February 14, 2014
Mystery Behind Water’s Role In Graphene Finally Tapped
April Flowers for redOrbit.com - Your Universe Online
With a vast range of unique properties, graphene has proven itself a wonder material. Among these properties, graphene's strange love affair with water has remained a mystery until now.
Graphene repels water, known as being hydrophobic. However, narrow capillaries made from graphene will vigorously suck in water, allowing for rapid permeation, that is if the water layer is the same thickness as the graphene itself—only one atom thick.
This bizarre contradictory property has attracted attention from both the academic world and industry with the intent to develop new water filtration and desalination technologies.
By piling layers of graphene oxide—a derivative of graphene—on top of each other, one-atom-wide graphene capillaries can be made easily and cheaply. This process results in multilayered stacks (laminates), which have a structure similar to nacre (mother of pearl). This makes the laminates mechanically strong.
In a 2012 study, University of Manchester researchers discovered that thin membranes made from such laminates were impermeable to all gases and vapors—except water. For example, helium is the hardest gas to block off, but it cannot pass through graphene's membranes. Water, however, still can.
The same team of researchers, led by Dr Rahul Nair and Prof Andre Geim, has conducted a new study to investigate how well the graphene membranes work as filters for liquid water. Their findings were described in a recent issue of Science.
The findings show that if the laminates are immersed in water, they become slightly swollen but still allow ultrafast flow of two monolayers of water.
The laminates will allow small salts with a size of nine Angstroms—less than one billionth of a meter—or less to flow with the water, but larger ions or molecules are blocked. An astonishingly accurate mesh allows the graphene filters to distinguish between atomic species that are only a few percent different in size.
Not only is this separation ultraprecise, it is ultrafast as well. The ions that are allowed to pass through do so with as much speed as if the graphene filters were ordinary coffee filters. This ultrafast effect is due to a property called "ion sponging." The graphene capillaries suck up small ions as powerful vacuums leading to internal concentrations. These concentrations 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 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."