Correlation between Polarity Parameters and Dielectric
Properties of [Na][TOTO] - a Sodium Ionic Liquid,
O. Zech, J. Hunger, J. R. Sangoro, C. Iacob, F. Kremer, W. Kunz and R. Buchner*
Phys. Chem. Chem. Phys. 12 (2010) 14341-14350.
Abstract
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 (293.15 - 333.15) K from measurements of Reichardt's
ENT value and the Kamlet-Taft
parameters is extraordinary 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 moities of the anion. This assignment, supported by
the derived effective dipole moments, argues against our previous hypothesis [Chem. Eur. J. 2009, 15, 1341]
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.
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