  An important question in understanding the structure of ionic liquids is
whether ions are truly "free" and mobile which would correspond to a
concentrated ionic melt, or are rather "bound" in ion pairs, that is a liquid
of ion pairs with a small concentration of free ions. Recent surface force
balance experiments from different groups have given conflicting answers to
this question. We propose a simple model for the thermodynamics and kinetics of
ion pairing in ionic liquids. Our model takes into account screened ion-ion,
dipole-dipole and dipole-ion interactions in the mean field limit. The results
of this model suggest that almost two thirds of the ions are free at any
instant, and ion pairs have a short lifetime comparable to the characteristic
timescale for diffusion. These results suggest that there is no particular
thermodynamic or kinetic preference for ions residing in pairs. We therefore
conclude that ionic liquids are concentrated, rather than dilute, electrolytes.
