  Liquid phase exfoliation (LPE) is the most promising method for the low-cost,
scalable production of two-dimensional nanosheets from their bulk counterparts.
Extensive exfoliation occurs in most solvents due to the huge amount of energy
introduced by sonication or shear mixing. However, the subsequent dispersion is
not always stable, with extensive reaggregation occurring in some solvents.
Identifying the optimal solvent for a particular layered material is difficult
and requires a fundamental understanding of the mechanism involved in
maintaining a stable dispersion. Here, we use molecular dynamics calculations
to show that when graphene is immersed in a solvent, distinct solvation layers
are formed irrespective of the choice of solvent and their formation is
energetically favourable for all considered solvents. However, energetic
considerations such as these do not explain the experimental solvent-dependence
of the dispersion concentration. Instead, we find that solvents with high
diffusion coefficients parallel to the graphene layer result in the lowest
experimental concentration of graphene in solution. This can be explained by
the enhanced ease of reaggregation in these solvents. Solvents with smaller
diffusion coefficients result in higher experimental graphene concentrations as
reaggregation is prevented. 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
exfoliation step. Based on this, we predict that the concentration of
exfoliated graphene in solvents such as benzaldehyde and quinoline, which have
low diffusion constants, can be increased dramatically by careful tuning of the
experimental sonication parameters.
