Understanding the Properties of Ionic Liquids: Electrostatics, Structure Factors, and Their Sum Rules

The
properties of room-temperature ionic liquids (ILs) may be viewed
as resulting from a balance of electrostatic interactions that can
be tuned at short range but constrained to satisfy universal, asymptotic
screening conditions. Short-range interactions and ion packing provide
ample opportunity for chemical tunability, while asymptotic sum rules
dictate that the long-range structure and charge oscillation be similar
to those of molten alkali halide salts. In this work, we study the
structure factors and long-range electrostatic interactions in six
ILs. The cation in all cases is 1-butyl-3-methylimidazolium (BMIM+), and we study six anions, namely, tetrafluoroborate (BF4–), hexafluorophosphate (PF6–), nitrate (NO3–), triflate
(CF3SO3–), bisfluorosulfonylimide
[(FSO2)2N–], and bistriflimide
[(CF3SO2)2N–].
To gain insight, we perform similar computer simulations of a primitive
molten salt model with and without electronic polarization. We emphasize
universal similarities among ionic liquids and molten salts in the
long-range ion ordering and the influence of electronic polarization
on the screening conditions while also characterizing important differences
in the short-range electrostatic interactions. We show that polarization
systematically reduces charge oscillations by as much as ∼0.5–1
ion per radial shell, which we argue is general to all room-temperature
ILs as well as molten salts. We suggest that a fundamentally important
distinction among BMIM-based ionic liquids (with different anions)
is the nature of the midrange, ∼1 Å–1 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.