Can the Cation Alkyl Side Chain Length Affect the Thermal and Transport Properties of Pyrrolidinium-TFSI Ionic Liquid Mixtures

There is growing interest in replacing the volatile and hazardous organic solvents, currently used in electrochemical devices, with ionic liquids (ILs), this pushing them far beyond the initially envisaged role of green solvents. However, the rational design of novel systems based on ILs requires a deep understanding of the intermolecular interactions and transport properties occurring in the ionic liquid bulk. The possibility of fine tuning the physicochemical properties of ionic liquids by forming IL blends is a very appealing characteristic [1]. For instance, the association and ordering of the ions in a mixture is more complicated than in the pure components, thus affecting the short-range aggregation motives and, consequently, the blend physicochemical properties. Ionic liquids are highly structured fluids with short-range aggregation motives, strongly correlated to the alkyl side chain length bounded to the nitrogen group of the cation [1].

Our basic idea is to investigate the length effect  of the cation main aliphatic side chain on the thermal and transport properties of binary ionic liquid mixtures formed by pyrrolidinium materials. Three sets of IL mixtures were prepared: i) PYR₁₁TFSI-PYR₁₄TFSI, ii) PYR₁₁TFSI-PYR₁₈TFSI and iii) PYR₁₄TFSI-PYR₁₈TFSI. The present study was performed by DSC, conductivity and self-diffusion coefficients (NMR measurements).

The DSC heating trace of the ionic liquid blends is shown in Figure 1. The PYR₁₁TFSI-PYR₁₄TFSI samples (panel A) exhibit a well-defined melting peak. The progressive enrichment in PYR₁₁TFSI was seen to hinder the crystallization process of the IL mixtures, as a result of the unfavorable ion packing likely ascribable to the different size of the (PYR₁₁)⁺ and (PYR₁₄)⁺ cations. Apart the x = 0.75 sample of panel B, the PYR₁₁TFSI-PYR₁₈TFSI (panel B) and PYR₁₄TFSI-PYR₁₈TFSI (panel C) mixtures do not display any feature, with exception of the glass transition one (T_g), indicating that these IL blends are not able to be crystallized. This suggests that the “ionic confusion”, resulting from the contemporaneous presence of different symmetry pyrrolidinium cations, is able to prevent the crystallization of the mixtures which, instead, solidify in a glassy state at very low temperatures.

Figure 1. DSC trace of PYR₁₁TFSI-PYR₁₄TFSI (panel A), PYR₁₁TFSI-PYR₁₈TFSI (panel B) and PYR₁₄TFSI-PYR₁₈TFSI (panel C) ionic liquid binary mixtures.

In Figure 2 is reported the ionic conductivity vs. PYR₁₄TFSI mole fraction dependence for (x)PYR₁₄TFSI-(1-x)PYR₁₈TFSI. At a first sight it is seen that the ionic conductivity decreases with the progressive impoverishment of the highest conductive material (PYR₁₄TFSI) fraction in the binary mixture. However, a more careful observation shows that this trend is true only at temperatures above the melting point of PYR₁₄FSI, e.g., -7 °C. At lower temperatures, the ionic conductivity for PYR₁₄TFSI mole fractions ranging from 0.25 to 0.75, is from two to four orders of magnitude higher than the neat IL materials.

Figure 2. Ionic conductivity vs. mole fraction behavior of (x)PYR₁₄TFSI-(1-x)PYR₁₈TFSI mixtures.

Acknowledgements

J.S.M. and G.B.A. thank the Italian Institute of Technology (IIT) for the financial support within the SEED Project “REALIST”.

References

[1] G.B. Appetecchi, M. Montanino, S. Passerini, Ionic liquid-based electrolytes for high-energy lithium batteries, in Ionic Liquids: Science and Applications, ACS Symposium Series 1117, A.E. Vissser, N.J. Bridges, R.D. Rogers eds., Washington, USA, 2013.