Interfacial activity of graphene oxide: Anisotropy, loading efficiency and pH-tunability

In this work, we use Langmuir film-balance techniques and high resolution Brewster angle microscopy (BAM) to investigate the surface activity of graphene oxide (GO) at the air-water interface. We focus on GO loading efficiency and packing at the air-water interface. We position two Wilhelmy plate tensiometers in different locations to simultaneously measure anisotropic forces as well as spatial inhomogenities in GO film coverage. We confirm our results with single sheet resolution images of GO at the air-water interface. Evidence is presented to show that GO does not significantly rearrange itself at the air-water interface over several hours. It is shown using a series of successive loading experiments that the GO surface loading efficiency decreases with each successive experiment. We also provide a detailed study of GO surface coverage with pH cycling of the subphase. In addition to subphase pH and GO sheet size we demonstrate the role of subphase pH history in determining GO surface coverage. Finally, we present a hypothesis to complement existing explanations of GO surface activity. This hypothesis explains the role of acidic pH in imparting surface active character to the GO and how GO is driven to the interface. We reconcile previous reports that while there is significant potential in the pH tunability of GO surface activity, there are also limitations pertaining to dynamics and size-selectivity. Our results present a comprehensive and high resolution perspective on the colloidal sheets packing of GO at the air-water interface and examples of how GO films are sensitive to preparation history.