Angstrom-Precise Vapor-Synthesized Polyamide Membranes for Ion–Ion Separation

Abstract Precise ion–ion separation using nanofiltration (NF) membranes remains a central challenge in water treatment and resource recovery. Conventional fabrication by interfacial polymerization (IP) provides limited control over the structure of the polyamide (PA) selective layer, thereby constraining ion–ion separation. Using a solvent-free, vapor-phase, layer-by-layer approach, we synthesized a PA selective layer for NF via molecular layer deposition (MLD) of trimesoyl chloride (TMC) and piperazine (PIP). This strategy enables angstrom-scale control over PA growth with a deposition rate of 1.3 Å per cycle and yields spatially uniform membranes with a mean pore size of 5.08 Å and a narrow geometric standard deviation of 1.18. MLD further enables precise control over surface chemistry through either TMC or PIP termination. Structural, chemical, and transport characterizations indicate an enhanced size-based exclusion and reduced Donnan exclusion in the MLD-grown membranes. MLD membranes with ∼39 nm-thick PA layer exhibited a water permeance of 13.0 L m–2 h–1 bar–1 and monovalent–divalent cation (Li+/Mg2+) and anion (Cl–/SO42–) selectivities of 36 and 92, respectively. This work establishes MLD-grown PA membranes as a model platform for precise PA structural control and mechanistic interrogation of ion–ion separation, providing design principles for more selective membranes.