Carbon-doped metal oxide interfacial nanofilms for ultrafast and precise separation of molecules

Ion and Molecular Sieving With Ultrathin Polydopamine Nanomembranes

In contrast to biological cell membranes, it is still a major challenge for synthetic membranes to efficiently separate ions and small molecules due to their similar sizes in the sub-nanometer range. Inspired by biological ion channels with their unique channel wall chemistry that facilitates ion sieving by ion-channel interactions, the first free-standing, ultrathin (10-17 nm) nanomembranes composed entirely of polydopamine (PDA) are reported here as ion and molecular sieves. These nanomembranes are obtained via an easily scalable electropolymerization strategy and provide nanochannels with various amine and phenolic hydroxyl groups that offer a favorable chemical environment for ion-channel electrostatic and hydrogen bond interactions. They exhibit remarkable selectivity for monovalent ions over multivalent ions and larger species with K+/Mg2+ of ≈4.2, K+/[Fe(CN)6]3- of ≈10.3, and K+/Rhodamine B of ≈273.0 in a pressure-driven process, as well as cyclic reversible pH-responsive gating properties. Infrared spectra reveal hydrogen bond formation between hydrated multivalent ions and PDA, which prevents the transport of multivalent ions and facilitates high selectivity. Chemically rich, free-standing, and pH-responsive PDA nanomembranes with specific interaction sites are proposed as customizable high-performance sieves for a wide range of challenging separation requirements.

Ultrafast Interfacial Self-Assembly toward Bioderived Polyester COF Membranes with Microstructure Optimization

The precise manipulation of the microstructure (pore size, free volume distribution, and connectivity of the free-volume elements), thickness, and mechanical characteristics of membranes holds paramount significance in facilitating the effective utilization of self-standing membranes. In this contribution, the synthesis of two innovative ester-linked covalent-organic framework (COF) membranes is first reported, which are generated through the selection of plant-derived ellagic acid and quercetin phenolic monomers in conjunction with terephthaloyl chloride as a building block. The optimization of the microstructure of these two COF membranes is systematically achieved through the application of three different interfacial electric field systems: electric neutrality, positive electricity, and negative electricity. It is observed that the positively charged system facilitates a record increase in the rate of membrane formation, resulting in a denser membrane with a uniform pore size and enhanced flexibility. In addition, a correlation is identified wherein an increase in the alkyl chain length of the surfactants leads to a more uniform pore size and a decrease in the molecular weight cutoff of the COF membrane. The resulting COF membrane exhibits an unprecedented combination of high water permeance, superior sieving capability, robust mechanical strength, chemical robustness for promising membrane-based separation science and technology.

Photo-Induced Geometry and Polarity Gradients in Covalent Organic Frameworks Enabling Fast and Durable Molecular Separations

Azobenzene, which activates its geometric and chemical structure under light stimulation enables noninvasive control of mass transport in many processes including membrane separations. However, producing azobenzene-decorated channels that have precise size tunability and favorable pore wall chemistry allowing fast and durable permeation to solvent molecules, remains a great challenge. Herein, an advanced membrane that comprises geometry and polarity gradients within covalent organic framework (COF) nanochannels utilizing photoisomerization of azobenzene groups is reported. Such functional variations afford reduced interfacial transfer resistance and enhanced solvent-philic pore channels, thus creating a fast solvent transport pathway without compromising selectivity. Moreover, the membrane sets up a densely covered defense layer to prevent foulant adhesion and the accumulation of cake layer, contributing to enhanced antifouling resistance to organic foulants, and a high recovery rate of solvent permeance. More importantly, the solvent permeance displays a negligible decline throughout the long-term filtration for over 40 days. This work reports the geometry and polarity gradients in COF channels induced by the conformation change of branched azobenzene groups and demonstrates the strong capability of this conformation change in realizing fast and durable molecular separations.

Polyester Nanofiltration Membranes for Efficient Cations Separation

Polyester nanofiltration membranes highlight beneficial chlorine resistance, but their loose structures and negative charge result in poor cations retention precluding advanced use in cations separation. This work designs a new monomer (TET) containing "hydroxyl-ammonium" entities that confer dense structures and positive charge to polyester nanofiltration membranes. The TET monomer undergoes efficient interfacial polymerization with the trimesoyl chloride (TMC) monomer, and the resultant TET-TMC membranes feature one of the lowest molecular weight cut-offs (389 Da) and the highest zeta potential (4 mv, pH: 7) among all polyester nanofiltration membranes. The MgCl2 rejection of the TET-TMC membrane is 95.5%, significantly higher than state-of-the-art polyester nanofiltration membranes (<50%). The Li+ /Mg2+ separation performance of TET-TMC membrane is on par with cutting-edge polyamide membranes, while additionally, the membrane is stable against NaClO though polyamide membranes readily degrade. Thus the TET-TMC is the first polyester nanofiltration membrane for efficient cations separation.