Surfactant sheets

The hydrophilic edges and hydrophobic centres of graphene oxide sheets mean they act as surfactants.

Credit: © 2010 ACS

Graphene oxide — a graphene sheet functionalized with oxygen-containing groups — can be produced by chemically exfoliating layers of graphite, and is an intermediate in some methods of producing graphene. It has been known for over 150 years and has been considered to be hydrophilic, because of the ease with which it can be dispersed in water, which is caused by the concentration of the oxygen atoms in carboxylic acid groups at the edge of the sheet. Jiaxing Huang and colleagues from Northwestern University now suggest¹ that graphene oxide is actually amphiphilic, because the main surface of the sheet contains polyaromatic rings.

On investigating this, Huang and colleagues found that the amphiphilic behaviour — in this case, floating on an air–water interface — was only observable after a few hours, because of the sheet's large molecular mass. They could speed this up by using carbonated water: the sheets stick to the bubbles and rise to the surface. Convection flows caused by heating had the same effect. To test whether the amphiphilicity was caused by a hydrophobic central plane, the team studied the effect of sheet size. They found that, as expected, larger sheets were more hydrophobic and floated more readily than smaller sheets. This could be used as a method of size-separating graphene oxide sheets.

Huang and co-workers also found that graphene oxide could act as a colloidal surfactant, by using it to emulsify small droplets of toluene in water, creating a suspension that was stable for months. The amphiphilic behaviour could be tuned by controlling the pH of the solution, because this affects the ionization, and therefore the hydrophilicity, of the carboxylic acid groups on the edge of the sheets.