Linking Structure to Dynamics in Protic Ionic Liquids: A Neutron Scattering Study of Correlated and Single-Particle Motions

Abstract

Coupling between dynamical heterogeneity of ionic liquids and their structural periodicity on different length-scales can be directly probed by quasielastic neutron scattering with polarization analysis. The technique provides the tools to investigate single-particle and cooperative ion motions separately and, thus, dynamics of ion associations affecting the net charge transport can be experimentally explored. The focus of this study is the structure-dynamic relationship in the protic ionic liquid, triethylammonium triflate, characterized by strong hydrogen bonds between cations and anions. The site-selective deuterium/hydrogen-isotope substitution was applied to modulate the relative contributions of different atom groups to the total coherent and incoherent scattering signal. This approach in combination with molecular dynamics simulations allowed us to obtain a sophisticated description of cation self-diffusion and confined ion pair dynamics from the incoherent spectral component by using the acidic proton as a tagged particle. The coherent contribution of the neutron spectra demonstrated substantial ion association leading to collective ion migration that preserves charge alteration on picosecond time scale, as well as correlation of the localized dynamics occurring between adjacent ions.

Enhanced Microscopic Dynamics of a Liver Lipid Membrane in the Presence of an Ionic Liquid

“…Our measurements suggest that the incorporation of the IL in the membrane modulates both of these dynamical processes. We note that the dynamics of ILs have also been investigated using the QENS technique (Triolo et al, 2006;Mamontov et al, 2009;Aoun et al, 2010;Burankova et al, 2018;Nemoto et al, 2018), however in this work we focus on the dynamics of the lipids and how they are affected by the presence of IL, and not on the dynamics of the IL themselves, since the concentration is small (see experimental details).…”

A Quantitative Comparison of the Counting Significance of van Hove Integral Spectroscopy and Quasielastic Neutron Scattering

“…Among the available techniques, neutron scattering is unique in probing the motion of atomic nuclei, rather than measuring the response of the electrons to the nuclear dynamics, which is the case for most other spectroscopies. This direct type of interaction makes the comparison between neutron scattering and classical molecular dynamics simulations a successful and straightforward combination [9][10][11] .…”

“…A common variant of neutron scattering that is routinely used to access system dynamics is quasi-elastic neutron scattering (QENS) 5,6,[11][12][13][14][15][16] . The output is an energy-spectrum of the number of neutrons scattered by the sample as a function of energy transfer, making QENS de facto an "inelastic" method for dynamics.…”

Ionic liquid dynamics in nanoporous carbon: A pore-size- and temperature-dependent neutron spectroscopy study on supercapacitor materials

“…It is reasonable to assume that cations exhibiting a sluggish translational diffusion are also those that correspondingly possess slow localized dynamics. The existence of several translational diffusive motions on different dynamic length and time scales is not unusual for ionic liquids in carbon nanoconfinement [13,14] and even in the bulk state [ [70][71][72], where it is often related to a nanoscale structural organization [73][74][75]. Also for bulk [BuPy][Tf 2 N] such a dynamic heterogeneity is found [40].…”

“…Also for bulk [BuPy][Tf 2 N] such a dynamic heterogeneity is found [40]. In these cases, one dynamic component is regarded as the diffusion of ions bound in ionic aggregates, while the other, long range diffusive motion is considered to take place in the free liquid between these clusters [40,70,[70][71][72]. In this connection, a disturbance of the Coulombic charge ordering inside conductive pores, like those of carbon materials, is found, e.g., by x-ray diffraction experiments and Monte Carlo simulations [76,77], at which this effect is stronger in smaller pores [26,76,78].…”