Nanostructured Membranes

Nanostructured Membranes

This technology enables the production of thin polymer membranes that are unique due to their precisely controlled nanometer-scale internal structure. Precise nanostructure control at such length scales opens up new possibilities for affordable and high-performance water purification, chemical separations, drug delivery, and a host of other applications.

Capabilities

Providing access to potable water is among the most critical challenges of our time. Membrane filtration provides an effective way of clean water production, but the current performance is limited by a trade-off between permeability and selectivity, due to the uncontrolled sieving structure of traditional fabrication technologies. This new technology provides a thin membrane that regulates the solvent transport through a precisely defined nanostructure to provide separation with improved permeability and selectivity.

Membranes based on self-assembled materials entail the use of nanostructures with near-monodisperse feature sizes. Therefore, this class of material has been considered an attractive way to realize highly selective separations without compromising permeability. In this regard, nanostructured materials with precisely defined and water-bicontinuous 1 nanometer-scale pores are highly sought after as advanced materials for next-generation filtration membranes. Such fabrication, however, represents a decidedly non-trivial challenge that has persisted for many years.

Lyotropic liquid crystal mesophase (i.e., a system that self-assembles into an ordered structure in solution) can address the aforementioned issues. In a manner similar to the formation of the lipid bilayer in biological systems, an appropriate surfactant precursor can be dissolved in a polar solvent system to generate a molecularly designed supramolecular matrix. Using this design principle, we use a quaternary ammonium functionalized building block to target a hexagonally ordered array of ~3 nm solid cylinders embedded in a continuous aqueous medium.

In case a non-volatile solvent such as glycerol is involved, it is possible to prepare membranes as thin as ~ 200 nm using solution-based methods. Interestingly, fabricating membranes as thin-films guarantees an efficient packing of cylinders on the surface to follow a parallel orientation, hence contributing to a more selective filtration membrane. We show the liquid crystal membranes exhibit high selectivity to organic solutes with a molecular weight higher than 300 g mol-1, effectively rejecting dissolved salts, and in particular, divalent species, while exhibiting water permeability 2 L m-2 h-1 bar-1 μm that rival or exceed the current state-of-the-art commercial membranes. We suggest these materials have the ability to address a broad range of separation-related applications, such as nanofiltration, reverse-osmosis pretreatment, membrane-based bio-reactor, or water softening.

Nanostructured membranes assembly

(Left) Schematic showing the arrangement of cylindrical nanostructures in a thin membrane sitting on top of a mechanical support membrane that has larger openings. The image on the left shows an image of a circular composite membrane (thin selective layer mounted on thicker mechanical support). (Right) Steps in the fabrication of the composite membrane. i) A solution of the self-assembling liquid crystal material is deposited on a sacrificial layer (PVP) on top of a mechanical support (ultrafiltration support). ii) The solvent is removed, and the material is cross-linked by UV light. iii) The sacrificial PVP layer is dissolved away using water.


Nanostructured Membranes filtration examples

(Left) Membrane operation in gas-pressure driven dead-end filtration stirred cells. The solutions in the test cells contain molecular dyes that are responsible for the red and blue color. The liquid produced after passage through the membranes in the cells, i.e. the permeate, is colorless as the dye molecules cannot pass through the small spaces in the nanostructure of the membrane. (Right) Nanofiltered whiskey. The left vial contains whiskey. The middle vial shows permeate from a membrane with approximately 1.3 nm pores. The right vial shows permeate from a membrane made with approximately 1 nm pores. The small difference in the pore size is precisely controlled by the composition of the liquid crystal from which the membrane is derived, and results in partial (middle) versus complete (right) removal of the colorants in the whiskey.

Challenges

  • The current system is processed by UV-induced photo-crosslinking that may pose a limitation to potential roll-to-roll membrane fabrications.
  • The process scale performance study of this new class of membrane, such as its fouling propensity or typical device life-span, is required.

References

X. Feng, Q. Imran, Y. Zhang, L. Sixdenier, X. Lu, G. Kaufman, U. R. Gabinet, K. Kawabata, M. Elimelech, C. O. Osuji. Precise nanofiltration in a fouling-resistant self-assembled membrane with water-continuous transport pathways. Science Advances, 5 (8), eaav9308 (2019). DOI: 10.1126/sciadv.aav9308

J. R. Werber, C. O. Osuji, M. Elimelech. Materials for next-generation desalination and water purification membranes. Nature Reviews Materials, 16018 (2016).

Richard W. Baker. Membrane Technology and Applications, 2nd ed. John Wiley & Sons (2004). DOI: 10.1002/0470020393

Y-Prize Kickoff presentation (Jan. 19, 2021)

Y-Prize Tech Briefing (Jan. 26, 2021)

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