Charge-patterned mosaic membranes, which consist of discrete positively-charged <i>and</i> negatively-charged domains patterned on a template, are capable of transporting dissolved salts more rapidly than neutral molecules of comparable size. For example, when tested in filtration experiments, this class of membrane is able to enrich the salt concentration in the permeate relative to the feed solution. This ability to facilitate the transport of whole salts is a direct result of the chemically-patterned surface. To date, however, few charge mosaics have been fabricated and as a result, their design and proliferation have lagged. Here, using inkjet printing techniques to precisely-pattern membrane chemistries, a series of experiments that helped to elucidate the membrane-solute interactions underlying the transport properties of charge-patterned membranes were executed. Membranes of varied charge (i.e., positively-charged, charge-patterned, and negatively-charged) were fabricated and their salt permeabilities as a function of concentration were quantified in diffusion cell experiments. Salt permeability increased with concentration for the single-charge membranes and decreased with concentration for the charge-patterned membrane. A theoretical framework capable of describing these trends was developed. In particular, it was shown that the structural charge heterogeneity of the patterned membrane surface modifies the local electrostatic potential, and in turn, availability of ions for transport. These insights informed experiments that examined the effects of pattern geometry (e.g., stripes, cubes, and hexagons) and feature size on membrane throughput. The variations in salt transport for this family of membranes could be captured using a single variable, the interfacial packing density, σ, that quantified the length of junctions between oppositely-charged domains in a unit area. In filtration experiments, the enrichment of symmetric salts (e.g., KCl) increased monotonically with σ. In contrast, the transport of asymmetric salts (e.g., MgCl<sub>2</sub>) was independent of σ. These results will enable further development of charge-patterned membranes.

Salt permeation mechanisms through charge-patterned mosaic membranes

2019 Spring National Meeting

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