Graphene’s love affair with water

14 February 2014

  Graphene is hydrophobic – it repels water – but narrow capillaries made from graphene vigorously suck in water allowing its rapid permeation, if the water layer is only one atom thick – that is, as thin as graphene itself. One-atom-wide graphene capillaries can now be made easily and cheaply by piling layers of graphene oxide – a derivative of graphene – on top of each other. The resulting multilayer stacks (laminates) have a structure similar to nacre (mother of pearl), which makes them also mechanically strong.

  Researchers at the University of Manchester led by Dr Rahul Nair and Prof Andre Geim have tested how good such graphene membranes are as filters for liquid water. They report that, if immersed in water, the laminates become slightly swollen but still allow ultrafast flow of not one but two monolayers of water.

  Small salts with a size of less than nine Angstroms can flow along but larger ions or molecules are blocked. Ten Angstroms is equivalent to a billionth of a meter.

  “Our ultimate goal is to make a filter device that allows a glass of drinkable water made from seawater after a few minutes of hand pumping..." Dr Irina Grigorieva, University of Manchester.

  The graphene filters have an astonishingly accurate mesh that allows them to distinguish between atomic species that are only a few percent different in size. On top of this ultraprecise separation, it is also ultrafast. Those ions that can go through do so with such a speed as if the graphene membranes were an ordinary coffee filter.

  The latter effect is due to a property that the Manchester scientists call “ion sponging”. Their graphene capillaries suck up small ions as powerful hoovers leading to internal concentrations that can be hundreds of times higher than in external salty solutions.

  Dr Nair said: “The water filtration is as fast and as precise as one could possibly hope for such narrow capillaries. Now we want to control the graphene mesh size and reduce it below nine Angstroms to filter out even the smallest salts like in seawater. Our work shows that it is possible.”

  Dr Irina Grigorieva, a co-author of the study, added: “Our ultimate goal is to make a filter device that allows a glass of drinkable water made from seawater after a few minutes of hand pumping. We are not there yet but this is no longer science fiction”.

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