Correlation between polarity parameters and dielectric properties of [Na][TOTO]—a sodium ionic liquid
The ionic liquid (IL) [Na][TOTO], with sodium as the cation and an oligoethercarboxylate as the anion, shows properties that differ significantly from conventional ionic liquids, like imidazolium salts. Its polarity, determined in the temperature range of (293-333) K from measurements of Reichardt's E value and the Kamlet-Taft parameters is extraordinarily low and matches the dielectric constant extrapolated from frequency dependent complex permittivity measurements. The dielectric spectra also reveal split dynamics with the dominating slow mode probably associated with the reorientation of -COO(-)...Na(+) ion pairs and the fast mode arising from the flexibility of the oligoethylene moieties of the anion. This assignment, supported by the derived effective dipole moments, argues against our previous hypothesis [O. Zech et al., Chem.-Eur. J., 2009, 15, 1341-1345] that salts, such as [Na][TOTO], have low melting points because of cation complexation in a pseudo crown ether-like fashion and resulting "intra-molecular" charge neutralization. The present data rule out a rigid chelate-like complex as the dominating species. Considering the present findings together with the viscosities of [TOTO](-) salts, a crosslinked structure of the ionic liquid with strong -COO(-)...Na(+) interactions and relatively weaker forces between cations and ether oxygen groups of the anions appears to be plausible.