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

Abstract

Based on temperature (T) dependent crystal structure data of seven organic salts, a radii-based scheme for the calculation of the van der Waals volume (V(vdw)) is analyzed. The obtained volumes (V(vdw,r), r=radius-based) are nearly T independent. An ion volume partitioning scheme is proposed by fixing the anion volumes of [Cl](-), [Br](-), [I](-), [BF(4)](-), [PF(6)](-), [OTf](-) and [NTf(2)](-). The van der Waals volumes (V(vdw,r) (+/-)) of 48 ions are established, with low standard deviations (0.2-3.6 Å(3), 0.1-4.5 % of V(vdw,r) (+/-)). The ion volumes are independent of the counterion and one crystal structure already suffices for their derivation. Correlations of the viscosity with V(vdw,r) via a Litovitz ansatz and our recently derived Arrhenius-type approach prove that these volumes are suitable for the volume-based description and prediction of IL properties. The corresponding correlation coefficient for the latter is R(2)=0.86 for 40 ILs (354 data points) in the T range of 253-373 K.

Size Matters! On the Way to Ionic Liquid Systems without Ion Pairing

“…We assume a packing fraction of 0.6 to be typical for liquid ILs. The packing index of IL crystals is around 0.65 close to room temperature 53. 54 However, on melting it further decreases and reaches values of roughly 0.60.…”

Ionic and molar volumes of room temperature ionic liquids

“…The values of P vary slightly among various classes of RTILs [4], but are, on the average, 0.6875 ± 0.0055, i. e., within b1%. Therefore, values of (v + + v -) may be reconstituted from the reported (v vdW+ + v vdW-) values in [4] by division of the latter by 0.6875.…”

“…The values of P vary slightly among various classes of RTILs [4], but are, on the average, 0.6875 ± 0.0055, i. e., within b1%. Therefore, values of (v + + v -) may be reconstituted from the reported (v vdW+ + v vdW-) values in [4] by division of the latter by 0.6875. Ionic volumes have been reported by several authors [3,[5][6][7] and a representative list for commonly encountered RTILs is shown in Table 1.…”

“…where N A is Avogadro's number, and the predicted densities ρ = M/V are obtained from these and the molar masses M. Rather than using ionic volumes, van der Waals volumes of the ions, v vdW +/− , have also been employed for this purpose [4]. These are related to the ionic volumes obtained directly from the crystal structures as:…”

Poly(ionic liquid)-carbon nanotubes self-supported, highly electroconductive composites and their application in electroactive devices

“…The ion's van der Waals volume varies from 26.5 Å 3 for Br -, 50 Å 3 for BF 4 -, to 147 Å for TFSI -. [20].…”