Charge-patterned mosaic membranes, which consist of discrete positively-charged and negatively-charged domains, are capable of transporting dissolved salts more rapidly than neutral molecules of comparable size. For example, under the action of pressure-mediated flow, this class of membrane is capable of enriching the salt concentration in the permeate solution relative to the feed solution. To date, however, few charge mosaic membranes have been fabricated and as a result, a fundamental understanding of the mechanisms controlling their unique transport properties has lagged. Here, inkjet printing was utilized to deposit reactive inks onto structural templates to form a family of charge-patterned membranes. These membranes were then characterized in a series of experiments that helped to elucidate the membrane-solute interactions underlying their transport properties. Specifically, nanostructured membranes of varied chemical composition (i.e., positively-charged, charge-patterned, and negatively-charged) were fabricated and their salt permeability as a function of salt identity and concentration were quantified in diffusion cell experiments. It was observed that the salt permeability increased with concentration for the single-charge membranes, and decreased with concentration for the charge-patterned membrane. A framework capable of describing the observed trends was developed by accounting for the structural charge heterogeneity of the patterned membrane surface. In particular, it was shown that the structural charge heterogeneity modified the local electrostatic potential, and in turn, the local availability of ions for transport. These insights informed a series of experiments that examined the effects of pattern geometry (e.g., stripe-patterned, cubically-patterned, and hexagonally-patterned) and feature size (i.e., 300 µm - 900 µm) on membrane throughput. The variations in salt transport over this family of membranes could be captured using a single variable, the interfacial packing density, that quantified the total length of the interfacial junctions between oppositely-charged domains in a unit area. In filtration experiments, the enrichment of symmetric salts (e.g., KCl and MgSO4) increased monotonically with interfacial packing density. These results will enable further development of charge-patterned membranes that can be deployed in the many established and emerging technologies where the selective transport of ionic solutes is desired.

Salt permeation through charge-patterned mosaic membranes

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