The Adsorption of Surfactant Exfoliated Graphene at Interfaces
Creators: name: Sham, Alison Yue Wan
Publisher: name: The Australian National University
Year: 2017
Subjects: graphene
ultrasonic exfoliation
surfactant
adsorption
interfaces
Pluronic F108
polyacrylic acid
polyethyleneimine
quartz crystal microbalance
atomic force microscopy
electrostatic
hydrogen bonding
multilayer films
LbL
Layer-by-Layer
qualitative nanomechanical mapping
foams
foam stability
SDS
CTAB
methylene blue
methyl red
organic dyes

description: Graphene, a single layer of highly ordered carbon atoms, has captured tremendous interest within academia and industry recently. This is due to its unique, two-dimensional structure which yields a combination of outstanding properties ideal for enhancing the performance of existing applications and enabling future disruptive technologies. Nevertheless, the production of graphene particles devoid of structural defects that can be easily integrated and used in products and applications on a commercial scale has proved exceptionally challenging. The first part of this problem can be addressed using the ultrasonic exfoliation of graphene in aqueous surfactant solutions, which offers an established method of producing defect-free graphene suspensions suited to large scale production processes. As the graphene exists in the liquid phase, a variety of techniques are possible for the transfer and integration of the particles into products and applications. Many of these techniques rely on adsorption, which involves the net accumulation of molecules from a liquid at an interface through attractive intermolecular interactions. The intermolecular interactions inherent to surfactant-assisted exfoliated graphene have been used to investigate the prospect of employing such particles in a series of applications governed by adsorption mechanisms. The outcomes of these studies are described here in this thesis. Suspensions of graphene particles were prepared using the ultrasonic exfoliation of graphite, with continuous surfactant addition in the presence of the non-ionic surfactant, Pluronic® F108. The resultant suspensions and particles were characterised using UV-Visible spectroscopy, Raman spectroscopy, and transmission electron microscopy. The prepared graphene suspensions exhibited single and bilayer defect-free particles ranging up to 1 μm in size. Zeta potential measurements indicated the graphene particles exhibited with a low residual negative electrostatic charge originating from edge defects rather than the adsorbed surfactant. Four studies reflecting proof-of-concept applications for surfactant stabilised graphene were then investigated. Each study focused on a different type of intermolecular interaction or interface involved in the underlying adsorption process. In one study, multilayer thin films were sequentially constructed through the electrostatic layer-by-layer (LbL) deposition of Pluronic F108 exfoliated graphene and the cationic polyelectrolyte, polyethyleneimine on silica surfaces. Multilayer assembly was monitored using a quartz crystal microbalance and was shown to be strongly influenced by conditions such as graphene concentration, pH, ionic strength and ionic species. Consequently, it was shown that the thickness of the films could be specified by altering the number of layers deposited, while the viscoelastic properties of the films could be tailored by altering film deposition conditions. Hydrogen-bonded multilayer films consisting of Pluronic F108 exfoliated graphene and the weak polyelectrolyte, polyacrylic acid were also constructed using the LbL technique at low pH. In this study, quartz crystal microbalance measurements and Raman spectra suggested a superlinear film growth regime, whilst atomic force microscopy qualitative nanomechanical mapping measurements indicated that the mechanical properties of the films differed with the number of layers adsorbed. The films also underwent partial deterioration when exposed to aqueous solutions at neutral and basic pH. Thus, the hydrogen-bonded thin films demonstrated a series of features appropriate to functional coatings, such as pH responsiveness, surface roughness and internal film structures, which could be altered depending on deposition conditions and number of layers adsorbed. The third study described foam stabilisation through adsorption of Pluronic F108 exfoliated graphene at the liquid-air interface. Particle surface activity was confirmed through surface tension measurements and was shown using the evolution of bubble size distributions to cause a reduction in destabilisation mechanisms. The addition of alkali metal chlorides also enhanced foam stability by altering the wettability of graphene particles through changes in the hydration of polyethylene oxide groups present on the stabilising surfactant. Thus, the high-aspect ratio particles and adaptable surface interactions possible with Pluronic F108 exfoliated graphene are able to stabilise and enhance bubble surfaces in foams. In the final study, ionic and non-ionic surfactant exfoliated graphene were used to adsorb ionic organic dyes from solution. Dye adsorption was maximised when attractive electrostatic interactions were possible with the graphene particles. In particular, the adsorption of methylene blue, a cationic dye, by anionic SDS exfoliated graphene particles was shown to be exceedingly rapid. This process was shown to be consistent with the pseudo-second order kinetics model and Freundlich adsorption isotherm. Consequently, the surface interactions caused by the presence of surfactant at the graphene particle surface are sufficient to drive the adsorption of dye from solution. These results collectively demonstrate the versatility of surfactant stabilised graphene adsorption at interfaces, which is made possible by the nature of the stabilising surfactant and properties inherent to the 2D graphene lattice. They illustrate the potential for surfactant exfoliated graphene to be used in a variety of applications such as functional and responsive thin films, as well as their ability to act as effective foam stabilisers and carbon-based adsorbents.