Graphene oxide membranes show an unusual behavior of water at the nanoscale

More pores in a strainer allow more liquid to flow through? As materials scientists have found, this seemingly simple question could yield an unexpected nanoscale answer — and it could have important implications for the development of water filtration, energy storage, and hydrogen production.

Researchers from UNSW Sydney, University of Duisburg-Essen (Germany), GANIL (France) and Toyota Technological Institute (Japan) experimenting with graphene oxide (GO) membranes have discovered that the opposite can occur at the nanoscopic level. The research, published in nano lettersshows that the chemical environment of the sieve and the surface tension of the liquid play a surprisingly important role in permeability.

The researchers observed that pore density does not necessarily result in higher water permeability — in other words, more tiny holes don’t always let water through at the nanoscale. The study, funded by the European Union and the Humboldt Research Foundation, sheds new light on the mechanisms that control the flow of water through GO membranes.

“As you create more and more holes in a sieve, you expect it to become more water permeable. But surprisingly, that’s the opposite of what happened in our experiments with graphene oxide membranes,” says Associate Professor Rakesh Joshi, senior author of the School of Study Materials Science & Engineering, UNSW Science.

Change in chemical environment

GO is an extremely thin form of carbon that has shown promise as a material for water purification. The chemical compound consists of a single layer of carbon atoms with oxygen and hydrogen atoms attached to them. If you imagine scattering LEGO bricks all over your soil, then the soil would be the carbon atoms and the oxygen and hydrogen atoms would be the LEGO bricks.

In chemistry, molecules can have so-called “functional groups” that are either hydrophobic (water-repellent) or hydrophilic (water-attracting). The pores in graphene can also be hydrophobic or hydrophilic.

“Surprisingly, it is not the number of pores that is more important for water flux (the passage of water through a membrane), but whether the pores are hydrophobic or hydrophilic,” says Tobias Foller, UNSW Scientia Ph.D. Candidate and lead author of the study. “This is very unexpected since the GO layers are only one atom thick. You expect the water to just flow through the pores, whether they attract or repel water.”

Despite the presence of many tiny holes in the GO filters used in the research, in the case of hydrophobic pores, they showed complete blockage of water.

“With filters you usually expect more water flow with more holes. But in our case, where we have more holes, the water flow is less and that is due to the chemical nature of the graphene oxide holes, which in this case are water-repellent,” says Prof. Marika Schleberger, co-author of the study from Duisburg.

Unusual effects of surface tension

The researchers also say that surface tension also contributes to how water interacts with the GO pores. Surface tension occurs because molecules like water want to stick to each other. Confined in a small enough space, the bonds between water (cohesion) and surrounding solid surfaces (adhesive force) can act to move the water. This explains how trees can overcome gravity to transport water from their roots through their capillaries to their leaves.

In GO membranes—where the “capillaries” in this case are pores fabricated on a scale of 1 millionth of a millimeter or smaller—the very forces that allow water to climb tree capillaries prevent it from flowing through membrane pores.

“If you enclose water in capillaries that are as small as possible – just a few atoms in size – the water molecules are attracted so strongly that they form a dense network. Undisturbed, this network is so strong that it does not allow the molecules to be released and pass through the sieve, even if the number of pores is increased,” says Mr. Foller.

Ultra-fine screens made of different materials have a wide range of possible uses. The researchers say their findings will help scientists fine-tune liquid transport in atomic sieves and could spur developments such as high-precision water filtration systems.

“By understanding which parameters increase or decrease water flux, we can optimize many possible applications of graphene oxide for water purification, energy storage, hydrogen production, and more,” says Mr. Foller. “We hope that other engineers and scientists can use this new knowledge to improve their own devices and lead to new developments in the future.”