Screening of highly charged ions in an ionic liquid; when will ion pairs form?

The properties of pairs of doubly charged solute ions are studied as a function of their separation in the ionic liquid, dimethylimidazolium chloride ([dmim][Cl]). Free energy (potential of mean force) profiles show that, as for singly charged ions, there is a barrier to oppositely charged ion pairs forming a contact ion pair. However for doubly charged ions this barrier is about twice as large (45 ± 10 kJ mol-1 rather than 20 ± 5 kJ mol-1). Contact ion pairs form when the short range repulsive force balances the direct interaction plus the screening force, and hence depend on the sizes of the solute ions. In order to understand the existence of the barrier and the extent of screening, local charge density distributions and various contributions to the energetics were examined. The barrier arises when the decrease in stabilisation of individual ions by their own solvation shells is balanced by the increase in other screening effects and the direct solute-solute interaction.