Molecular Insight into the Ionic Conduction of Quaternary Ammonium and Phosphonium Cation-Based Ionic Liquids Using Dielectric and Spectroscopy Analyses

Abstract

We analyzed the primary properties of ionic liquids (ILs) comprising quaternary phosphonium cations and bis(trifluoromethylsulfonyl) amide anions and compared them with those of corresponding quaternary-ammonium-cation-based ILs. Broadband dielectric spectroscopy was used to confirm the coupling between the translational and orientational motions of ions, and our results demonstrated that the high ionic conductivity of the phosphonium-based ILs was attributed to their fast rotational dynamics. The differences between ILs with different cations were further evaluated using vibrational (Raman and terahertz) spectroscopy. The Raman spectroscopy data revealed that the cation structure affected the conformation and flexibility (conformational change) of the anion. Furthermore, terahertz spectroscopy allowed us to evaluate the relationship between ion transport and intermolecular interactions between the cation and anion of ILs.

Dynamics in Quaternary Ionic Liquids with Non-Flexible Anions: Insights from Mechanical Spectroscopy

“…At 25 °C, the viscosities for the ammonium ionic liquids are higher than those of the phosphonium analogues for a given cation, while the [DCA] – samples have significantly higher viscosities than the [TCM] – ionic liquids. The trend of the higher viscosity of ammonium ionic liquids compared to the isostructural phosphonium is a quite general finding, and this has also been observed for the bis(trifluoromethanesulfonyl)imide anion [NTf 2 ] – (also termed [TFSI] – ), with both comparably small [ 38 , 39 ] and comparably large cations [ 40 ], as well as for the tetracyanoborate [B(CN) 4 ] − anion [ 33 ]. This is rationalized by the stronger interactions in the ammonium samples [ 35 ] resulting from the lower flexibility of the ions [ 41 ].…”