Interactions and structure of ionic liquids on graphene and carbon nanotubes surfaces

Using quantum methods it was possible to build an atomistic force field for ionic liquids interacting with a graphene surface. Density functional calculations of 1-ethyl-3-methylimidazolium cation and thiocyanate anion interacting with a cluster of carbon atoms representing a graphene surface were performed, at a series of distances and orientations, using the BLYP-D functional. The DFT interaction energies were successfully fitted to a site-site potential function. The developed force field accounts also for the polarization of the graphene surface, therefore the use of induced dipoles to reproduce the interaction energy between charges and a conductor surface is not required. We report molecular dynamics results on the structure of the interfacial layer of the ionic liquid 1-ethyl-3-methylimidazolium thiocyanate at a flat graphene surface and inside single-wall carbon nanotubes of different diameters, including analyses of the positional and orientational ordering of the ions near the surface, and charge density profiles. Both anions and cations are found in the first ordered layer of ions near the surface, with the interfacial layer being essentially one ion thick.