Ion-ion interactions among room-temperature ionic liquids are believed to play an important role in determining the transport properties of ionic liquids.  Thus, there is an incentive to measure charge organization in these materials to better understand the correlation between the liquid structure of ionic liquids and their transport properties.  Charge organization measurements are typically performed with highly specialized techniques that are not easily accessible.  As an attainable and cost-efficient alternative, we propose using a combination of ATR and transmission FT-IR spectroscopy to quantitatively assess the amount of charge organization in an ionic liquid.  The heart of the strategy is to compare two independent measurements of the dipole moment derivative of select vibrational modes.  The first is derived from the optical constants of the ionic liquid, which are obtained from a Kramers-Kronig transform of ATR IR spectrum.  The second comes from applying dipolar coupling theory to the degree of TO-LO band splitting for a vibrational mode in the infrared absorption spectrum.  Dipolar coupling theory necessarily assumes the quasilattice of the molten salt is perfectly organized.  In contrast, no assumptions about the nature of the quasilattice are required to estimate the dipole moment derivate from the ATR spectrum.  Thus, disparities between the two measurements can be quantified and show how close the ionic liquid is to satisfying the assumption of a perfect crystalline lattice.  We have previously applied the methodology to 1-alkyl-3-methylimidazolium trifluoromethanesulfonate ionic liquids, and we are now extending this work to the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide family of ionic liquids.  As expected, charge organization decreases as the size of the alkyl side chain increased.  Furthermore, binary solutions of 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide (high amounts of charge organization) and 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide (low amounts of charge organization) show a gradual variation in charge organization as the composition is varied from one end member to the other.

Charge organization of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids measured by infrared spectroscopy

247th National Meeting (2014)

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