Free volume investigation of imidazolium ionic liquids from positron lifetime spectroscopy

Abstract

Highlights Free volume thermal expansion correlates to the molecular volume for ionic liquids. A schematic free volume model is proposed to explain the volume correlation. The ratio, mean hole volume to molecular volume, correlates to transport properties.

On the temperature dependence of the double layer capacitance of ionic liquids

“…Thus the data points presented as circles should be multiplied by a factor that could in principle, be as small as 0.63 [7]. Upon doing so, a value of roughly 0.2 is found, which is in good agreement with experiments of RTILs [69][70][71][72][73][74][75][76][77][78]. The free volume over a threshold can be determined with the use of a probe, which has a Figure 9: Fitted differential capacitance curves show reasonable agreement over the whole voltage domain with the potential rescaled differential capacitance equation.…”

“…11 displays the distribution of free volume as a function of displacement from the interface, which shows that the region near the electrodes has a smaller fraction of free volume. Typically, glassy RTILs (a disordered solid phase) have free volume fractions smaller than the value of liquid RTILs [74,[76][77][78]. Hence, the free volume is consistent with the observation of oriented ion pairs and neutral aggregates at the electrode interface [41].…”

“…The free volume fraction, v f , (free volume per unit volume) is commonly reported in experiments and MD simulations [69][70][71][72][73][74][75][76][77][78][79][80][81][82][83]105]. Here, we investigate two kinds of free volume: that without a probe and with an argon probe (radius of 0.188 nm), which is a test for a void (volume on a similar footing to ions).…”

“…Firstly, there have been numerous studies into the fraction of free volume (free volume per unit volume) in RTILs [69][70][71][72][73][74][75][76][77][78][79][80][81][82][83][84]. These have generally reported that the fraction of free volume is in the range of 0.05-0.2 [69][70][71][72][73][74][75][76][77][78]. For free volume to be considered as a void it must be treated on the same footing as ions; that being, it has a similar volume.…”

“…There has been a great deal of literature concerned with the free volume in RTILs. It has been reported the that fraction of free volume, v f , in bulk RTILs is in the range of 0.05-0.2 [69][70][71][72][73][74][75][76][77][78][79][80][81][82][83]. The volume of an average void is expected to be smaller than the volume of ions.…”