Layer-by-Layer Assembled Hydrogen-Bonded Multilayer Poly(2-oxazoline) Membranes for Aqueous Separations

Preparation of multilayer films using the layer-by-layer (LbL) technique via hydrogen bonding has recently gained increasing research interest and has potential in diverse areas of research, especially in the biomedical field. However, the scope of these hydrogen-bonded multilayer films in separation science is quasi-unexplored. In this work, ultrathin multilayer membranes were prepared by the LbL deposition of tannic acid (TA) and poly(2-alkyl-2-oxazoline)s via hydrogen-bonding (HB) and hydrophobic interactions. The potential of these membranes in aqueous filtrations was demonstrated as they show high retention (>95%) for both positively (malachite green oxalate) (MGO) and negatively charged (Rose Bengal) (RB) dyes and even 95% retention for the much smaller and neutral micropollutant bisphenol A (BPA) (228 g/mol). Nanofiltration (NF) performances of three variants of poly(2-alkyl-2-oxazoline), namely, poly(2-methyl-2-oxazoline) (PMEOx), poly(2-ethyl-2-oxazoline) (PEtOx), and poly(2-n-propyl-2-oxazoline) (PPropOx), were compared revealing an increase in retention and decrease in permeance with increasing hydrophobicity of the poly(2-alkyl-2-oxazoline)s. Even though the reported HB multilayer membranes appear to be promising candidates for aqueous filtrations, their permeance is currently still about an order of magnitude too low for real-life applications due to the formation of rather dense hydrogen-bonded layers.