Abstract
For nearly half a century, thin-film composite reverse osmosis membranes have served as key separation materials for desalination. However, the precise structure of their polyamide selective layer under hydrated conditions and its relationship to membrane transport remain poorly understood. Using cryo-electron tomography, we successfully reconstructed the three-dimensional structure of six commercial polyamide membranes under hydrated conditions, revealing a fully swollen nodular network. The highly heterogeneous nodules, measuring 17.2 ± 2.8 nanometer in thickness, were directly connected to the pores of the underlying polysulfone substrate. The nodules occupied most of the surface area compared to the 75.9 ± 26.8-nanometer-thick dense layer of the polyamide film. Key structural parameters of the nodules, including surface area index and wall thickness, were correlated with the water permeance of an additional 16 polyamide membranes, validating the major role of these nodules in water transport. This study enhances our understanding of the heterogeneous structure of desalination membranes and its role in membrane transport.