Role of Aryl Amphiphile Hydrophobe Size on the Concentration and Stability of Graphene Nanoplatelet Dispersions

Graphene nanoplatelets (GNPs) are stable and relatively inexpensive compared to single-layer graphene sheets and carbon nanotubes and are useful in diverse electronic, optoelectronic, and mechanical applications. Solution-state processing of the active material is desired in most of the applications mentioned above, and thus, there is great interest in increasing the concentration and stability of GNP suspension. Herein, to elucidate the role of the stabilizer structural parameters on the concentration and stability of GNP dispersions, we synthesized and used a series of aryl amphiphiles (ArAs) of varying aryl hydrophobe sizes and geometries. The ArAs were found to generate GNP dispersions with concentrations ranging from 0.05 to 0.13 mg mL^-1 depending on the size of the aryl hydrophobe. The ArAs' hydrophobe size played a key role in determining the concentration of GNP suspension, while ArAs' critical aggregation concentration and solubility limits had no impact on the GNP suspension concentration. Most of the studied ArAs work similar to methylcellulose, the previously reported best performing stabilizer . Moreover, the ArAs stabilized the GNP suspension better than methylcellulose and were able to form stable dispersions for up to 6 h. Raman studies indicate that the quality of the GNPs did not degrade during the dispersion process. These findings will aid in the development of design rules for next-generation stabilizers.

Initial Studies Directed toward the Rational Design of Aqueous Graphene Dispersants

This study presents preliminary experimental data suggesting that sodium 4-(pyrene-1-yl)butane-1-sulfonate (PBSA), 5, an analogue of sodium pyrene-1-sulfonate (PSA), 1, enhances the stability of aqueous reduced graphene oxide (RGO) graphene dispersions. We find that RGO and exfoliated graphene dispersions prepared in the presence of 5 are approximately double the concentration of those made with commercially available PSA, 1. Quantum mechanical and molecular dynamics simulations provide key insights into the behavior of these molecules on the graphene surface. The seemingly obvious introduction of a polar sulfonate head group linked via an appropriate alkyl spacer to the aromatic core results in both more efficient binding of 5 to the graphene surface and more efficient solvation of the polar head group by bulk solvent (water). Overall, this improves the stabilization of the graphene flakes by disfavoring dissociation of the stabilizer from the graphene surface and inhibiting reaggregation by electrostatic and steric repulsion. These insights are currently the subject of further investigations in an attempt to develop a rational approach to the design of more effective dispersing agents for rGO and graphene in aqueous solution.

Formation of Two-Dimensional Micelles on Graphene: Multi-Scale Theoretical and Experimental Study

Graphene and related two-dimensional (2D) materials possess outstanding electronic and mechanical properties, chemical stability, and high surface area. However, to realize graphene's potential for a range of applications in materials science and nanotechnology there is a need to understand and control the interaction of graphene with tailored high-performance surfactants designed to facilitate the preparation, manipulation, and functionalization of new graphene systems. Here we report a combined experimental and theoretical study of the surface structure and dynamics on graphene of pyrene-oligoethylene glycol (OEG) -based surfactants, which have previously been shown to disperse carbon nanotubes in water. Molecular self-assembly of the surfactants on graphitic surfaces is experimentally monitored and optimized using a graphene coated quartz crystal microbalance in ambient and vacuum environments. Real-space nanoscale resolution nanomechanical and topographical mapping of submonolayer surfactant coverage, using ultrasonic and atomic force microscopies both in ambient and ultrahigh vacuum, reveals complex, multilength-scale self-assembled structures. Molecular dynamics simulations show that at the nanoscale these structures, on atomically flat graphitic surfaces, are dependent upon the surfactant OEG chain length and are predicted to display a previously unseen class of 2D self-arranged "starfish" micelles (2DSMs). While three-dimensional micelles are well-known for their widespread uses ranging from microreactors to drug-delivery vehicles, these 2DSMs possess the highly desirable and tunable characteristics of high surface affinity coupled with unimpeded mobility, opening up strategies for processing and functionalizing 2D materials.