Molecular Layer-by-Layer Assembled Reverse Osmosis Membranes for Water Desalination
Joung-Eun Gu, Seunghye Lee, Wansuk Choi, Jong Suk Lee, Bo-Young Kim, Edwin P. Chan, Christopher M. Stafford, Jun-Young Chung, Joona Bang, Jung-Hyun Lee
Here, we report on the successful design, construction, and performance of membranes constructed using the mLbL process and show that these membranes exceed the performance of membranes synthesized through conventional interfacial polymerization. In our approach, we employ a thin interlayer coating that inhibits the reactive monomer solutions from penetrating the porous support. We then conduct mLbL assembly using traditional monomers in RO membrane fabrication (TMC and MPD), which are rigid aromatic monomers that display relatively low fractional free volume. The sequential mLbL deposition produced highly selective PA active layers with controlled thickness, minimal roughness, and well-defined chemical composition. As a result, only 15 cycles of mLbL assembly were needed to achieve the targeted NaCl rejection (R ≈ 98+ %) while the flux was ≈ 75 % greater than a traditional interfacially polymerized membrane. The high salt rejection demonstrates that the molecular topology from mLbL is sufficiently similar to traditional interfacial polymerization, while the reduced thickness of the active layer (≈25 nm) equates to a reduced hydraulic resistance and shorter diffusive path length for water to pass through the membrane. Additionally, we show that the reduced surface roughness and chemical homogeneity achieved by our mLbL films mitigated fouling on the membrane surface.