Pressure and Temperature Dependence of Local Structure and Dynamics in an Ionic Liquid

Abstract

A detailed understanding of the local dynamics in ionic liquids remains an important aspect in the design of new ionic liquids as advanced functional fluids. Here, we use small-angle X-ray scattering and quasi-elastic neutron spectroscopy to investigate the local structure and dynamics in a model ionic liquid as a function of temperature and pressure, with a particular focus on state points ( P , T ) where the macroscopic dynamics, i.e., conductivity, is the same. Our results suggest that the initial step of ion transport is a confined diffusion process, on the nanosecond timescale, where the motion is restricted by a cage of nearest neighbors. This process is invariant considering timescale, geometry, and the participation ratio, at state points of constant conductivity, i.e., state points of isoconductivity. The connection to the nearest-neighbor structure is underlined by the invariance of the peak in the structure factor corresponding to nearest-neighbor correlations. At shorter timescales, picoseconds, two localized relaxation processes of the cation can be observed, which are not directly linked to ion transport. However, these processes also show invariance at isoconductivity. This points to that the overall energy landscape in ionic liquids responds in the same way to density changes and is mainly governed by the nearest-neighbor interactions.

Structure and dynamics of highly concentrated LiTFSI/acetonitrile electrolytes

“…For ionic liquids QENS has revealed a complex picture of the dynamics with several different processes contributing to the local dynamics. [27][28][29][30][31][32] However, to the best of our knowledge no studies have been reported on the local dynamics in highly concentrated electrolytes using QENS.…”

How Temperature, Pressure, and Salt Concentration Affect Correlations in LiTFSI/EMIM-TFSI Electrolytes: A Molecular Dynamics Study

“…Experimental works and computational modeling cover a range of temperatures of possible operating conditions between the melting and boiling points of the IL. 15 − 20 Studies on pressure effects on ILs focus mainly on structural and conformational changes or glass transitions, 21 − 31 but there are also several works investigating transport properties—diffusion coefficients or conductivity. 21 , 26 , 32 − 35 …”

“…In search of optimized electrolytes, ionic liquids (ILs) gained significant attention, − owing to their several advantages, such as stability and nonvolatility. Experimental works and computational modeling cover a range of temperatures of possible operating conditions between the melting and boiling points of the IL. − Studies on pressure effects on ILs focus mainly on structural and conformational changes or glass transitions, − but there are also several works investigating transport propertiesdiffusion coefficients or conductivity. ,,− …”

Confinement Effects in Dynamics of Ionic Liquids with Polymer‐Grafted Nanoparticles

“…A three‐fold jump model is usually applied for the rotation of methyl groups in the cation [13,29] . The random jump model describes the intramolecular rotation of alkyl side chain of imidazolium cation [13] or pyrrolidinium cation [30] . Another model describes diffusive motion within a restricted space, like the vibrational motion of cation rings [13,30] or the localized confinement [7d] .…”

“…The random jump model describes the intramolecular rotation of alkyl side chain of imidazolium cation [13] or pyrrolidinium cation [30] . Another model describes diffusive motion within a restricted space, like the vibrational motion of cation rings [13,30] or the localized confinement [7d] . We will use this last model which captures the cation diffusion inside a confined sphere with radius a using the Equation: [21a,29a,31] …”