  Forthcoming applications in electronics and optoelectronics make phosphorene
a subject of vigorous research efforts. Solvent-assisted exfoliation of
phosphorene promises affordable delivery in industrial quantities for future
applications. We demonstrate, using equilibrium, steered and umbrella sampling
molecular dynamics, that the 1-ethyl-3- methylimidazolium tetrafluoroborate
[EMIM][BF4] ionic liquid is an excellent solvent for phosphorene exfoliation.
The presence of both hydrophobic and hydrophilic moieties, as well as
substantial shear viscosity, allows [EMIM][BF4] simultaneously to facilitate
separation of phosphorene sheets and to protect them from getting in direct
contact with moisture and oxygen. The exfoliation thermodynamics is moderately
unfavorable, indicating that an external stimulus is necessary. Unexpectedly,
[EMIM][BF4] does not coordinates phosphorene by p-electron stacking with the
imidazole ring. Instead, the solvation proceeds via hydrophobic side chains,
while polar imidazole rings form an electrostatically stabilized protective
layer. The simulations suggest that further efforts in solvent engineering for
phosphorene exfoliation should concentrate on use of weakly coordinating ions
and grafting groups that promote stronger dispersion interactions, and on
elongation of nonpolar chains.
