Zaneta WOJNAROWSKA1, Marian PALUCH1, Beibei YAO1, Malgorzata SWADZBA-KWASNY2
1University of Silesia in Katowice, Chorzow, Poland
2The QUILL Research Centre, School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Belfast, United Kingdom
When ionic liquid (IL) is cooled below the melting point, it usually enters into a supercooled state and subsequently into a non-equilibrium amorphous phase. The characteristic feature of this transformation is a continuous increase of density, accompanied by a slowing down of ions mobility. Alternatively, crystallization of IL can be observed at decreasing temperature. Recently, it has been shown that in addition to these two well-known transitions, selected ILs are capable of unique first-order liquid-liquid transformation (LLT).
The liquid-liquid phase transitions in a family of phosphonium ionic liquids will be discussed in this talk. Using the dielectric spectroscopy we will provide an insight into ion dynamics above and below the LLT. These data will be compared with mechanical and calorimetric measurements. We found that at room temperature i.e., above the temperature of LLT (TLL), the charge transport is fully controlled by viscosity, like for conventional aprotic ILs (so-called vehicle mechanism), whereas cooling below TLL makes the cations partially frozen into positions, while anions are free to move in channels of non-polar domains. Consequently, ion diffusion (τσ) and structural dynamics (τα) are decoupled below TLL (in liquid 2), which constitutes the first example of conductivity independent from viscosity in a liquid phase of neat aprotic ILs. Furthermore, quenching liquid 2 leads to a conductive glass. High-pressure measurements and MD simulations reveal that nanostructure of liquid 2 depends on the anion size and varies nonmonotonically with pressure. For small anions, increasing pressure shapes immobile alkyl chains into lamellar phases, leading to increased anisotropic diffusivity of anions through channels. Bulky anions drive the formation of interconnected phases with continuous 3D curvature, which renders ion transport independent of pressure.
This research was funded in whole by National Science Centre, Poland [Opus 21, 2021/41/B/ST5/00840].