Predicting Effective Solvents for Graphene Stabilization in Nonaqueous Dispersions*

Solvents play an essential in the liquid phase exfoliation (LPE) of two-dimensional (2D) materials. The concentration of monolayers in solution after the exfoliation process depends critically on the chosen solvent [1]. The physical origin of this dependence is difficult to determine experimentally. Screening for effective solvents currently relies on matching solubility parameters between the solvent and 2D material. This makes the approximation that thermodynamic effects alone determine the nanosheet concentration. It is quite a blunt tool, with some notable failures [2].
Here, density functional theory and classical molecular dynamics calculations are used to show that energetic effects alone will not be sufficicent as a screening descriptor, and instead explicit interactions at the interface between the solvent and solute play a critical role. We find that distinct solvation layers form around the nanosheet with molecular-level structural and orientational ordering. These interfacial solvent layers exhibit extremely low dielectric constants due to their surface confinement irrespective of the polar nature of solvent.
Kinetic effects, and in particular the diffusion behaviour of these solvation shells parallel to the graphene sheet, are then reliable predictive descriptors determining the stability of graphene monolayers in solution. Solvents with high diffusion coefficients lead to enhanced layer reaggregation. Solvents with smaller diffusion coefficients correspond to higher experimental graphene concentrations. In the low diffusion limit however, this relationship breaks down. We suggest that here the concentration of graphene in solution depends primarily on the separation efficiency of the initial LPE step.

[1] Hernandez et al., Nature Nanotech 3, 563 (2008)
[2] Hernandez et al., Langmuir 26, 3208 (2010)