Interaction-based ion selectivity exhibited by self-assembled, cross-linked zwitterionic copolymer membranes

Abstract

Water filtration membranes with advanced ion selectivity are urgently needed for resource recovery and the production of clean drinking water. This work investigates the separation capabilities of cross-linked zwitterionic copolymer membranes, a self-assembled membrane system featuring subnanometer zwitterionic nanochannels. We demonstrate that selective zwitterion–anion interactions simultaneously control salt partitioning and diffusivity, with the permeabilities of NaClO4, NaI, NaBr, NaCl, NaF, and Na2SO4 spanning roughly three orders of magnitude over a wide range of feed concentrations. We model salt flux using a one-dimensional transport model based on the Maxwell–Stefan equations and show that diffusion is the dominant mode of transport for 1:1 sodium salts. Differences in zwitterion–Cl− and zwitterion–F− interactions granted these membranes with the ultrahigh Cl−/F− permselectivity (PCl-/PF- = 24), enabling high fluoride retention and high chloride passage even from saline mixtures of NaCl and NaF.

Ultra‐Fast Click Modification of Self‐Assembled Zwitterionic Copolymer Membranes for Enhanced Ion Selectivity

“…The average permeance of the un‐cross‐linked TCZ‐0 membrane was 5.5 ± 0.9 L m −2 .h −1 .bar −1 . As expected from recent studies, [ 19,41 ] cross‐linking of the hydrophobic domains of P(AM‐ r ‐SBMA)lead to a decrease in the effective pore size, as demonstrated by a decrease in water permeance along with an increase in the rejection of solutes. Increasing UV curing time from 10 to 40 s leads to a permeance decrease of ≈80% compared to the un‐cross‐linked system ( Figure ), with the change plateauing at only ≈30–40 s exposure time.…”

“…As discussed in a recent study, membranes with highly cross‐linked ZAC selective layers exhibit anion selectivity associated with steric effects as well as zwitterion‐ion interactions. [ 19,41 ] Therefore, it is reasonable to expect selectivity between salt ions in the thiol‐ene cross‐linked membranes discussed here. To test this hypothesis, we measured the rejection of various salts, specifically NaCl, MgSO 4 , and Na 2 SO 4 , using 20 mM solutions at 2–4 bar transmembrane pressure ( Figure ).…”

“…In other words, both the size of ions and their affinity to SBMA affect selectivity. [ 41 ] In this case, the difference in trends may arise from differences in cation partitioning into the zwitterionic nanochannels, which also affects sulfate permeability due to electroneutrality. At higher degrees of cross‐linking, magnesium rejection increases due to size exclusion.…”

“…Furthermore, adapting ZAC‐based membranes to exhibit NF‐like selectivity would enable us to circumvent major limitations arising from the inherent chemical structure of interfacially polymerized polyamide NF membranes, including chlorine sensitivity [ 19,20 ] and fouling. [ 19,20,23,41 ]…”

“…However, this process also requires 5–20 min of UV exposure for sufficient cross‐linking. [ 19,41 ] These time scales are not feasible in roll‐to‐roll systems, where residence times on the order of min are not possible within reasonable equipment sizes and operating parameters. Therefore, to truly impact large scale applications, post‐fabrication techniques to tune and enhance membrane performance must occur in time scales measured in seconds, and are simple to apply and execute.…”