Correlation between polarity parameters and dielectric properties of [Na][TOTO] - a sodium ionic liquid

Abstract

The ionic liq. (IL) [Na][TOTO], with sodium as the cation and an oligoethercarboxylate as the anion, shows properties that differ significantly from conventional ionic liqs., like imidazolium salts. Its polarity, detd. in the temp. range of (293-333) K from measurements of Reichardt's ENT value and the Kamlet-Taft parameters is extraordinarily low and matches the dielec. const. extrapolated from frequency dependent complex permittivity measurements. The dielec. spectra also reveal split dynamics with the dominating slow mode probably assocd. 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 m.ps. because of cation complexation in a pseudo crown ether-like fashion and resulting "intramol." 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 liq. with strong -COO-···Na+ interactions and relatively weaker forces between cations and ether oxygen groups of the anions appears to be plausible.