(376c) Elucidating the Effects of Asymmetric Charge Patterning on Ion Transport through Charge Mosaic Membranes

Charge mosaic membranes consist of cationic and anionic domains that traverse the membrane thickness. Due to their charge-patterned structure, these membranes are capable of transporting ionic solutes more rapidly than neutral particles of comparable or smaller sizes. To date, the majority of the understanding that has been developed regarding the novel transport properties of charge mosaic membranes has been generated using 1:1 monovalent salts (e.g., KCl, NaCl) permeating through mosaic membranes with equal coverage of cationic and anionic domains. The ability of mosaic membranes to separate asymmetric, multivalent salts (e.g., MgCl₂, Na₂SO₄), however, has not been investigated in detail due to limited control over membrane properties provided by existing patterning processes. Here, using a recently-developed inkjet printing process that enables the ready fabrication of charge mosaic membranes with controlled patterns, we investigate the throughput of asymmetric salts by adjusting the areal coverage of the charge-functionalized domains. First, parent nanofiltration membranes based on a poly(acrylonitrile-co-[oligo(ethylene glycol) methyl ether methacrylate]-co-[3-azido-2-hydroxypropylmethacrylate]) [P(AN-OEGMA-AHPMA)] copolymer were prepared. These parent membranes possess pore walls lined by reactive azido moieties that made them amenable to post-synthetic modification via a printing device. Next, charge patterning was accomplished through the selective deposition of alkynyl-terminated reactants on the surface of the azido-functionalized parent membranes. In this way, cationic and anionic domains in patterns of alternating stripes were generated via the copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) reaction mechanism. The use of the striped pattern allowed asymmetric charge-patterns to be generated by implementing different values of the stripe thickness. Single solute rejection experiments using magnesium chloride and sodium sulfate as model solute were executed using these membranes with asymmetric patterns. The fundamental knowledge developed by studying charge patterned membranes with controlled areal coverages of the charge-functionalized domains will enable further development of charge mosaic membranes that can be deployed in the many established and emerging technologies where the selective transport of ionic solutes is of critical importance such as water softening and protein purification.