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A new sulfonated diamine monomer, 4,4′-((1,4-phenylenebis(methylene))bis(azanediyl))dibenzenesulfonic acid (PMABSA) was synthesized and used as the sole aqueous reactant to fabricate a novel reverse osmosis (RO) membrane with trimesoyl chloride (TMC), in place of conventional m -phenylene diamine (MPD). The separation performance of the PMABSA/TMC membrane was optimized by the response surface method (RSM) by means of evaluating the pure water permeability (PWP) and salt rejection (R) to NaCl, and optimal results of PWP of 1.18 ± 0.03 L m −2 h −1 bar −1 , and R of 98.2 ± 0.4%, respectively, were achieved at 1.55 MPa. Owing to the rigid benzene ring in the center of the molecule, which may have a blocking effect during the interfacial polymerization (IP) process, the resulting barrier layer was less compact and had low thickness. Both molecular dynamics (MD) simulation and positron annihilation spectroscopy (PAS) revealed the creation of sufficient free volumes in the sulfonated polyamide matrix. Regardless of the slightly lower water permeation, the PMABSA/TMC membrane exhibited a superior rejection ability to all the ions compared to that of commercial SW30HR (Dow Chemical) in a model seawater separating test. Overall, all the results indicated the great potential of the novel membrane as a new choice of non-MPD RO membrane for seawater desalination. [ABSTRACT FROM AUTHOR]