Redox potentials and screening in ionic liquids: Effects of sizes and shapes of solute ions

Simulations of a model ionic liquid, [dmim][PF(6)] (dimethylimidazolium hexafluorophosphate), containing solute ions of different sizes and shapes have been used to investigate the changes in redox potentials of and screening around solute ions of different sizes and shapes. The effective solute size of spherical ions increases with the actual solute size although more slowly than expected. The effective solute size of tetrahedral or square planar ions varies little with actual ligand size. These results are clarified by reference to the charge density in the solvent around the ions, which is also used to calculate the potential within the solvent. Screening is essentially complete within 1 nm of the solute ion although charge density oscillations propagate further into the liquid. The results are compared to theoretical models and the implications for experiments are discussed.