A new sulfonated diamine monomer, namely 3,3'-(ethane-1,2-diylbis(azanediyl))bis(2,6-dimethyl-benzene-sulfonic acid) (EDADMBSA), was designed and synthesized in order to fabricate a novel thin-film composite (TFC) reverse osmosis (RO) membrane by using the interfacial polymerization (IP) technique with trimesoyl chloride (TMC). In particular, EDADMBSA was employed as the sole reactant in the aqueous phase during the IP process, instead of the conventional m-phenylene diamine (MPD). Due to the presence of two methyl groups next to a phenylamino group that could exert a strong steric hindrance influence, the rotation of the polyamide main chain was significantly enforced, as clearly illustrated by a molecular dynamics simulation. In this manner, sufficient free volumes were generated in the polymer matrix, allowing for a lower water penetration resistance. The optimal pure water permeability (PWP) attained with this EDADMBSA/TMC membrane was as high as 1.9 +/- 0.2 Lm(-2) h(-1) bar(-1), which is 2.6-2.7 times greater than that of a self-made MPD/TMC membrane under the same test conditions. Furthermore, the unique methyl substituents could also serve as void occupiers, allowing the TFC membrane to retain a reasonably high water-solute selectivity. Compared to commercial BW30FR (Dow Chemical), the novel membrane displayed a superior ability to reject all the ions in a simulated desalination test, revealing its promising potential as a separation medium for brackish water desalination.