In this work, the effect of applying layers of polyethyleneimines with different molecular weights (1300 and 25 000 Da), polystyrene sulfonic acid or sodium polyacrylate on the surface of the heterogeneous membrane MK-40 on the selectivity to singly charged ions was investigated. It is shown that the modification leads to an increase in the selectivity coefficients P(Na+/Ca2+) and P(Li+/Mg2+), and in membranes based on polyethyleneimine with a molecular weight of 1300 Da this effect is more pronounced. For a couple of Na+/Ca2+, the highest selectivity value (3.95) was obtained using a membrane with two layers of polyethyleneimine with a molecular weight of 1300 Da and polystyrene sulfonic acid.
In this work, asymmetric flat-sheet membranes based on the commercial polyetherimide–siloxane copolymer Siltem, whose gas permeability is one order of magnitude higher than that of polyetherimide Ultem 1000, were first obtained. Based on the solubility analysis and Hansen solubility parameters, N‑methyl-2-pyrrolidone was selected as the principal solvent, as it provides the best thermodynamic compatibility with both polymers. It was shown that Siltem STM1700 is soluble in a significantly wider range of organic solvents, including tetrahydrofuran and N-butylpyrrolidone. Furthermore, solutions of Siltem in N‑methyl-2-pyrrolidone exhibit substantially lower viscosity compared to those of Ultem, which allows the preparation and processing of polymer solutions at concentrations up to 36 wt
This work investigates the competitive transport of acetic and citric acids from model solutions containing both salt and molecular forms of the acids. The study examines two membrane systems: a Ralex AMH heterogeneous membrane with a total concentration of 0.1 mol-eq/L of each acid and a Ralex AMH/MF-4SK bilayer membrane with a total concentration of 0.5 mol-eq/L of each acid. Experiments were conducted at solution pH values of 1.0, 4.8, and 9.0. The separation of organic acids by electromembrane processes is a key challenge when these acids are obtained from mixtures. The results show that electrodialysis extraction of citric acid from solution containing 0.1 mol-eq/L of organic acid ions and molecules, using Ralex AMH membrane at current density of about 3 A/dm2 and pH 9.0, yields a specific selective permeability coefficient PCit/Ac equal to 4 for citric acid compared to acetic acid. Electrodialysis removal of acetic acid from solution containing 0.5 mol-eq/L of organic acid ions and molecules using the Ralex AMH/MF-4SK membrane at current density of 0.17 A/dm2 and pH 9.0 results in PCit/Ac value of 0.025.
A series of composite ion-exchange membranes based on the heterogeneous anion-exchange strongly basic membrane MA-41 and polyaniline was obtained via oxidative polymerization of aniline. This was achieved through sequential diffusion of an oxidizer (ammonium persulfate) and a monomer (aniline sulfate) through the membrane into water with varying contact times with the monomer solution (from 2 to 6 h). The penetration depth of the modifier into the membranes was determined to be independent of the polyaniline synthesis time and to be 30–35
Mixed ionic-electronic conductors (MIECs) are essential for high-temperature electrochemical devices, such as solid oxide fuel cells and oxygen separation membranes. While it is difficult to optimize both ionic and electronic transport simultaneously in single-phase materials, composite systems offer a promising strategy by separating these functionalities into distinct and percolating phases. This study reports the functional properties of the nominal composition BaCe0.5Fe0.5O3 – δ (BCF), which forms a dual-phase composite due to the mismatch in ionic radii between the Ce4+ and Fe3+/4+ cations. The BCF material was synthesized via a single-step citrate-nitrate combustion route to ensure a homogeneous dual microstructure state consisting of an orthorhombic BaCeO3-based phase and a cubic BaFeO3-based phase. This was confirmed by Rietveld refinement and electron microscopy analyzes. Electrical characterization revealed moderate total conductivity at 300–750°C, which did not exceed 1 S cm–1. Despite its restricted electronic conductivity, the composite exhibited acceptable oxygen permeation fluxes, reaching 2.0 × 10–3 mL min–1 cm–2 at 750°C for a 650 µm-thick membrane. The calculated specific oxygen permeation flux exhibited thermally activated behavior with an activation energy ( 0.75 eV) comparable to that of similar iron-containing perovskite systems. A comparative analysis with literature data showed that the BCF performance is acceptable for the 600–750°C range. These results highlight the effectiveness of the in situ formed composite, in which the Ce-rich phase facilitates ionic transport and compensates for the moderate electronic conductivity of the Fe-rich phase. Consequently, the BCF composite can be considered a promising material for oxygen transport membranes.
