Free volume in ionic liquids: a connection of experimentally accessible observables from PALS and PVT experiments with the molecular structure from XRD data

Abstract

In the current work, free volume concepts, primarily applied to glass formers in the literature, were transferred to ionic liquids (ILs). A series of 1-butyl-3-methylimidazolium ([C4MIM](+)) based ILs was investigated by Positron Annihilation Lifetime Spectroscopy (PALS). The phase transition and dynamic properties of the ILs [C4MIM][X] with [X](-) = [Cl](-), [BF4](-), [PF6](-), [OTf](-), [NTf2](-) and [B(hfip)4](-) were reported recently (Yu et al., Phys. Chem. Chem. Phys., 2012, 14, 6856-6868). In this subsequent work, attention was paid to the connection of the free volume from PALS (here the mean hole volume, <v(h)>) with the molecular structure, represented by volumes derived from X-ray diffraction (XRD) data. These were the scaled molecular volume Vm,scaled and the van der Waals volume V(vdw). Linear correlations of <v(h)> at the "knee" temperature (<v(h)>(T(k))) with V(m,scaled) and V(vdw) gave good results for the [C4MIM](+) series. Further relationships between volumes from XRD data with the occupied volume Vocc determined from PALS/PVT (Pressure Volume Temperature) measurements and from Sanchez-Lacombe Equation of State (SL-EOS) fits were elaborated (V(occ)(SL-EOS) ≈ 1.63 V(vdw), R(2) = 0.981 and V(occ)(SL-EOS) ≈ 1.12 V(m,scaled), R(2) = 0.980). Finally, the usability of V(m,scaled) was justified in terms of the Cohen-Turnbull (CT) free volume theory. Empirical CT type plots of viscosity and electrical conductivity showed a systematic increase in the critical free volume with molecular size. Such correlations allow descriptions of IL properties with the easily accessible quantity V(m,scaled) within the context of the free volume.

WITHDRAWN: Ionic liquid mixtures with tunable physicochemical properties

“…Also, the VTF diagram slope of the pyrrolidinium binary blends is seen to increase with increasing the mole fraction of the (pyrrolidinium) cation sharing the shorter alkyl side main chain, indicating a progressively rising activation energy for the ion transport. A possible explanation could be given by the "free volume" (V v ), which represents the available void space for the ion transferring through the binary IL mixtures [39]. The ion movement occurs if the free volume medium value, V c m , exceeds a critical volume V c accordingly to the following equation:…”

Physical and Electrochemical Properties of Some Phosphonium-Based Ionic Liquids and the Performance of Their Electrolytes in Lithium-Ion Batteries

“…The addition of a salt to an ionic liquid can only increase the short-range, ionion coulombic interactions but there is evidence that, similar to other amorphous materials and glass-forming liquids (molten salts, polymer electrolytes, and ionic glass, concentrated liquid electrolytes), free volume, the free space surrounding molecules or ions in the liquid state, might be responsible for ion transport. 28 Recently, the original ideas of Doolittle and Cohen-Turnbull on free volume have been applied by Krause-Rehberg and coworkers to ionic liquid electrolytes and Abu-Lebdeh and co-workers to molecular liquid electrolytes and new relationships that correlate viscosity and conductivity to free volume have been introduced. 28,29 A correlation with the salt-solvent phase diagram has also been introduced in the case of molecular liquid electrolytes where the concentration of maximum conductivity was correlated to the eutectic composition, 29 Unfortunately this analysis is out of the scope of this work and will be reported separately, however it clearly demonstrates that there is little known about the structure and transport mechanism in both molecular and ionic liquid electrolytes.…”

Establishing Consistent van der Waals Volumes of Polyatomic Ions from Crystal Structures

“…The outlined approach has been used recently for the establishment of connections between V vdw and free-volume-related quantities. [46] The free volume is the key property determining transport phenomena in glass formers, and the corresponding concepts were shown to be valid for ILs. [46][47][48] The van der Waals volumes were calculated for the T dependent crystal structures presented in this work.…”

“…[46] The free volume is the key property determining transport phenomena in glass formers, and the corresponding concepts were shown to be valid for ILs. [46][47][48] The van der Waals volumes were calculated for the T dependent crystal structures presented in this work. Crystal structures exhibiting disorder or having structural problems were excluded from the analysis.…”