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Alternating deposition of oppositely charged polyelectrolyte (PEs) can create stable solvent-resistant nanofiltration (SRNF) membranes with very high flux and selectivity. A new combination of PEs (poly(diallyldimethylammonium chloride) and sulfonated poly(ether ether ketone) is reported from which supported membranes consisting of 5, 10, 15, and 20 bilayers are prepared via the layer-by-layer method. The morphology of the membranes was studied in detail by SEM and AFM, showing rougher membrane surfaces with increasing bilayer number. For the first time, polyelectrolyte complex (PEC) based membranes were also applied in the filtration of organic solvents, where they form an excellent new tool to study the role of charges in solvent filtrations. Due to Donnan exclusion, the multilayered PEC membranes showed very good retentions up to 99% for charged solutes in the pressure-driven filtration of isopropanol solutions. Moreover, they were found to show excellent prospective use in polar aprotic solvents, like dimethylformamide and tetrahydrofuran, which have been extremely troublesome so far for most existing SRNF membranes.

This study presents a methodology for an in-depth characterization of six representative commercial nanofiltration membranes. Laboratory-made polyethersulfone membranes are included for reference. Besides the physical characterization [molecular weight cut-off (MWCO), surface charge, roughness and hydrophobicity], the membranes are also studied for their chemical composition [attenuated total reflectance Fourier spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS)] and porosity [positron annihilation spectroscopy (PAS)]. The chemical characterization indicates that all membranes are composed of at least two different layers. The presence of an additional third layer is proved and studied for membranes with a polyamide top layer. PAS experiments, in combination with FIB (focused ion beam) images, show that these membranes also have a thinner and a less porous skin layer (upper part of the top layer). In the skin layer, two different pore sizes are observed for all commercial membranes: a pore size of 1.25-1.55 angstroms as well as a pore size of 3.20-3.95 angstroms (both depending on the membrane type). Thus, the pore size distribution in nanofiltration membranes is bimodal, in contrast to the generally accepted log-normal distribution. Although the pore sizes are rather similar for all commercial membranes, their pore volume fraction and hence their porosity differ significantly.

Ellipsometric porosimetry was used to determine the adsorption isotherms of toluene, methanol, and water on b-oriented Silicalite-1 coatings with a thickness of less than ca. 250 nm and to obtain adsorption kinetics. The adsorption isotherms are of sufficient quality to reveal several aspects of the pore structure such as the adsorbate capacity and the adsorbate/framework affinity. The use of a combination of different molecular probes in ellipsometric porosimetry to elucidate the molecular accessibility of Silicalite-1 pores is demonstrated. It is shown that ellipsometric porosimetry is an appropriate technique for probing the influence of aging of the Silicalite-1 coating and of planarization polishing on the porosity, pore accessibility, and adsorbate/framework affinity.

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