Internal structure of the polyamide thin film, as revealed by HAADF-STEM. Image: Enrique Gomez, Penn State.
Internal structure of the polyamide thin film, as revealed by HAADF-STEM. Image: Enrique Gomez, Penn State.

Careful sample preparation, electron microscopy and quantitative analysis of three-dimensional models (3D) can provide unique insights into the inner structure of the reverse osmosis membranes widely used for salt water desalination and wastewater recycling, according to a team of US chemical engineers. They describe these insights in a paper in the Proceedings of the National Academy of Sciences.

Reverse osmosis membranes comprise several layers of material, including an active aromatic polyamide layer that allows water molecules through but screens out between 99% and 99.9% of salt and other contaminants.

"As water stresses continue to grow, better membrane filtration materials are needed to enhance water recovery, prevent fouling and extend filtration module lifetimes while maintaining reasonable costs to ensure accessibility throughout the world," said Enrique Gomez, professor of chemical engineering at Penn State. "Knowing what the material looks like on the inside, and understanding how this microstructure affects water transport properties, is crucial to designing next-generation membranes with longer operational lifetimes that can function under a diverse set of conditions."

Gomez and his team investigated the internal structure of the polyamide layer using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). HAADF-STEM's image intensity is directly proportional to the density of the material, allowing the layer to be mapped at nanoscale resolution.

"We found that the density of the polyamide layer is not homogeneous," said Gomez. "But instead varies throughout the film and, in this case, is highest at the surface."

This discovery changes the way engineers think about how water moves through this material, because it means the resistance to flow is not homogeneous and is highest at the membrane surface.

HAADF-STEM also allowed the researchers to construct 3D models of the membrane's internal structure. With these models, they can analyze the membrane’s structural components and determine which characteristics must remain for the membrane to function, and which could be manipulated to improve longevity and antifouling and enhance water recovery.

Another characteristic revealed by HAADF-STEM was the general absence of previously reported enclosed voids. Researchers thought that the polyamide membrane’s fine structure would contain enclosed void spaces that could trap water and alter flow patterns, but the 3D models show that there are few closed voids in the state-of-the-art polyamide material investigated in this study.

"Local variations in porosity, density and surface area will lead to heterogeneity in flux within membranes, such that connecting chemistry, microstructure and performance of membranes for reverse osmosis, ultrafiltration, virus and protein filtration, and gas separations will require 3D reconstructions from techniques such as electron tomography," the researchers report in the paper.

Leading on from this study, the researchers would now like to push the resolution of HAADF-STEM to below 1nm. "We don't know if sub nanometer pores exist in these materials and we want to be able to push our techniques to see whether these channels exist," said Gomez. "We also want to map how flow moves through these materials to directly connect how the microstructure affects water flow, by marking or staining the membrane with special compounds that can flow through the membrane and be visualized in the electron microscope."

This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.