GRAPHENE OXIDE membranes were once thought to be permeable only to aqueous solutions, but researchers in Manchester, UK, have developed a new form of graphene oxide membrane that can filter organic solvents.
Rahul Nair from the National Graphene Institute at the University of Manchester led the team, which included researchers from Manchester’s school of chemical engineering and analytical science, and the school of materials science and engineering. Researcher Yang Su said that the new membranes, with precise sieve size and high flux rates, offer new opportunities for separating molecules from organic solvents. Existing polymer membranes are unstable in organic solvents, unlike the graphene oxide membranes, which are stable. The development could open up the use of graphene-based membranes to organic solvent nanofiltration, not just seawater desalination.
Nair explained to The Chemical Engineer that the properties of the membrane come from its unique laminar structure. Generally, graphene oxide membranes are only permeable to water molecules but to make them permeable to organic solvents, Nair and the team begin with graphene oxide flakes that are larger than usual. During the vacuum filtration process to make the membranes, the large individual flakes form an ultrathin but very layered structure.
“In each layer, there are many pinholes. Below a critical thickness, these pinholes pierce through the membrane and hence we do not get any sieving effect. However, after a critical thickness of 8 nm (eight layers of graphene oxide), these pinholes are bridged by 1 nm wide graphene nanochannels, which provide an atomic-scale sieve,” said Nair.
Solvent can then pass through the filter, but solute molecules are left behind. Because the membrane is so thin, the solvent flow rate through it is high.
The researchers tested the membrane by filtering dye molecules as small as 1 nm in size dissolved in organic solvents. The filtered solvents were pure with no dye molecules remaining. In a fun “Friday night experiment”, they even attempted to filter whisky.
“The membrane allowed the alcohol to pass through but removed the larger molecules, which gives the amber colour. The clear whisky smells similar to the original whisky,” said Nair.
Nair said the membrane’s properties could allow for a number of applications in the chemical pharmaceutical and petrochemical industries.
“For example, in pharmaceuticals, adequate high-quality separation and concentration processes are critical to obtain very high purity products from dilute suspensions, which currently add most of the production cost of therapeutic molecules. More efficient separation membranes could significantly bring the cost down,” he said.
Nature Materials doi.org/cf75
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