Physical Properties and Low-Frequency Polarizability Anisotropy and Dipole Responses of Phosphonium Bis(fluorosulfonyl)amide Ionic Liquids with Pentyl, Ethoxyethyl, or 2-(Ethylthio)ethyl Group

Abstract

This study compared the physical properties, e.g., glass transition temperature, melting point, viscosity, density, surface tension, and electrical conductivity, and the low-frequency spectra under 200 cm-1 of three synthesized ionic liquids (ILs), triethylpentylphosphonium bis(fluorosulfonyl)amide ([P2225][NF2]), ethoxyethyltriethylphosphonium bis(fluorosulfonyl)amide ([P222(2O2)][NF2]), and triethyl[2-(ethylthio)ethyl]phosphonium bis(fluorosulfonyl)amide ([P222(2S2)][NF2]), at various temperatures using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and terahertz time-domain spectroscopy (THz-TDS). The [P222(2S2)][NF2] had the highest viscosity and glass transition temperature, whereas the [P222(2O2)][NF2] had the lowest. Among the three ILs, the [P222(2S2)][NF2] had the highest density and surface tension, and the [P222(2O2)][NF2] had the highest electrical conductivity. The RIKES and THz-TDS spectral line shapes for the three ILs varied significantly. For the [P2225][NF2], molecular dynamics simulations successfully reproduced the line shapes of the experimental spectra and indicated that the RIKES spectrum was mainly due to the cation and cross-term and their rotational motions, whereas the THz-TDS spectrum was mainly due to the anion and its translational motion. This shows that it is desirable to utilize both fs-RIKES and THz-TDS methods to reveal molecular motions at the low-frequency domain. The [P222(2S2)][NF2] had higher frequency peaks and broader bands in the low-frequency spectra via fs-RIKES and THz-TDS than those for the [P2225][NF2] and [P222(2O2)][NF2].