(100c) Zwitterionic Amphiphilic Copolymers as Scalable, Fouling-Resistant, Highly Selective Membranes

Membranes offer a highly energy-efficient, simple to operate, scalable and portable separation method for many applications, from water treatment to oil and gas processing to pharmaceutical manufacturing. Yet, their broader use is often limited by insufficient selectivity and/or fouling with complex feeds. We have attempted to address these two key challenges through the rational design of novel, self-assembling polymeric membrane materials that can be integrated easily into roll-to-roll manufacturing for easier commercialization. Our first approach utilized the self-assembly of zwitterionic amphiphilic copolymers (ZACs), synthesized from a hydrophobic monomer and a zwitterionic monomer. When ZACs are coated onto a support to form a thin film composite (TFC) membrane, self-assembled zwitterionic domains act as a network of nanochannels for water permeation. Our first ZAC-based thin film composite (TFC) membranes were size-selective with an effective pore size of ~1.3-1.5 nm. These membranes are exceptionally fouling resistant, exhibiting complete resistance to irreversible fouling with a wide range of simulated and realistic wastewater feeds. This initial data motivated the founding of ZwitterCo, Inc., a start-up initiated at Tufts University, which licensed ZAC membrane technology and has successfully scaled it up for applications in treating high-organic wastewater streams, water reuse, recovery of nutrients and other resources from waste, and others. Our group has since shown that cross-linkable ZACs (X-ZACs) can access smaller effective pore sizes, down to ~0.9 nm, where ion separations are possible. Initial X-ZAC membranes with the smallest pore sizes exhibited unprecedented selectivity between equally charged anions, including the highest Cl^-/F^- selectivity in the literature. This selectivity arises from zwitterion-ion interactions, which affect both ion partitioning and ion diffusivity, further emphasized through nanoconfinement. This opens the door to novel membranes with novel selectivity between molecules and ions of similar size and charge, mediated through channel-solute interactions. More recently, we have been exploring new avenues to prepare membranes the self-assembled nanopores and fouling resistance of ZAC-based membranes, but expand the range of separations accessible. We have developed amphiphilic polyampholytes (APAs), where hydrophobic, anionic, and cationic monomers form a random/statistical terpolymer that is insoluble in water. This approach allows access to a very broad array of functional groups lining the effective nanopores of these membranes, opening the door for complex separations. Alternatively, we have formed amphiphilic polyelectrolyte complex (APEC) membranes by coating consecutive layers of two amphiphilic polyelectrolytes (i.e. water-insoluble copolymers combining a hydrophobic monomer with either an anionic or a cationic monomer). Interestingly, these bilayer membranes exhibit very small effective pore sizes as well as higher permeances, implying selectivity arises from the formation of polyelectrolyte complexes at a thin interface between these layers. These approaches demonstrate a versatile and highly customizable approach for developing novel high-performance membranes that can be brought to market with minimal technical risk in scale-up.

COI Note: A. Asatekin owns a small equity in ZwitterCo and is its Senior Scientific Advisor. Some of the work that will be presented was funded as collaborative projects with ZwitterCo.