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 + /Mg 2+ up to 112.5 and for Cl -/SO4 2up 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.

Measurements of ion transport and water flux across ultrathin multilayered membranes of polyelectrolytes were carried out under nanofiltration and reverse osmosis conditions. The polyelectrolyte membranes were prepared upon alternating electrostatic layer-by-layer adsorption of polyvinylamine (PVA) and polyvinyl sulfate (PVS) on porous supports. The pressure-driven transport of aqueous electrolyte solutions containing NaCl, Na2SO4, MgCl2, and MgSO4 in 1 and 10 mM concentration was investigated. For MgCl2 and MgSO4, a complete rejection was observed independently from the concentration of the feed solution and the pressure applied. For NaCl and Na 2SO4, the rejections were 84 and 96% at 5 bar, and 93.5 and 98.5% at 40 bar, respectively. The hydraulic permeability of the composite membrane was 113.7 mL/(m 2 h bar). It was only little affected by the presence of salt. At low and moderate pressure the membranes are suitable for water softening applications, while at pressures of 40 bar or higher they can be used for water desalination. Effects of the stirring of the feed solution on the membrane characteristics are also discussed.

A series of amphiphilic diacetylene monocarbonic acids was synthesized, and their ability to form monolayers at the air–water interphase was investigated. Acids with total number of C atoms ≥20 and mp >45°C form surface states suitable to be used for buildup of multilayers by the Langmuir–Blodgett technique. Using the LB technique, multilayers of defined thickness were built up on quartz substrates. The multilayers were polymerized by exposure to a UV light source according to the mechanism of solid‐state polymerization of diacetylenes without destruction of the layer structure and with retention of the packing in the individual layers. Thus well‐defined polymer multilayers were obtained. The monomer and polymer multilayers were characterized by UV spectra, x‐ray small‐angle diffraction, and interference microscopic and electron microscopic techniques. The polyconjugated backbones of the polymer chains are all stretched out in the plane of the substrate but not over macroscopic dimensions because the multilayers have a crystalline domain structure. The polymer multilayer films are quite stable under ambient conditions.

Ultrathin films of metal hexacyanometalates were prepared upon multiple sequential adsorption of metal cations M m + (Fe3+, Fe2+, Co2+, and Ni2+) and hexacyanometalate anions [M(CN)6] n - (Fe(CN)6 3-, Fe(CN)6 4-, and Co(CN)6 3-) on solid supports. The layer-by-layer deposition led to the formation of films of the metal complex salts with monolayer precision. The films were characterized using UV and IR spectroscopic methods and cyclic voltammetry. The alternating adsorption of Fe3+ and Fe(CN)6 4- ions led to dense and defect-free films of Prussian Blue, which were useful as membranes for ion separation. The porous, zeolitic structure of Prussian Blue was permeable for ions with a small Stokes radius such as Cs+, K+, and Cl-, whereas large hydrated ions such as Na+, Li+, Mg2+, or SO4 2- were blocked. The effect of ion sieving increased with the thickness of the membrane. After a hundred dipping cycles, high separation factors α(CsCl/NaCl) and α(KCl/NaCl) of 6.5 and 6.2, respectively, were found. Corresponding membranes of cobalt and nickel hexacyanoferrate were also useful for ion separation, but the α-values were lower. Possible reasons for the differences in selectivity are discussed.

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