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.

Structure, morphology, and elemental composition as well as the size-selectivity of the ion transport behavior of ultrathin membranes of iron(III) hexacyanoferrate(II) (FeIIIHCFII), iron(II) hexacyanoferrate(III) (FeIIHCFIII), cobalt(II) hexacyanoferrate(III) (CoIIHCFIII), and nickel(II) hexacyanoferrate(III) (NiIIHCFIII) are described. The membranes were prepared upon multiple sequential adsorption of metal cations and hexacyanometalate anions on porous polymer supports. Scanning electron and scanning force microscopy indicate that the membranes of the complex salts consist of a multitude of small, densely packed particles with diameter in the 10−100 nm range. Energy-dispersive X-ray analysis indicates that the iron hexacyanoferrate (Prussian blue) membranes consist of the potassium-rich, so-called “soluble” modification, KFe[Fe(CN)6], while the membranes of the analogous complex salts consist of a mixture of the potassium-rich and potassium-free modification. The porous, zeolitic structure of the inorganic complex salts 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. Ion separation became progressively more effective, if the number of complex layers increased. The highest separation factors α(CsCl/NaCl) and α(KCl/NaCl) of 7.7 and 5.9, respectively, were found for the FeIIIHCFII membrane subjected to a hundred dipping cycles. Membranes of iron(II), cobalt(II), and nickel(II) hexacyanoferrate(III) were also useful for ion separation, but the α values were lower. Effects on the ion flux rates caused by the feed concentration and the polyelectrolyte precoating of the support are also discussed.

The organic–inorganic hybrid polymers poly(metal tetrathiooxalates), [MII(C2S4)]n, have so far only been available as insoluble and infusible powders, which restricts their applications. The first successful preparation—by a reactive self‐assembly process—of ultrathin films of the polymers (see Figure) is described and their interesting electrical and optical properties are presented.

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