Rinse-free deposition of molecular layer-by-layer (mLbL) polyamide reverse osmosis membranes

• mLbL deposition of polyamide membranes traditionally requires multiple rinse steps • We demonstrate that we can eliminate rinse step by reducing monomer concentrations • Rinse-free mLbL achieved ∼20 nm membrane thickness at 0.01 % monomer concentration • Achieved steady-state faster than conventional mLbL and commercial RO membranes • Enables large-scale RO membrane production with resource and time efficiency The molecular layer-by-layer (mLbL) deposition technique enables the fabrication of polyamide-based reverse osmosis membranes with low surface roughness and tunable membrane thickness, but scale-up of the process is challenged by the necessity for rinsing away unreacted monomers after each deposition step. By sensible tuning of monomer concentrations during deposition, we can eliminate the rinsing steps while still producing polyamide membranes with comparable thickness and surface properties. This approach markedly shortens fabrication time and improves reagent efficiency. By measuring the growth rate of the polyamide membrane as a function of deposition cycles and monomer concentration, we demonstrate that we can deliver a more targeted quantity of monomer to the growing film surface while maintaining a linear growth rate profile. Desalination tests reveal that mLbL membranes produced at the lowest monomer concentration (membrane thickness of ≈20 nm) achieved 99.9 % salt rejection within 4 h, while maintaining a flux of 0.98 L m⁻² h⁻¹ bar⁻¹, indicating that the crosslink density of the membrane remains high. This modified mLbL approach significantly reduces membrane fabrication time without compromising performance, offering a path towards scalable production of polyamide-based membranes with controlled thickness and low roughness while prioritizing both time and resource efficiency.