More pores in a sieve let more liquid flow through it. But the opposite can be the case at the nanoscopic level, researchers experimenting with Graphene Oxide (GO) membranes have found.
The research from UNSW Sydney, University of Duisburg-Essen (Germany), GANIL (France) and Toyota Technological Institute (Japan) demonstrates how the chemical environment of the sieve, as well as the surface tension of the liquid, have surprisingly key roles in permeability. In other words, having more tiny holes doesn’t always allow water to flow through at the nanoscale.
The nanoscopic scale (nanoscale) usually refers to structures with a length scale between 1–100 nanometers, which is one billionth of a metre.
Graphene oxide is an extremely thin form of carbon that has shown promise as a material for water purification. The chemical compound comprises a single layer of carbon atoms with oxygen and hydrogen atoms attached.
If you visualize a scattering of LEGO bricks on your floor, then the floor would be the carbon atoms, and the oxygen and hydrogen atoms would be the LEGO bricks.
In chemistry, molecules can have what’s known as “functional groups” that are either hydrophobic (water repelling) or hydrophilic (water-attracting). The pores in graphene can also be hydrophobic or hydrophilic.
“Surprisingly, more important for the water flux (flow of water through a membrane) isn’t the number of pores, but whether the pores are hydrophobic or hydrophilic. That’s very unexpected as the GO layers are only one atom thick. One expects the water to just pass through the pores, no matter if they attract or repel water,”
said lead author Tobias Foller, UNSW Ph.D. candidate.
Regardless of the presence of many minuscule holes in the graphene oxide atomicfilters used in the study, they displayed a complete blockage of water in the case of hydrophobic pores.
“With filters, you usually expect more water flow with more holes. But in our case, where we have more holes, water flow is lower, and that’s due to the chemical nature of the graphene oxide holes which are in this case water-repelling,”
co-author Prof. Marika Schleberger said.
Surface Tension And Tree Roots
The researchers say that surface tension also contributes to the water interaction with the GO pores. Surface tension occurs because molecules, like water, want to stick together.
When confined in a sufficiently small 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 take water from their roots, up their capillaries, to their leaves.
In GO membranes — where the capillaries, in this case, are pores made at the scale of 1 millionth of a millimetre or less — the same forces that allow water to climb tree capillaries prevent it from flowing through membrane pores.
“When you confine water in the smallest possible capillaries, just the size of a few atoms, the water molecules attract themselves so much they form a tight network. Undisturbed, this network is so strong that it doesn’t allow the molecules to be released and pass through the sieve, even if you increase the number of pores,”
said Mr. Foller.
Ultrafine sieves made of different materials have a diverse range of applications. The researchers say optimizing liquid transport in atomic sieves could advance developments such as highly precise water filtration systems, energy storage, and hydrogen production.
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