(402f) Local Density and Free Volume Inhomogeneities Govern Transport Properties in Reverse Osmosis Membranes

Developing a mechanistic description of how the 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. Previous work has demonstrated that three-dimensional (3D) reconstructions over two-dimensional projections are crucial in this regard, as they provide a more complete and less ambiguous representation of the complex polyamide morphology. In this work, we studied a series of reverse osmosis membranes, all of which show systematic increases in water permeance over the currently available membrane materials without sacrificing water-salt selectivity. We quantified the complex internal morphology of the polyamide active layers using scanning transmission electron tomography, where 3D reconstructions were obtained and conventional parameters such as void fraction and surface area were measured. Tomogram intensity analysis revealed the nanometer-scale density and free volume distributions, which have been used as input parameters, alongside 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.