Graphene Oxide-Stabilized Oil-in-Water Emulsions: pH-Controlled Dispersion and Flocculation

Oil-in-water emulsions stabilized by graphene oxide (GO) can be flocculated by either an increase or a decrease in pH. At highly acidic pH, fully reversible flocculation of emulsion droplets can be achieved, whereas when adjusted to high pH, the flocculation is irreversible, which we interpret as a permanent chemical change in the GO. We correlate the effectiveness of GO as a stabilizer to its aqueous aggregation state and explore the effects of the GO surface charge in determining emulsion properties. By directly measuring the interaction forces between two emulsion droplets coated in GO using the atomic force microscope, we demonstrate the basis for the pH-dependent flocculation behavior. It is shown that interfacial charge of the GO and oil-water interface is the overriding drive for the exceptional stability of acidic GO emulsions.