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Keywords:

  • anions;
  • cations;
  • ionic volumes;
  • organic salts;
  • X-ray diffraction

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 (Vvdw) is analyzed. The obtained volumes (Vvdw,r, r=radius-based) are nearly T independent. An ion volume partitioning scheme is proposed by fixing the anion volumes of [Cl], [Br], [I], [BF4], [PF6], [OTf] and [NTf2]. The van der Waals volumes (Vvdw,r+/−) of 48 ions are established, with low standard deviations (0.2–3.6 Å3, 0.1–4.5 % of Vvdw,r+/−). The ion volumes are independent of the counterion and one crystal structure already suffices for their derivation. Correlations of the viscosity with Vvdw,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 R2=0.86 for 40 ILs (354 data points) in the T range of 253–373 K.