On the relation between reorientation and diffusion in glass-forming ionic liquids with micro-heterogeneous structures

Abstract

We investigate complex structure-dynamics relations in glass-forming ionic liquids comprising 1-alkyl-3-methylimidazolium cations and bis(trifluoromethylsulfonyl)imide anions. In doing so, we exploit the microheterogeneous structures emerging when the alkyl length is increased in the range n = 1–12 and use that 1H and 2H NMR give information about cation dynamics, while 19F NMR reports on anion motions. Furthermore, we combine spin-lattice relaxation analysis, including field-cycling relaxometry, with stimulated-echo experiments to follow reorientation dynamics related to structural relaxation in wide dynamic ranges and we apply static field gradients to probe translational diffusion. The resulting correlation times τ and diffusion coefficients D show Vogel-Fulcher-Tammann temperature dependence. Moreover, they indicate a moderate slowdown of both cation and anion dynamics with increasing alkyl length n. However, the relative diffusivities of the ionic species depend on the cation size, where cations are more mobile for n < 6 and anions for n > 6. Finally, we relate rotational and translational motions in the framework of the Stokes-Einstein-Debye (SED) approach. We find that the SED relation is obeyed for anion dynamics in all samples, while it breaks down for cation dynamics when n is increased. The origin of this SED breakdown is shown to differ fundamentally from that reported previously for conventional glass formers. We argue that an emergence of cation clusters causes a retardation of cation diffusion relative to cation reorientation upon cooling, i.e., the studied ionic liquids show a complex interplay of structural and dynamical properties.

Equilibrium Solvation, Electron-Transfer Reactions, and Stokes-Shift Dynamics in Ionic Liquids

“…The single-particle translational dynamics are often faster than collective rotational dynamics, leading to the well-documented violation of the Stokes–Einstein equation. In fact, recent NMR measurements report retardation of cation’s translational diffusion relative to rotational diffusion in ionic liquids, in contrast to the conventional phenomenology of glass formers. In contrast to these established views, our perspective on collective relaxation of charge density does not separate the ionic species and points to structural, related to positions of ions, origin of the much slower relaxation time τ 2 ( k ).…”

Pulsed field gradient NMR diffusion measurement in nanoporous materials

“…It was the associated loss in sensitivity that has given rise to the development of the PFG NMR [29,67,68], followed by a most impressive progress in the application of NMR diffusometry, making it the possibly most versatile technique of diffusion measurement quite in general. Continuous progress in sensitivity enhancement and the options provided by novel generations of superconducting magnets have meanwhile initiated a renaissance of NMR diffusometry with also constant magnetic field gradients [69][70][71][72]. Compared to PFG NMR, however, the acquisition of a sufficiently large amount of data is still connected with a much greater expenditure of time.…”

Molecular Dynamics in Ionic Liquid/Radical Systems

“…Several studies on IL systems have been performed using NMRD, giving valuable information on the dynamics of these solvents. − Cations and anions in ILs usually contain different NMR-sensitive nuclei. This offers the possibility to study the different ionic species by observing the relaxation properties of the respective nucleus.…”