Expanding the toolbox for microfluidic-based in situ membrane characterization via microscopy

Polyamide thin film composite membranes are the commercial standard for aqueous nanofiltration and reverse osmosis. Establishing their synthesis-structure-performance relationships (SSPs), needed for rational membrane design, is hampered by the small scale and high reaction rate of interfacial polymerization (IP). Microfluidic devices, compatible with microscopic real-time visualization of IP and performance testing of the formed film, are interesting within this respect. In this study, a new microfluidic design and operational protocol for in situ characterization of IP is developed. Difficulties encountered with microfluidics and coping strategies are highlighted. The outcome of the optimization study proves that a parylene-coated PDMS-glass chip comprising a channel lay-out with 4 inlets, 2 outlets, a channel height of 20 μm, and a reaction channel length ≤50 μm is most compatible with IP and performance testing. Varying synthesis conditions show changing film morphology and water flux in line with trends for dip-coated membranes. Addition of NaHCO₃ and ethyl acetate induce morphological features and increase water flux. Increasing TMC concentrations decrease water flux until an excess is generated. By combining the developed protocol and microfluidic device with an online measurement technique to probe film formation dynamics, such as fluorescence microscopy, SSPs can be derived in the future.