Quantification of Nanovoids in Fully-Aromatic Thin-Film Composite Polyamide Membranes Using a Quartz Crystal Microbalance Equipped with a Humidity Cell

The design of thin-film composite (TFC) membranes is crucial for efficient separation, as the transport of water and solutes is governed by the intrinsic microstructure of its polyamide selective layer. The presence of nanovoids in the top polyamide selective layer significantly impacts membrane separation performance and regulates water transport pathways through the film. In this study, we present a novel approach that employs a quartz crystal microbalance (QCM) equipped with a humidity cell to characterize the size and distribution of nanovoids in the polyamide layers of five commercial TFC membranes. By utilizing the Kelvin equation and accurately measuring the mass changes under a controlled humidity environment, we quantified the nanovoids, detecting small voids often overlooked by traditional methods. Our findings offer critical structural parameters, including void size distribution, total void volume, and void fraction, providing a practical and accessible alternative to complex methods. The findings were also validated and compared with traditional microscopic analysis, revealing that the smaller voids are often overlooked by the transmission electron microscopy characterization. Overall, this study demonstrates the feasibility of employing QCM humidity cells to quantify nanovoids and provides a useful tool for membrane characterization, fostering future advancements in membrane design and performance optimization.