Amphiphilic Polyampholytes for Fouling-Resistant and Easily Tunable Membranes

Abstract

The versatility of membranes is limited by the narrow range of material chemistries on the market, which cannot address many relevant separations. Expanding their use requires new membrane materials that can be tuned to address separations by providing the desired selectivity and robustness. Self-assembly is a versatile and scalable approach to create tunable membranes with a narrow pore size distribution. This study reports the first examples of a new class of membrane materials that derives stateof-the-art permeability, selectivity, and fouling resistance from the self-assembly of random polyampholyte amphiphilic copolymers. These membranes feature a network of ionic nanodomains that serve as nanochannels for water permeation, framed by hydrophobic nanodomains that preserve their structural integrity. This copolymer design approach enables precise selectivity control. For example, sodium sulfate rejections can be tuned from 5% to 93% with no significant change in the pore size or fouling resistance. Membranes developed here have potential applications in wastewater treatment and chemical separations.

Amphiphilic Polyelectrolyte Complexes for Fouling-Resistant and Easily Tunable Membranes

“…All APEC membranes, B-, B0, and B+ displayed exceptional fouling resistance, demonstrated by no flux decline during the fouling test and after water rinse. In contrast, the commercial NF membrane irreversibly lost half of its initial flux upon exposure to the oil emulsion (Figure ), consistent with previous studies using various commercial membranes. ,,, This loss was not recovered by a water rinse, indicating a chemical cleaning process would be required for any flux recovery.…”

“…The copper ions are expected to only diffuse across the water-permeable ionic domains. This method of free-standing film staining followed by sectioning and TEM analysis has been utilized in several studies using similar amphiphilic copolymers. , The brightfield TEM image of B0 is shown in Figure S3. In agreement with previous studies, , the image shows a disordered bicontinuous morphology, with the dark regions representing the percolated network of the water permeable ionic channels stained with copper ions, and the light regions representing the continuous network of hydrophobic domains.…”

“…16 r-PACs with an overall zero net charge (i.e., equal numbers of anionic and cationic monomers) were very similar to r-ZACs in terms of their self-assembly, pore size, and fouling resistance, especially when the anionic and cationic functional groups in the two polymers were similar. 16 These findings support the hypothesis that ion−ion interactions are key to driving the self-assembled morphologies, whether the charged groups are located on the same monomer unit or not. It also opens the door to a range of charged ion combinations to control membrane selectivity.…”

Salt-Screened Transition toward Bulk-Like Water Dynamics near Polymeric Zwitterions

“…Polymeric zwitterions are a promising class of new materials defined by a stoichiometrically balanced set of positive and negative charges, tethered to a polymer backbone. The unique combination of high hydrophilicity and overall charge neutrality provides exceptional and nonspecific fouling resistance across a wide range of length scales appropriate for a diverse range of applications. − Ultralow protein binding makes these materials undetectable by the immune system, offering enhanced biocompatibility for implants and long-lasting drug formulations. , Sustained hydration, even in the presence of high salt concentrations, enables the design of environmentally benign marine antifouling coatings for ocean vessels. , Demonstrated high water permeability and metal ion selectivity in zwitterion functionalized membranes suggest potential for processing of industrial produced water streams. − The performance and design of zwitterionic materials for each of these applications is directly related to the modulation of ion-dipole interactions between the zwitterion and surface water molecules, which act as a thermodynamic barrier to foulant surface adsorption. Thus, understanding of the structure and dynamics of water molecules at the zwitterion surface is critical to the development of zwitterionic materials for targeted applications.…”

“…Zwitterion selectivity toward specific ion types has previously been reported in simulations and experiments. , Further study of salt interaction effects on water dynamics is necessary to potentially use zwitterionic groups as powerful tools to mitigate salt screening and provide improved ion selectivity in separation processes …”