Pushing The Limits of Size-Selectivity in Nanoscale Solute Separations

Transport of a spherical solute through a cylindrical pore has been modeled for decades using well-established hindered transport theory, predicting solutes with a size smaller than the pore to be rejected nonetheless because of convective and diffusive hindrance; this rejection mechanism prevents extremely sharp solute separations by a membrane. While the model has been historically verified, solute transport through near-perfect isoporous membranes may finally overcome this limitation. We achieve encouraging solute rejections using nanofabricated, defect-free, thin silicon nitride isoporous membranes. The membrane is challenged by a recirculated feed of dextran molecules to increase the opportunity for interactions between solutes and the pore array. Results show the membrane completely rejects solutes with greater size than the pore size while effectively allowing smaller solutes to permeate through. A steep size-selective rejection curve takes shape in distinct contrast to the conventional S-shape rejection curve predicted by hindered transport theory. With this traditional hurdle overcome, there is new promise for unprecedented membrane separations through judicious process design and extremely tight pore-size distributions.