A recent collaboration between users at the University of Chicago and the University of Illinois at Chicago with the Center for Nanoscale Material's Electronic & Magnetic Materials & Devices Group has produced the thinnest nanofiltration membrane achieved thus far, at ~30 nm, made of just four layers of nanoparticles.
A separation membrane is a key component in both nanofiltration and reverse osmosis filtration systems. Typically they are microns-thick polymer films. Reducing the thickness of the membrane reduces the pressure that needs to be applied across the membrane in order to achieve a certain amount of flux, which is a major operational cost in these devices. The filtration coefficient of this membrane for aqueous solutions is two orders of magnitude larger than for typical polymer-based nanofiltration systems. Near only 80 kPa pressure, the membrane exhibits pronounced charge sensitivity for a variety of dyes and other molecules, while rejecting molecules greater than 1.7 nm in size. Guided by atomistic molecular dynamics simulations, it was found that molecular transport occurs through pore-like regions between close-packed nanoparticles and that dielectric exclusion dominates the charge-dependent rejection.
Close-packed nanoparticle monolayers self-assembled from dodecanethiol-ligated gold nanocrystals. TEM image (left) and atomistic simulation of tryptophan transport through a pore.
This research opens up new possibilities for using nanoparticles in nanofiltration and separation. As the particle size, surface ligand type, and packing geometry in the membrane can all be adjusted, it is potentially possible to further adjust the cut-off size and robustness of the membrane for a variety of filtration applications.