Interactions and Ordering of Ionic Liquids at a Metal Surface

An atomistic force field for ionic liquids interacting with a metal surface is built on the basis of quantum methods. Density functional calculations of alkylammonium cations and alkylsulfonate anions interacting with a cluster of iron atoms were performed, at a series of distances and orientations, using the M06 functional that represents noncovalent interactions. A site-site potential function was then adjusted to the BSSE-corrected DFT interaction energies. Finally, the polarization of the metal by the ions was taken into account using induced dipoles to reproduce the interaction energy between charges and a conductor surface. When combined with a molecular force field for the ionic liquid and a suitable potential for metals, our model allows the computer simulation of heterogeneous systems containing metal surfaces or nanoparticles in the presence of ionic liquids. Our aim is to study tribological systems with ionic lubricants. We report molecular dynamics results on the structure of the interfacial layer of several alkylammonium alkylsulfonate ionic liquids at a flat iron surface, 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 oxygen atoms of the sulfonyl groups interacting more strongly with the metal. The interfacial layer is essentially one ion thick, except for very short chain ionic liquids in which a second layer is observed. The effects of different lengths of the nonpolar alkyl side chains on the cation and the anion are different: whereas butyl chains on the sulfonate anions tend to be directed away from the surface, those on ammonium cations lie more parallel to the surface.