Self-assembled films of Prussian blue and analogues: Structure and morphology, elemental composition, film growth, and nanosieving of ions

Structure, morphology, and elemental composition as well as the size-selectivity of the ion transport behavior of ultrathin membranes of iron(III) hexacyanoferrate(II) (Fe^IIIHCF^II), iron(II) hexacyanoferrate(III) (Fe^IIHCF^III), cobalt(II) hexacyanoferrate(III) (Co^IIHCF^III), and nickel(II) hexacynoferrate(III) (Ni^IIHCF^III) 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 consists of a multitude of small, densely packed particles with diameter in the 10-100 nm range. Energy-dispersive X-ray analysis indicates that the ion 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 consists 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 small Stokes radius such as Cs⁺, K⁺, and Cl^-, whereas large hydrated ions such as Na⁺, Li^-, Mg^2-, 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 Fe^IIIHCF^II 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.