Membranes are compelling candidates for many separation-based processes. Having uniform nanoscale pore size, isoporous membranes have attracted wide research interest in water treatment, biosensing, and pathogen isolation. Transport of a solute through a cylindrical pore has been modeled using well-established hindered transport theory, predicting solutes with a smaller size than pore size to be rejected because of the convective and diffusive hindrance; this rejection mechanism prevents extremely sharp solute separations. While the model has been historically verified using numerous solutes and membrane materials, solute transport through near-perfect isoporous membranes in a cross-flow filtration system may finally overcome this limitation. Here, we propose an emerging binary rejection model governing the transport of poly(ethylene oxide) (PEO) with varying molecular weight through nanofabricated, defect-free silicon nitride (SiN<sub>x</sub>) isoporous membranes. By challenging the membrane with a recirculated feed solution, we achieve a high number of interactions between the PEO molecules and the isoporous membrane. As a result, the membrane strongly rejected PEOs with greater size than the pore size while effectively allowing smaller PEOs to permeate through. A step-function shape molecular weight cut-off (MWCO) curve takes shape in distinct comparison to the conventional S-shape MWCO curve predicted by hindered transport theory. Overall, the study highlights a sharp size-selective separation that an isoporous membrane can achieve and promises exciting opportunities for their future applications.

Exploring the limits of membrane processes: Isoporous membranes for precision separations

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ACS Fall 2022

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