Graphene Oxide Sheets at Interfaces

TLDR

Graphene oxide, produced by chemical exfoliation of graphite, has been known for over a century and is generally considered hydrophilic due to its colloidal stability. The study reports that graphene oxide is amphiphilic, with hydrophilic edges and a hydrophobic basal plane. The amphiphilicity arises from the molecule‑colloid duality of GO sheets and can be tuned by pH through ionization of edge carboxyl groups. GO functions as a surfactant, reducing interfacial tension, forming stable Pickering emulsions, dispersing insoluble materials, and its amphiphilicity is tunable by pH and size.

Abstract

Graphite oxide sheet, now called graphene oxide (GO), is the product of chemical exfoliation of graphite and has been known for more than a century. GO has been largely viewed as hydrophilic, presumably due to its excellent colloidal stability in water. Here we report that GO is an amphiphile with hydrophilic edges and a more hydrophobic basal plane. GO can act like a surfactant, as measured by its ability to adsorb on interfaces and lower the surface or interfacial tension. Since the degree of ionization of the edge -COOH groups is affected by pH, GO's amphiphilicity can be tuned by pH. In addition, size-dependent amphiphilicity of GO sheets is observed. Since each GO sheet is a single molecule as well as a colloidal particle, the molecule-colloid duality makes it behave like both a molecular and a colloidal surfactant. For example, GO is capable of creating highly stable Pickering emulsions of organic solvents like solid particles. It can also act as a molecular dispersing agent to process insoluble materials such as graphite and carbon nanotubes in water. The ease of its conversion to chemically modified graphene could enable new opportunities in solution processing of functional materials.