Effects of Polyamide Chemistry on Solution Permeance in Molecular Layer-By-Layer Desalination Membranes

We present a series of polyamide membranes synthesized via molecular layer-by-layer (mLbL) deposition using a combination of two acid chlorides, trimesoyl chloride (TMC) and isophthaloyl chloride (IPC), and one of two diamines, m-phenylenediamine (MPD) or 3,5-diaminobenzoic acid (BA). These monomer combinations permit comparison of the conventional reverse osmosis chemistry, TMC combined with MPD, against a variant with a higher concentration of polar acid groups, TMC combined with BA, and a variant with reduced cross-link density via the addition of a linear monomer, a 50%/50% mixture by mass fraction of TMC and IPC combined with MPD. Water permeance, NaCl rejection, and water swelling are compared across the series. Although efficient NaCl rejection appears to be dependent on the formation of a sufficiently cross-linked polyamide network, we find that water permeance is more strongly controlled by the concentration of hydrophilic moieties in the membrane than by the cross-link density. The enhanced permeance in polar membranes is associated both with increased membrane swelling and with elevated water mobility.