All-Atom Molecular Dynamics Simulations of Electrotunability in Ionic Liquid Lubrication: Mechanism and Anion Specificity.

We report an all-atom molecular dynamics study of the electrotunable friction of ionic liquids in systems featuring a single ion layer confined between a hemispherical model silica tip and a flat graphene substrate. The ion specificity of friction is probed through two ionic liquids with the same cation, [EMIM]+, but different sulfonyl-based anions, [TFSI]- and [TfO]-. We show that the friction force vs surface charge density is V-shaped, with minimum friction occurring at -0.12 C/m2. In the curves' right branch, friction for [EMIM]-[TfO] is considerably smaller than for [EMIM]-[TFSI]. In the left branch, the friction for the two ionic liquids shows only a minor difference. Analysis reveals that the instantaneous friction force exhibits steady rises and sudden drops due to the stick-slip motion of the tip-bound ion layer over the graphene substrate. Such a stick-slip response arises from ions becoming temporarily trapped in, and subsequently escaping from, the energy valleys associated with nearby graphene atoms. The number of trapped ion atoms is identified as the decisive factor governing the friction force. Quantifying and then tracing them to the composition and orientation of ions confined under the tip apex enables a mechanistic understanding of the observed electrotunability of friction and its ion specificity.