A major goal in the field of ionic liquids is correlating transport property trends with the underlying liquid structure of the compounds, such as the degree of charge organization among the constituent ions. Traditional techniques for experimentally assessing charge organization are specialized and not readily available for routine measurements. This represents a significant roadblock in elucidating these correlations. We use a combination of transmission and polarized-ATR infrared spectroscopy to measure the degree of charge organization for ionic liquids. The technique is illustrated with a family of 1-alkyl-3-methylimidazolium trifluoromethansulfonate ionic liquids at 30°C. As expected, the amount of charge organization decreases as the alkyl side chain is lengthened, highlighting the important role of short-range repulsive interactions in defining quasilattice structure. Inherent limitations of the method are identified and discussed. The quantitative measurements of charge organization are then correlated with trends in the transport properties of the compounds to highlight the relationship between charge and momentum transport and the underlying liquid structure. Most research laboratories possess infrared spectrometers capable of conducting these measurements, thus, the proposed method may represent a costeffective solution for routinely measuring charge organization in ionic liquids.

It has come to our attention that the equation describing the transverse optic-longitudinal optic (TO-LO) mode splitting is incorrectly written in our paper. The equation for TO-LO splitting was first derived by Decius 1 and was written in terms of angular frequency, ω, with units of rad s −1 . In later papers, however, Decius and co-workers 2−5 favored the symbol ν in lieu of ω for the vibrational frequency (see especially refs 4 and 5). Unfortunately, we did not realize the author's notation change from ω to ν was not accompanied by a change in units from rad s −1 to s −1 . Therefore, our eq 1 should be written Addition/Correction pubs.acs.org/JPCB

Solutions of lithium tetrafluoroborate dissolved in dimethyl carbonate (DMC) are confined within MCM-41, a mesoporous silica matrix. Thermal measurements indicate that the melting points of pure DMC and the DMC solutions are significantly reduced when confined within the pores of MCM-41 compared to unconfined samples; this is an observation that is consistent with the Gibbs−Thomson equation. The melting point onsets of confined solutions are slightly lower than that of pure DMC, suggesting the dissolved salts impact the phase-transition temperature of DMC when confined within mesoporous silica. Rotational dynamics of the confined solutions are explored by doping DMC with small quantities of Tempone, an electron paramagnetic resonance (EPR)-active spin probe. Tempone rotational correlation times are an order of magnitude slower for confined liquids compared to unconfined solutions. Temperature-dependent conductivity measurements of the composite materials suggest that the liquid electrolyte solution is distributed among the MCM-41 pores and the intergrain voids between individual MCM-41 particles. Ionic conductivities of confined electrolyte solutions remain above 0.01 mS•cm −1 for temperatures greater than −50 °C. However, the ionic conductivity of the unconfined solutions (i.e., solution occupying the spaces between the MCM-41 particles) rapidly decreases over subzero temperatures. Limitations associated with directly implementing these materials as low-temperature ion conductors are discussed.

Solutions of lithium hexafluorophosphate dissolved in dimethyl carbonate (DMC) are confined within a nano-porous silica matrix (MCM-41). Thermal measurements indicate that the melting points of pure DMC and a 1.0 m LiPF6 DMC solution are significantly reduced when confined within the nanopores compared to bulk samples; an observation that is consistent with the Gibbs-Thompson equation. Temperature-dependent conductivity measurements of the confined solution show a sharp decrease in solution conductivity between 0 and -10oC. The measured conductivity values below -10oC are not sufficient to operate a lithium ion battery at sub-zero temperatures.