Screening of pairs of ions dissolved in ionic liquids

The properties of pairs of solute ions in the ionic liquid, dimethylimidazolium chloride or [dmim][Cl], are studied as a function of their separation. The potential of mean force curves show that there is only a small stabilisation of ion pairs with opposite charges followed by a barrier to separation. Ion pairs with the same charge are also stabilised by solvent screening due to induced polarisation of the solvent. In both cases screening is essentially complete outside the first shell, but is large even when there is no solvent between the ions. Charge distributions in the solvation shells around ion pairs are shown and the results interpreted with the aid of a simple model of two ions in an spheroidal cavity in a conducting medium. The actual or Madelung potential experienced by the second solute ion is found to decay rapidly with distance and does not show the oscillations found in the Poisson potential around a single solute ion. We conclude that ionic liquids provide very effective electrostatic screening between solute molecules.