We have investigated the screening of solute ion--electrode interactions in two ionic liquids (1-butyl 3-methylimidazolium tetrafluoroborate [BMIm][BF?4] 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 which correlate with the layered structure of the ionic liquid.  In both ionic liquids there are high barriers impeding the motion of charged probes to oppositely charged surface. We examined the local liquid structure around the probes and found that the free energy minima correspond to positions where 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. We extended this work to mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate with lithium tetrafluorobate and potassium tetrafluorobate where we determined the density profiles and free energy profiles of lithium and potassium ions as a function of distance from the electrode. We find that there are again oscillations in the density and free energy which extend to 1-2nm  from the electrode. We found that lithium ions came in contact with a negative wall in the most concentrated solutions investigated (25 mole percent) but not in 10 mole percent solutions.

Simulations of ionic liquids near charged walls

248th National Meeting (2014)

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