Molecular layer-by-layer assembled forward osmosis membranes

A recently devised, molecular layer-by-layer (mLbL) approach based on the alternative cross-linking of monomers was employed to fabricate high-performance thin film composite (TFC) forward osmosis (FO) membranes with excellent rejection toward monovalent NaCl salt. An ultrathin and highly dense polyamide (PA) selective layer with a precisely controlled structure was created on a tailored porous support via mLbL. The intrinsic separation properties of the mLbL-assembled TFC membranes were tuned by adjusting the mLbL cycle number to optimize the FO performance. The best FO performance was achieved at 10 mLbL cycles (mLbL-10), where the permeability and selectivity were properly balanced. Importantly, the mLbL-10 membrane exhibited superior FO performance compared to the commercial HTI FO membranes as well as hand-cast TFC membranes prepared by the conventional interfacial polymerization: the mLbL-10 membrane showed ~3.5 times higher water flux, ~60% lower reverse salt flux and ~85% lower specific salt flux compared to the cellulose triacetate HTI membrane, with 0.5. M NaCl draw solution and DI water feed solution in FO mode. The stability and the associated membrane performance of the mLbL-assembled membrane depending on the ionic strength of the environment were explained by the swelling behavior of the polyelectrolyte-assembled interlayer adhered underneath the PA selective layer.