Layer-based thermal migration of an ionic liquid nano-droplet on a graphene surface: a molecular dynamics study

Abstract

ABSTRACT The migration behaviour of an ionic liquid (IL) nano-droplet on a graphene surface under temperature gradients was studied using non-equilibrium molecular dynamics (MD) simulations. 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) was used in the study. The migration of the IL nano-droplet from the hot end to the cold end of the graphene surface was observed under all temperature gradients considered in the current work. The migration was found to be faster under higher temperature gradients. Detailed analysis reveals that, instead of rolling like a ball, the IL nano-droplet migrates via a layer-based motion, in which the molecules have limited displacement in the direction perpendicular to the graphene surface. The migration of the IL nano-droplet was also found to be coupled with spreading on the graphene surface. The spatial organisation of the cations and anions within the IL nano-droplet was also analysed. It was found that the ions forming the IL nano-droplet organised themselves in layers parallel to the graphene surface, and the angles of cation rings on different layers also tend to be specific. The cation closest to the graphene surface tends to align with their rings parallel to the surface, while those away from the surface are more disordered.