Screening of Ion-Graphene Electrode Interactions by Ionic Liquids: The Effects of Liquid Structure

We have investigated the screening of solute ion electrode interactions in two ionic liquids (1-butyl 3-methylimidazolium tetrafluoroborate [BMIm][BF4] and 1,3-dimethylimidazolium chloride [MMIm]Cl) by constructing free energy profiles for dissolved charged probes as a function of distance from a charged surface (graphene). The free energy profiles for three types of mutual interactions (surface and solute with opposite charges, solute and uncharged surface, and surface and solute with the same charges) differ from each other, but are remarkably similar in the two ionic liquids. They all show oscillations rather than the monotonic behavior predicted by Debye-type screening models. In both ionic liquids, there are high barriers impeding the motion of charged probes to the oppositely charged surface. We examined the local liquid structure around the probes and found that the free energy minima correspond to positions in which the solvation layers induced by the surface charge and the solvation shells around the probes enhance each other while barriers occur where they perturb each other.