Chemical membranes are a vital part of industry. Like tiny sieves they filter out one chemical from another at a molecular scale. Using the driving force of a hydrostatic pressure gradient some components in a solution permeate the membrane, while others do not. The result is a separation technique that can remove unwanted chemicals or extract those that are needed.

But as nanotechnology takes chemistry to a smaller and smaller level, new techniques are needed to separate chemical products, faster.

In response to this need, science has once again turned to nature and copied the nanoscale channels that allow water to flow in and out of cells.

By mimicking the hourglass shape of the biological channels known as aquaporins, nanochemists from KAUST (the King Abdullah University of Science and Technology in Saudi Arabi) have created a structure made of graphene-oxide layers that works as an ultrathin membrane for the quick separation of chemical mixtures.

Using current technology, chemical engineers are able to separate chemical molecules from a solution, but it is a slow process. As Shaofei Wang, a postdoctoral researcher at KAUST explains, “In making pharmaceuticals and other chemicals, separating mixtures of organic molecules is an essential and tedious task.”

The problem is defined by the ‘permeance-rejection trade off’. To allow for a faster flow of solution, chemists could make the holes in the membrane larger, but this will lower the quality as some unwanted molecules will pass through. Making the holes smaller would improve separation performance but would result in a much slower flow of solvent through the membrane.

The ‘permeance-rejection trade off’ solved with an hourglass shaped membrane

Nature had already found a solution, using an hourglass-shaped channel. A tube which is wide at each end but has a narrow hydrophobic central section would allow a fast flow of solvent through the membrane while still maintaining high selectivity.

Schematic of a dual-spacing channel graphene oxide (GO) membranes intercalated with (3-aminopropyl)triethoxysilane [APTES] nanoparticles. 

Inspired by this, the KAUST researchers, including the head of the Nanostructured Polymeric Membrane Laboratory Prof. Suzana Nuñes, Wang and other team members synthesized a membrane that widens and narrows in different places.

(b) Photograph of different membranes. (c) SEM image of the GO–Si2 membrane surface. The inset shows the surface of the nylon support. (d) Cross-sectional SEM image of the GO–Si2 membrane, prepared on the nylon support. The inset shows a lower magnification of the whole membrane cross-section. (e) AFM image of the GO–Si2 membrane.

As the scientific journal, Phys.org describes, “The membrane is made from flakes of a two-dimensional carbon nanomaterial called graphene oxide. The flakes are combined into sheets several layers thick with graphene oxide. Organic solvent molecules are small enough to pass through the narrow channels between the flakes to cross the membrane, but organic molecules dissolved in the solvent are too large to take the same path. The molecules can therefore be separated from the solvent.”

To aid flow team also added spacers between the graphene-oxide layers to widen some parts of the channel without compromising the quality of the process.

These spacers are made of a silcon-based molecule which has been treated with sodium hydroxide to form an in situ silicon-dioxide nanoparticle.  

“The hydrophilic nanoparticles locally widen the interlayer channels to enhance the solvent permeance,” Wang explains.

TEM image from the top surface

The team’s finding have now been published in the Journal of Materials Chemistry A, where they announced their successful results, stating, “Notably, a high methanol permeance of 290 L m−2 h−1 bar−1, and a higher than 90% rejection of dyes with sizes larger than 1.5 nm was reported for the membrane, making it highly attractive for nanofiltration in pharmaceutical processes.”

Cross sectional SEM image of GO–Si2 membranes. The orange arrows denote the SiO2 nanoparticles

At the same time the flow of solvent was increased 10-fold.

Additionally, the silica nanoparticles increased the durability and strength of the membranes.

The researchers hope that their discovery can be further developed to make even more efficient membranes, as well as for real world use in other applications, such as drug delivery, catalysis, and adsorption.

As Nuñes notes, “The next step will be to formulate the nanoparticle graphene-oxide material into hollow-fiber membranes suitable for industrial applications.”

Given the importance of chemical membranes for all manner of processes throughout the chemical industry, any advancement that improves speed or efficiency could be very valuable. By developing a nanoscale chemical membrane that is both more efficient and faster the KAUST research team have truly created something exceptional.


Title picture: A nanoscale membrane created at KAUST by Suzana Nunes and her team during a 2017 work

Photo credit: Phys.org, & Journal of Materials Chemistry A