Regulation of the polyamide network structure with adamantane-1,3-diamine to adjust the selectivity of SWRO membranes

Seawater reverse osmosis (SWRO) membranes have served as the gold standard technology in seawater desalination over the past half-century. Despite their widespread application, the intrinsic separation mechanisms within the selective layer remain inadequately understood, highlighting the need for a new insight research. This study investigated the incorporation of adamantane-1,3-diamine (Ad), a secondary monomer characterized by its three-dimensional cage-like structure, into the aqueous phase to enhance its integration into the polyamide network. This affected the stacking morphology and spacing of the benzene rings as well as the folding and compaction structures. The fractal dimension of fully aromatic polyamide (APA) aggregates remained at about 1.70, indicating that Ad incorporation did not change the aggregation morphology, thus maintaining the consistent water permeability of the SWRO membrane. However, the rigid Ad molecules with obstructive effects increased the shoulder-to-shoulder packing spacing d₁ of benzene rings in the polyamide network at high Ad concentrations. This change altered the twisting of polyamide chains, reducing their folding and colloidal particle compaction, thereby regulating the salt retention performance. As the composition, morphology, and thickness of APA layer remained largely unchanged, the packing spacing d₁ is likely a key factor influencing the salt rejection performance of SWRO membranes.