTROLYTE MULTILAYERS
Gas Permeation
At first, studies on the transport behavior across poly-electrolyte multilayers were concerned with gas perme-ation.[30-33] Membranes were prepared from porous supports [Celgard,[30-32] Isopore,[32] and polyacrylonitrite/ polyethylene terephthalate (PAN/PET)[32]], as well as nonporous supports (silicone[30] polymethylpentene,[31] and Nafion,[33]). Poly(allylamine hydrochloride) (PAH), and poly(styrenesulfonate sodium salt) (PSS) were mostly used as the polyelectrolytes. Up to a hundred layer pairs were deposited, which caused a reduction of the membrane permeability by more than a factor of 103. Theoretical selectivities in gas permeation were generally low. For CO2/N2 separation, a selectivity of 2.4 was re-ported,[32] for oxygen/nitrogen permeation the highest selectivity was 3.5.[33] This value was obtained by using a polyelectrolyte couple of PSS and poly(N-octadecyl-2-ethynylpyridinium bromide), a substituted polyacetylene derivative. The selectivity was ascribed to the high affinity of the unsaturated C=C bonds to oxygen.
Only recently, better results were obtained, when PAH and polyamic acids were alternately deposited followed by heat-induced imidization at 250°C for 2 hr.[34] The resulting polyimide multilayer membranes showed O2/N2 selectivities of up to 6.9 and CO2/CH4 selectivities of up to 68.
Pervaporation of Alcohol/Water Mixtures
Polyelectrolyte multilayer assemblies were also tested as pervaporation membranes. An initial study was reported by van Ackern et al.[32] Using PAH/PSS separating membranes on a PAN/PET support, they tried to separate ethanol/water and benzene/cyclohexane mixtures. The separation was only modest, but after a careful adjustment of several parameters the ethanol-water separation could be considerably improved.[35-40] It was found that membranes with high efficiency in alcohol/water pervaporation separation require the following.
• The use of polyelectrolytes of high charge density such as polyethyleneimine (PEI),[37,38] polyvinylamine (PVA),[38-40] polyvinylsulfate (PVS),[38-40] polyacryl-ic acid (PAA),[40] or polyvinylsulfonate (PVSu).[40] These polyelectrolytes form complexes with small pore size and large hydrophilicity, which favors the transport of water across the membrane.
• The use of polyelectrolyte solutions of either low pH (1.7) with a high concentration of supporting electrolyte (e.g., 1 M NaCl),[38,40] or neutral pH without supporting electrolyte for preparation of the mem-branes.[40] In the former case, very thick polyelectrolyte layers are adsorbed,[2,3] a thick membrane with low flux and good separation is obtained. In the latter case, very thin and smooth layers are adsorbed.[16] A thin but very dense membrane with good separation efficiency is obtained.
• Deposition of a large number of layer pairs (approximately 60) to avoid voids and defects in the mem-brane.[35,40]
• Annealing[35-38] of the membrane for 1 hr at 90°C to smoothen the surface and to heal defects in the poly-electrolyte multilayer, which was especially useful in case of PEI/PVS membranes.[37]
• A pervaporation temperature of about 60°C or higher.
The use of polyelectrolytes of high charge density[37-40,42] is especially crucial for obtaining high selectivities. This is demonstrated in Fig. 2, which shows a plot of the total flux and water content in the permeate vs. the charge density pc of the polyelectrolyte complex. It can be seen that the lowest flux and the highest water content in the permeate are obtained for the membranes made from polyelectrolytes of the highest pc values. The separation behavior was explained in terms of the nanopores model[86,87] shown in Fig. 3. The alternately adsorbed polycations and polyanions form a network structure, in which the polymer-bound ion pairs represent the cross-linking units. For polyelectrolytes of high charge density, a much closer network is obtained as for poly-electrolytes of low charge density. Because each pore of the network contains an ion pair, a network with narrow pores will be more hydrophilic and preferentially permeable for the small and polar water molecules, while a network with larger pore size will be less hydrophilic and more permeable for the bigger, more hydrophobic alcohol molecules. The correlation between pc and the membrane selectivity holds for most of the polyelectrolytes and can even be used to predict the separation capability of unknown polyelectrolyte couples. This is surprising, because specific structural elements, such as the presence of aromatic rings, chain branching, or nature of the ionic groups, have not been taken into account yet. More detailed studies on structure/property relationships were undertaken by Toutianoush and Tieke,[40] when they studied influences of the nature of the anionic polyelectrolyte groups on the separation behavior. For this purpose, three multilayer membranes with same high charge density, but different anionic substituent groups were investigated. The poly-cation was always PVA, the polyanion was either PVSu, PVS, or PAA. With increasing acidity of the polyanion, i.e., from PAA to PVSu, the membranes became increasingly hydrophilic. Therefore PVA/PVSu membranes were especially suited to remove small amounts of water from organic solvents, while the less hydrophilic PVA/PAA membranes showed the best performance at high water content of the mixture.
Fig. 2 Separation characteristics of polyelectrolyte multilayer membranes (60 layer pairs) as a function of the charge density p of the polyelectrolyte complex. Feed solution, ethanol/water (93.8:6.2) (w/w); pervaporation temperature, 58.5°C. PAH: poly(allylamine); P4VP: poly(4-vinylpyridine); CHI: chitosan; PVA: poly(vinylamine); PDADMA: poly(diallyldimethylammo-nium bromide); PEI: branched poly(ethyleneimine); PSS: poly(styrenesulfonate); DEX: dextrane; PVS: poly(vinylsulfate); PAA: poly(acrylic acid).
Using the PVA/PVSu membrane, a series of alcohol/ water mixtures with alcohols of different hydrophobicity were studied.[40] As can be derived from Fig. 4, the flux and separation increased, if the hydrophilicity of the alcohols decreased. For a t-butanol/water (9:1) mixture, for example, a total flux J of about 2.18 kg- 2 h- \ a water content in the permeate of 99.9%, a separation factor a of about 9000, and a separation efficiency (pervaporation separation index, PSI) a J of 1.96 x 107 were found, the pervaporation temperature being 58.5°C. The PVA/PVS membrane showed similar behavior.[39] The excellent separation is primarily a result of the lower solubility of the more hydrophobic alcohols in the multilayer membrane, and secondly, a weaker hydrogen bonding between the more hydrophobic alcohol and water molecules.
Fig. 3 Simplified structure model of polyelectrolyte multilayer membrane of high (a) and low (b) charge density p. Note that the nanopores become larger and less hydrophilic if p decreases. The model does not take into account possible chain interdigitation and incomplete ionization.
Fig. 4 Separation characteristics of a PVA/PVSu membrane (60 bilayers) for various alcohol/water feed mixtures. Plot of water content in permeate (a), total flux (b), separation factor a (c), and separation efficiency (d) vs. water content in feed mixture. Pervaporation temperature, 58.5°C.
Ion Permeation
From earlier studies, it is known that the sequential solution casting of a cationic and an anionic polyelectro-lyte leads to a so-called bipolar membrane. In such a membrane, permeating ions receive strong repulsive forces from the equally charged parts (Donnan rejection) and attractive forces from the oppositely charged parts (Donnan attraction) of the membrane. For divalent permeating ions, the interactions are much stronger than for mono-valent ones and thus, a high selectivity in ion transport occurs. Polyelectrolyte multilayers resemble in their architecture the bipolar membranes except that the multilayer structure causes a multibipolar character of the membrane. A corresponding structure model[43] is shown in Fig. 5 and indicates that divalent ions will receive much stronger repulsive forces than monovalent ones. The model agrees with experimental results showing that the permeation rate of sodium chloride across a PAH/PSS membrane is at least 15 times higher than for magnesium chloride.[36] After careful adjustment of several parameters, Krasemann and Tieke[43] reported theoretical selectivities a(Na+/Mg2+) of up to 112.5 and a(Cl~/SO2 ") of up to 45. The PR values were dependent on the number of deposited layers, the charge density of the polyelec-trolytes, pH, and ionic strength of the polyelectrolyte solutions used for membrane preparation. The high transport selectivity was confirmed by the study of Harris et al.[44] on anion permeation. The authors used porous aluminum oxide as support, the pore size in the skin layer was about 20 nm. The inorganic substrate was coated with several PAH/PSS layers, the a(Cl-/SO2-) and a(Cl-/ Fe(CN)6-) values being 7 and 310, respectively. For PAH/PAA membranes, the selectivity was comparable, but the anion flux was three times smaller.
Fig. 5 Rejection model of the multibipolar polyelectrolyte multilayer membrane.
In a more recent study, Toutianoush and Tieke[45] investigated the ion transport across highly charged PVA/ PVS membranes. In agreement with the transport model based on Donnan exclusion/inclusion, it could be shown that the permeation rates of alkali and earth alkaline metal cations increase from Li to K, and from Mg to Ba, that is in the direction of decreasing charge density of the naked ions (Fig. 6). In the study of the permeation of NaCl, MgCl2, Na2SO4, and MgSO4, it was found that PR values increased in the sequence 2.2-electrolyte < 2.1-electro-lyte ffi 1.2-electrolyte < 1.1-electrolyte, again in the direction of decreasing charge density of the ions. Furthermore, it was found that the influence of the surface charge on the permeation rates was negligible for thick membranes consisting of more than 50 bilayers. The effect of salt concentration on PR was also studied. Up to a concentration of 0.3 mol L- \ the PR values of NaCl or MgCl2 changed only slightly, and the selectivity of the membrane was essentially maintained. However, at higher electrolyte concentration, a significant increase in ion flux was found and the selectivity dropped strongly. The effect can be explained by considering the equilibrium (1) of polyelectrolyte complex formation[46,80,83]
with P+ and P- being charged segments of the cationic and anionic polyelectrolytes. At high concentration of permeating M+ and X- ions, the equilibrium begins to shift to the left side. Dissociation of the polymer-bound P+P- pairs sets in, the pore size of the membrane increases and the flow of the electrolyte solution becomes progressively higher and less selective.
