No abstract

Membranes that can separate compounds based on molecular properties can revolutionize the chemical and pharmaceutical industries. This study reports membranes capable of separating organic molecules of similar size based on their electrostatic charge. These membranes feature a network of carboxylate-functionalized 1-3 nm nanochannels, manufactured by a simple, scalable coating process: a porous support is coated with a packed array of polymer micelles in alcohol, formed by the self-assembly of a water-insoluble random copolymer with fluorinated and carboxyl functional repeat units. The interstices between these micelles serve as charged nanochannels through which water and solutes can pass. The negatively charged carboxylate groups lead to high separation selectivities between organic solutes of similar size but different charge. In single-solute diffusion experiments, neutral solutes permeate up to 263 times faster than negatively charged compounds of similar size. This selectivity is further enhanced in experiments with mixtures of these solutes. No permeation of the anionic compound was observed for over 24 h. In filtration experiments, these membranes separate anionic and neutral organic compounds while exhibiting water fluxes comparable to that of commercial membranes. Furthermore, carboxylate groups can be functionalized, creating membranes with nanopores with customizable functionality to enable a broad range of selective separations.

Chemical separations account for about 50% of costs and energy use associated with chemical and petrochemical manufacturing, corresponding to about 10% of all energy use in the U.S. Membrane separations are highly energy efficient, simple to operate, scalable, and portable. Broader use of membranes is limited by the selectivity of available membranes, mostly confined to the separation of species about an order of magnitude or more different in size in the liquid phase. This perspective focuses on new approaches for creating liquid filtration membranes that can perform more challenging separations. We first discuss the selectivity mechanisms of currently available membranes and compare them with the operation of biological systems that exhibit enhanced selectivity. Then, we review some approaches for creating isoporous membranes with narrow pore size distributions for enhanced size-based selectivity. We discuss biological systems that exhibit selectivity based on factors beyond size and how they can inspire the design of membranes capable of complex separations. After a review of approaches for creating membranes for separating similarly sized solutes, based on their charge, we discuss the development of membranes that can perform even more challenging separations, differentiating between solutes of similar size and charge based on other molecular criteria. This burgeoning area of research promises to transform chemical and pharmaceutical manufacturing if membranes with sufficient selectivity and permeability for realistic separations can be prepared using scalable manufacturing methods.

Separating oil–water mixtures is a common obstacle in many processes from wastewater treatment to biofuel manufacture to cleanup of oil spills. There is an urgent need for new, fast, and simple technologies for such separations. In this work, we describe a simple and practical route for creating superoleophilic electrospun membranes that are capable of selectively passing oil and organic compounds at very high rates in a gravity-driven system while retaining water. To prepare these membranes, we blended a new, highly fluorinated random copolymer (FCP), poly­(methyl methacrylate-random-perfluorodecyl methacrylate), P­(MMA-r-FDMA), with the commodity polymer poly­(vinylidene fluoride) (PVDF) and prepared electrospun membranes from their mixture. Membranes composed of nonwoven fibers with uniform and bead-free morphology were obtained upon electrospinning of PVDF blended with this FCP. The PMMA segments provided anchors to the PVDF matrix, resulting in significant enhancement in the mechanical properties with up to 7 times higher Young’s modulus for the blend membranes. Moreover, the self-organization of the long, pendant FDMA side groups within the PVDF matrix resulted in fluorine-rich, highly hydrophobic and superoleophilic surface. As a result, the FCP-containing membranes exhibited up to 17 times faster permeation of oil and organic solvent, compared with pure PVDF membrane in gravity-driven filtration experiments. Their performance was highly stable during a 70 min continuous gravity-driven filtration experiment for oil/water separation, reflecting their excellent fouling resistant properties. This easy-to-implement and cost-effective approach, combined with the high porosity and re-entrant structure created by the electrospinning, can create membranes with excellent mechanical properties and fouling resistance.

No abstract