Decoupling rheology from particle concentration by charge modulation: Aqueous graphene-clay dispersions

Hypothesis: Aqueous graphene dispersions are usually obtainable by treating the surface of graphene chemically or physically. In these dispersions, the rheological properties (e.g., viscosity) are governed by a direct coupling to the graphene concentration, which limits their applicability. An alternative approach for dispersing graphene is trapping them in a viscoelastic-network formed by a co-dispersed charged fibrous-clay, Sepiolite. Contrary to surface treatment, the rheological properties of these dispersions are set by the clay particles. The rheology of charged-colloidal dispersions is governed by various parameters, including interparticle interactions. Hence, the rheology of the dispersion could be modulated by changing the clay surface charge without compromising the dispersed graphene concentration.Experimental: The surface charge of Sepiolite was modulated either by charge-screening (by NaCl added to the solution) or by surface-charging (by attachment of highly charged ions, e.g., HexaMetaPhosphate, HMP-) and the effect on rheology and graphene concentration was assessed. In particular, loading the dispersion with HMP- yielded low viscosity, storage, and loss moduli (two orders of magnitude lower than the corresponding HMP-- free dispersion) while the graphene concentration was maintained. We demonstrate that by this charge- modulation approach, reaching the rheological requirements of different applications without compromising on graphene concentration is plausible.