2, University of Virginia, Charlottesville, Virginia, United States
Liquid-phase exfoliation of graphene from graphite is an economical and increasingly popular method of graphene production. Shear exfoliation processes have achieved some of the highest production rates of few-layer graphene and show great promise for industrial scale up. In addition to manufacturing graphene for use by itself, the systems of graphene dispersed in liquid directly produced by exfoliation have great potential as (multi-)functional fluids, inks for circuit and/or 3D printing, and can be an attractive precursor to polymer nanocomposites. Successful realization of these applications depends on the colloidal stability of these systems over practical time scales and under mechanical stresses experienced during processing and use.
However, attempts at predicting and explaining the stability of graphene dispersions in a variety of liquids are still limited. Much early work on the exfoliation and dispersion of graphene focused on matching the surface energy of graphene/graphite to that of the exfoliating or dispersing liquid. Recent work suggests that while surface tension may be appropriate for exfoliation, liquids with similar surface tension have been found to have different dispersibilities of graphene. This has led new research to consider the use of Hansen solubility parameters for predicting the dispersibility of graphene in liquids, which allows for the separate consideration of different interactions. However, it is hard to predict these parameters for arbitrary solvents. The dispersion solubility parameter has been found to be the most important of the three Hansen parameters for the stability of graphene sols, and we propose an alternative approach based on this. In classical colloid theory, the dispersion interaction is predicted by the dielectric constants and refractive indices of the particle and dispersing medium. We propose to study the stability of graphene dispersions in various organic solvents as a function of these physical properties, which are more easily determined than Hansen parameters. We predict that liquids with dielectric constants and refractive indices closer to that of graphite should show improved dispersibility of graphene. Ultraviolet-visual spectroscopy will be used to measure dispersed graphene concentration and changes over time will provide an understanding of storage stability. Rheological measurements will probe the stability of the dispersions under stress and provide information on the strength of interparticle interactions.