Correlation between configurational entropy, excess entropy, and ion dynamics in imidazolium-based ionic liquids: Test of the Adam-Gibbs model.

ABSTRACT

The Adam-Gibbs (AG) model, linking thermodynamics with molecular dynamics of glass-forming liquids, plays a crucial role in the studies of the glass transition phenomenon. We employ this approach to investigate the relationship between ion dynamics and thermodynamics in three imidazolium-based ionic liquids in the current work. We show that the AG relation, -log10σdc ∝ (TSc)-1 (where σdc, T, and Sc denote the dc-conductivity, absolute temperature, and configurational entropy, respectively), does not work when the whole supercooled liquid state is considered. Meanwhile, a linear relationship between -log10σdc and (TSe)-1 (where Se denotes the excess entropy) was observed in the entire supercooled range. On the other hand, the generalized AG model log10σdc ∝ (TSc α)-1 with an additional free parameter α successfully describes the relation between σdc and Sc. The determined α values being less than unity indicate that the configurational entropy is insufficient to govern the ion dynamics. Meanwhile, we found a systematical decrease in α with the elongation of the alkyl chain attached to the imidazolium ring.