Permeation through nanometer pores is important in the design of materials for filtration and separation techniques and because of unusual fundamental behavior arising at the molecular scale. We found that submicrometer-thick membranes made from graphene oxide can be completely impermeable to liquids, vapors, and gases, including helium, but these membranes allow unimpeded permeation of water (H 2 O permeates through the membranes at least 10 10 times faster than He). We attribute these seemingly incompatible observations to a low-friction flow of a monolayer of water through two-dimensional capillaries formed by closely spaced graphene sheets. Diffusion of other molecules is blocked by reversible narrowing of the capillaries in low humidity and/or by their clogging with water.

/pdf/unimpeded-permeation-of-water-through-helium-leak-tight-50chaotsy2.pdf

Unimpeded permeation of water through helium-leak-tight graphene-based membranes.

Membranes have an increasingly important role in alleviating water scarcity and the pollution of aquatic environments. Promising molecular-level design approaches are reviewed for membrane materials, focusing on how these materials address the urgent requirements of water treatment applications.

Materials for next-generation desalination and water purification membranes

Design strategies for water-soluble small molecular chromogenic and fluorogenic probes.

A method of fabricating ultrathin (≈22–53 nm thick) graphene nanofiltration membranes (uGNMs) on microporous substrates is presented for efficient water purification using chemically converted graphene (CCG). The prepared uGNMs show well packed layer structure formed by CCG sheets, as characterized by scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. The performance of the uGNMs for water treatment was evaluated on a dead end filtration device and the pure water flux of uGNMs was high (21.8 L m−2 h−1 bar−1). The uGNMs show high retention (>99%) for organic dyes and moderate retention (≈20–60%) for ion salts. The rejection mechanism of this kind of negatively charged membranes is intensively studied, and the results reveal that physical sieving and electrostatic interaction dominate the rejection process. Because of the ultrathin nature of uGNMs, 34 mg of CCG is sufficient for making a square meter of nanofiltration membrane, indicating that this new generation graphene-based nanofiltration technology would be resource saving and cost-effective. The integration of high performance, low cost, and simple solution-based fabrication process promises uGNMs great potential application in practical water purification.

Ultrathin Graphene Nanofiltration Membrane for Water Purification

Membranes with unprecedented solvent permeance and high retention of dissolved solutes are needed to reduce the energy consumed by separations in organic liquids We used controlled interfacial polymerization to form free-standing polyamide nanofilms less than 10 nanometers in thickness, and incorporated them as separating layers in composite membranes Manipulation of nanofilm morphology by control of interfacial reaction conditions enabled the creation of smooth or crumpled textures; the nanofilms were sufficiently rigid that the crumpled textures could withstand pressurized filtration, resulting in increased permeable area Composite membranes comprising crumpled nanofilms on alumina supports provided high retention of solutes, with acetonitrile permeances up to 112 liters per square meter per hour per bar This is more than two orders of magnitude higher than permeances of commercially available membranes with equivalent solute retention

http://science.sciencemag.org/content/sci/348/6241/1347.full.pdf

Sub–10 nm polyamide nanofilms with ultrafast solvent transport for molecular separation

Chemical, petrochemical, energy, and environment-related industries strongly require high-performance nanofiltration membranes applicable to organic solvents To achieve high solvent permeability, filtration membranes must be as thin as possible, while retaining mechanical strength and solvent resistance Here, we report on the preparation of ultrathin free-standing amorphous carbon membranes with Young's moduli of 90 to 170 gigapascals The membranes can separate organic dyes at a rate three orders of magnitude greater than that of commercially available membranes Permeation experiments revealed that the hard carbon layer has hydrophobic pores of ~1 nanometer, which allow the ultrafast viscous permeation of organic solvents through the membrane

https://science.sciencemag.org/content/sci/335/6067/444.full.pdf

Ultrafast viscous permeation of organic solvents through diamond-like carbon nanosheets

Thin-film composite membranes comprising a polyamide nanofilm separating layer on a support material are state of the art for desalination by reverse osmosis. Nanofilm thickness is thought to determine the rate of water transport through the membranes; although due to the fast and relatively uncontrolled interfacial polymerization reaction employed to form these nanofilms, they are typically crumpled and the separating layer is reported to be ≈50-200 nm thick. This crumpled structure has confounded exploration of the independent effects of thickness, permeation mechanism, and the support material. Herein, smooth sub-8 nm polyamide nanofilms are fabricated at a free aqueous-organic interface, exhibiting chemical homogeneity at both aqueous and organic facing surfaces. Transfer of these ultrathin nanofilms onto porous supports provides fast water transport through the resulting nanofilm composite membranes. Manipulating the intrinsic nanofilm thickness from ≈15 down to 8 nm reveals that water permeance increases proportionally with the thickness decrease, after which it increases nonlinearly to 2.7 L m-2 h-1 bar-1 as the thickness is further reduced to ≈6 nm.

/pdf/water-transport-through-ultrathin-polyamide-nanofilms-used-53od9vk86z.pdf

Water Transport through Ultrathin Polyamide Nanofilms Used for Reverse Osmosis.

wileyonlinelibrary.com Existing state of the art polymeric membranes are either integrally skinned asymmetric (ISA) or thin fi lm composite (TFC) membranes. [ 2 ] ISA membranes are produced by the phase inversion technique which leads to a dense separation layer a few hundred nanometres thick being formed on a highly porous support structure several microns in thickness. Of the ISA membranes, crosslinked polyimide (PI) membranes are probably the most widely used for OSN due to their ease of fabrication, high mechanical strength, and high stability even in harsh solvents such as tetrahydrofuran or dimethylformamide. [ 3–5 ] However, physical aging and compaction remains a challenge for ISA membranes, resulting in permeance reduction over time in operation, often by more than 50% in a few days. [ 6 ]

Santanu Karan

Papers

Sub–10 nm polyamide nanofilms with ultrafast solvent transport for molecular separation

Ultrafast viscous permeation of organic solvents through diamond-like carbon nanosheets

Water Transport through Ultrathin Polyamide Nanofilms Used for Reverse Osmosis.

Ultrathin Polymer Films with Intrinsic Microporosity: Anomalous Solvent Permeation and High Flux Membranes

Ultraselective and Highly Permeable Polyamide Nanofilms for Ionic and Molecular Nanofiltration