(663a) Mechanisms of Selectivity in Zwitterionic Amphiphilic Copolymer Membranes

Zwitterions, functional groups with equal numbers of positive and negative charges, strongly resist fouling, defined as performance loss due to the adsorption and adhesion of feed components onto the membrane. Our group has shown that random/statistical copolymers of zwitterionic and hydrophobic monomers, termed zwitterionic amphiphilic copolymers (ZACs), self-assemble to create bicontinuous disordered hydrophilic/zwitterionic nano-domains that act as a network of nanochannels for water permeation. When coated onto a support to form a thin selective layer, ZACs result in membranes with extraordinary selectivity and fouling resistance. Our initial efforts focused on copolymers of trifluoroethyl methacrylate (TFEMA) with zwitterionic monomers sulfobetaine methacrylate (SBMA) and methacryloxy phosphorylcholine (MPC). This resulted in size-selective membranes with an effective pore size or ~1-1.5 nm, closely matching the size of self-assembled zwitterionic nanodomains. These membranes are exceptionally fouling resistant, showing little to no flux decline during the filtration of a wide range foulants including oil, and complete flux recovery with a water rinse. Additional work has shown that the use of a cross-linkable hydrophobic monomer such as allyl methacrylate (AMA) can enable us to tune the effective pore size of the membrane, down to ~0.8 nm based on the rejection of uncharged solutes. These membranes also exhibited exceptional selectivity between SO₄^2- and Cl^-, even in highly saline mixtures simulating seawater. We hypothesized that the confinement of water permeation to zwitterionic nanodomains, unique to this polymer system, differentiate its selectivity from common nanofiltration membrane chemistries (e.g. cross-linked polyamides). To better understand these effects, we modeled the relative contributions of diffusion and convection through the membrane nanopores. We then performed a series of salt rejection experiments at various trans-membrane pressures and ionic strengths, and fit this data to deconvolute contributions of different permeation mechanisms. Our findings indicate that salt permeation through cross-linked ZAC membranes occurs through both convective and diffusive transport. MgSO₄ rejection is not heavily influenced by feed ionic strength, indicating electrostatic interactions are not the dominant mechanism of rejection for this salt. However, rejection is also not purely size-based. Our results imply that ion-zwitterion interactions may influence the selectivity of cross-linked ZAC membranes, but also show that deeper investigation of transport mechanisms in these intriguing systems is needed. This need is emphasized by their promise in highly relevant industrial applications such as sulfate removal and the treatment of challenging wastewater streams.