Microscopy and microanalysis of reverse-osmosis and nanofiltration membranes

The polyamide active layers of commercial reverse-osmosis and nanofiltration membranes are examples of nanoscale functional materials that challenge the state of the art of materials characterization. The active layer is only ∼100 nm thick, and because the active layer is formed by a process of interfacial polymerization, the structure and composition of the membrane is highly inhomogeneous. Even such basic physical and chemical properties of the membrane as the atomic density, swelling in water, distribution of charged species, and the mobility of water and ions, are poorly understood. In this article, we briefly review progress in the characterization of polyamide separation membranes using transmission electron microscopy, atomic force microscopy, vibrational spectroscopy, positron annihilation, nuclear magnetic resonance, and Rutherford backscattering spectrometry. Advances in the microanalysis methods applicable to these complex materials will advance fundamental understanding of the structure-property relationships of polymer membranes and further the long-term goal of synthesizing membranes with improved performance.