Local density and free volume inhomogeneities govern transport properties in reverse osmosis membranes

Developing a mechanistic description of how microstructure affects membrane properties could lead to the development of next-generation materials for desalination. Quantification of the internal microstructure of fully-aromatic polyamide thin-films, which serve as the active layer in state-of-the-art desalination membranes, is a crucial component to developing such descriptions. Here, we studied a series of reverse osmosis membranes showing systematic increases in water permeance over currently available membrane materials without sacrificing water-salt selectivity. We quantified the internal morphology of the polyamide active layers via scanning transmission electron tomography, where 3D reconstructions were obtained and parameters, such as void fraction and surface area, were measured. Tomogram intensity analysis revealed the nanometer-scale density and free volume distributions, used in conjunction with the 3D polyamide models to model water transport properties in such materials. The combination of density and free volume distributions determined from electron tomography with water transport modeling has provided a robust approach towards the development of structure-property relationships in reverse osmosis membranes.