(538f) Membranes with Cross-Linked Zwitterionic Nanochannels Achieve Sub-Nanometer Selectivity

Self-assembly is the process by which ordered structures are spontaneously formed from simpler building blocks. Self-assembling copolymers have attracted great interest for membrane applications, due to facile processing and the exceptional selectivity provided by their ordered transport pathways. Achieving effective pore sizes less than 1.0 nm has been a significant challenge, however, and the sub-nanometer selectivity needed for the broad application space of nanofiltration has not yet been demonstrated. Random zwitterionic amphiphilic copolymers (r-ZACs) are strong candidates to meet this challenge. These copolymers can self-assemble to form zwitterionic nanochannels through which water and small solutes can permeate. Membranes derived from r-ZACs demonstrate exceptional fouling resistance, low ion retention, and a size-based cutoff of ~ 1.5 nm. Slight reductions in the effective pore size would enable r-ZAC membranes to filter divalent ions and sub-nanometer neutral solutes.

In the present work, we employed a novel cross-linking technique to reduce the size of the zwitterionic nanochannels. The effective pore size was less than 1.0 nm, as indicated by the rejection of sugars (sucrose rejection 99.6%; Stokes diameter = 0.96 nm). The membranes also demonstrated outstanding divalent-monovalent selectivity when challenged with artificial seawater (sulfate rejection = 99.2%, sulfate-chloride separation factor = 75). The membranes were also extremely fouling resistant, which was due to the strong hydration barrier surrounding the zwitterions. These results illustrate a promising route to achieving sub-nanometer separations with self-assembling copolymer membranes.