All-nanoparticle layer-by-layer surface modification of micro- and ultrafiltration membranes.

Layer-by-layer (LbL) deposition using primarily inorganic silica nanoparticles is employed for surface modification of polymeric micro- and ultrafiltration (MF/UF) membranes to produce novel thin film composite (TFC) membranes intended for nanofiltration (NF) and reverse osmosis (RO) applications. A wide variety of porous substrate membranes with different surface characteristics are successfully employed. This report gives detailed results for polycarbonate track etched (PCTE), polyethersulfone (PES), and sulfonated PES (SPEES) MF/UF substrates. Both spherical (cationic/anionic) and eccentric elongated (anionic) silica nanoparticles are deposited using conditions similar to those in prior works for solid substrates (e.g., Lee et al.). Appropriate selection of the pH for anionic and cationic particle deposition enables construction of nanoparticle-only layers 100-1200 nm in thickness atop the original porous membrane substrates. The surface layer thickness appears to vary linearly with the number of bilayers deposited, i.e., with the number of anionic/cationic deposition cycles. The deposition process is optimized to eliminate drying-induced cracking and improve mechanical durability via thickness control and postdeposition hydrothermal treatment. "Dead-end" permeation tests using dextran standards reveal the hydraulic characteristics and separations capability for the PCTE-based TFC membranes. The results show that nanoparticle-based LbL surface modification of MF and UF rated media can produce TFC membranes with NF capabilities.