Complex Permittivity of Ionic Liquid Mixtures Investigated by Terahertz Time-Domain Spectroscopy

Abstract

Ionic liquids are salts found in their liquid state at ambient temperature. The physicochemical properties of ionic liquids can be tailored by selecting constituent cation and anion from numerous available ions. The physicochemical properties can be further tuned by mixing different neat ionic liquids. Reported data of ionic liquid mixtures reveal that frequently investigated properties such as density, viscosity, and thermal stability follow corresponding mixing laws. Complex permittivity in the interval of terahertz frequencies is of great importance to understand the molecular interactions and the solvation dynamics which drive the macroscopic properties of ionic liquids; however, to the best of our knowledge, there are few reports about the mixing behavior of complex permittivity in ionic liquid mixtures. In this contribution, binary mixtures of 1-butyl-3-methylimidazoulium iodide ([C4C1im]I) and 1-butyl-3- methylimidazoulium bis(trifluoromethylsulfonyl)imide ([C4C1im][NTf2]) are investigated in the terahertz spectral range, and the resulting low-energy spectra are analyzed in order to clarify the mixing laws at play. The results show that the complex permittivity of mixtures of [C4C1im]I and ([C4C1im][NTf2] obeys a linear mixing law.

Pseudo-Optical Modes in Room-Temperature Ionic Liquids

“…Reichenbach et al proposed that the two components included within the broad low-frequency band seen in the FIR spectra of RTILs (see the right panel of Figure ) arise from LO–TO splitting. Considering once more the Lydane-Sachs-Teller relation and ε s and ε ∞ values for [C 4 C 1 Im]­[NTf 2 ], , the resulting value for (ω LO /ω TO ) 2 is 3.83 ± 0.39, which should be compared to the corresponding value (3.05 ± 1.12) for the two components of the curve fit of the FIR spectrum. As shown in Figure S1, we remark that including or not the TO mode has only a marginal impact on the quality of the fit of FIR spectrum of [C 4 C 1 Im]­[NTf 2 ].…”

Characteristic Spectroscopic Features because of Cation–Anion Interactions Observed in the 700–950 cm–1 Range of Infrared Spectroscopy for Various Imidazolium-Based Ionic Liquids

“…17−20 The low-frequency intermolecular vibrational modes have been studied by using far IR (FIR) spectroscopy and terahertz time-domain spectroscopy (THz-TDS). 21−29 In determining the center frequencies of the broad absorption band below 150 cm –1 for some imidazolium-based ILs, the major contribution of the force constant in the angular frequency of the harmonic oscillator, ω = ( k /μ) 1/2 , and the significance of the local and directional hydrogen bond were pointed out. 22−24 On the other hand, we previously reported that THz-TDS and FIR spectroscopy can directly explore the intermolecular vibrations because of the cation–anion interactions in methylimidazolium ILs that originated in Coulomb interactions and hydrogen-bond-type interactions and argued that the central frequencies of intermolecular vibrations are on the essential contribution of the reduced mass μ calculated from the respective masses of the methylimidazolium ring cation [mim + ] and the anion [A – ] as well as the force constant k, which was established for a wide variety of methylimidazolium ILs.…”

TeraVision: A LabVIEW Software for THz Hyper-Raman Spectroscopy

“…The importance of THz spectroscopy in fundamental science is obvious because many important low-energy excitations are present within this range in practically all kind of materials: emerging materials [2], semiconductors [3], ferroelectrics [4], superconductors [5], polymers [6], photonic crystals [7], liquids [8][9][10], gases [11] and biological systems [12]. Moreover, THz technology has a high impact in many different areas too.…”