The properties of room-temperature ionic liquids (ILs) may be viewed as resulting from a balance of electrostatic interactions that are tunable at short-range, but constrained to satisfy universal, asymptotic screening conditions. Short-range interactions and ion packing provides ample opportunity for chemical tunability, while asymptotic sum rules dictate that the long-range structure and charge oscillation is similar to that in molten alkali-halide salts. We quantitatively illustrate how the liquid structure itself is directly modulated by electrostatic screening conditions. A consequence of the exact Stillinger-Lovett second moment condition is that, at fixed density, any pairwise-additive, non-polarizable force field will necessarily predict artificially enhanced long-range ion structuring, as the screening condition is set by the infinite-frequency dielectric response. A consequence is that one has to use polarizable force fields to correctly reproduce the optical dielectric constant. We show that polarization systematically reduces charge oscillations in ionic liquids by as much as 0.5-1 ions per radial shell, which is general to all room-temperature ionic liquids as well as molten salts. We also suggest that a fundamentally important distinction among ionic liquids is the nature of the mid-range peak in the charge-correlation structure factor; while this correlation is straightforward to analyze in computer simulations, it may often be hidden in X-ray and/or neutron scattering structure factors.

Influence of electronic polarization on the structure of ionic liquids

ACS Fall 2020 Virtual Meeting

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