Relating uncharged solute retention of polyelectrolyte multilayer nanofiltration membranes to effective structural properties

A novel way of making Nanofiltration (NF) membranes is to apply the Layer-by-Layer method, where polyelectrolytes are alternatingly coated on top of a porous Ultrafiltration membrane to form a separation layer with a controllable thickness in the nanometer range, also known as Polyelectrolyte Multilayer (PEM). An important precondition to make use of the variety this fabrication method offers for membrane optimization is knowledge of design rules. Therefore, the structural properties of PEMs and their relation to both coating conditions and membrane performance is an ongoing field of research. In this work, the separation performance of NF PEM membranes, based on PDADMAC/PSS and PAH/PAA, towards uncharged molecules is related to PEM structure. The structure of the membrane is represented by a nanoporous film with distribution in pore size. As, up to date, no experimental technique is available to directly measure pore sizes in the nanometer range of a wet film, a representative pore size distribution is estimated via the use of a theoretical transport description fitted to experimental data. Here, the studied PEM systems showed distinctive differences in both film thickness (PDADMAC/PSS: 44 nm/BL, PAH/PAA: 689 nm/BL) and mean pore size (PDADMAC/PSS: 0.44 nm, PAH/PAA: 0.27 nm). Within the range of layer numbers, the pore size of both PEMs in the layer dominated regime was independent of layer number. This indicates that there is an optimum layer number for PEM NF membranes regarding uncharged solute retention. Surprisingly, despite forming much thicker layers the PAH/PAA system closes off the support membrane pores at a higher bilayer number.