Ionic liquids are usually organic salts with a low melting point near room temperature. Because of various exceptional properties they have enormous application potential. For example, they are known as green solvents and nonflammable electrolytes.

Tailor-made properties

The major advantage of ionic liquids is that their properties can be tailored to a desired application by the choice of an appropriate pair of anions and cations. However, the relations between the complex microscopic structures and dynamics and the macroscopic properties of ionic liquids are still subject to intensive research.

Picture: AG Vogel

Simulations of ionic liquids

Many ionic liquids show very prominent structural and dynamical heterogeneities. Molecular dynamics simulations enable detailed analyses of the space-time characteristics of these inhomogeneitites.

Our simulation results reveal that various ionic liquids with amphiphilic components feature extended polar and nonpolar regions, which grow upon cooling and form bicontinous phases at sufficiently low temperatures. Moreover, it is possible to distinguish ions with very different dynamics on intermediate time scales between ballistic and diffusive motions. Because studies of these spacious structural and dynamical heterogeneities require time-consuming simulations of very large systems, we collaborate with other research groups to develop computationally less expensive coarse-grained models of ionic liquids in the framework of a DFG funded collaborative research center (TRR 146).

Polar and nonpolar regions in molecular dynamics simulations of an ionic liquid
Polar and nonpolar regions in molecular dynamics simulations of an ionic liquid
Picture: AG Vogel

Experiments on ionic liquids

Nuclear magnetic resonance (NMR) experiments are ideally suited to investigate the complex dynamical behaviors of ionic liquids. The isotope selectivity of the method allows for separate analyses of anion and cation dynamics.

Furthermore, NMR provides access to both the local reorientation and the translational diffusion of the ions. We exploit these capabilities, e.g., to ascertain how the properties depend on the chemical structure of the constituents and how structural inhomogeneities on nanometer scales affect the interplay of short-range and long-range dynamics. Our studies show that, unlike that of the anions, the size of the cations, e.g., the length of their alkyl tails, determines the absolute and relative diffusivities of the ionic components. Moreover, we find that a segregation into extended polar and nonpolar regions can lead to a breakdown of the Stokes-Einstein-Debye relation, which is a hallmark of liquids and predicts a coupling of rotational motion and translational diffusion.

Temperature-dependent diffusion coefficients of anions and cations in various ionic liquids. The diffusion of the ionic species depends differently on the length of the alkyl tail of the cations.
Temperature-dependent diffusion coefficients of anions and cations in various ionic liquids. The diffusion of the ionic species depends differently on the length of the alkyl tail of the cations.

Selected publications

  • Manuel Becher, Elisa Steinrücken, and Michael Vogel, “On the Relation between Reorientation and Diffusion in Glass-Forming Ionic Liquids with Micro-Heterogeneous Structures,” The Journal of Chemical Physics 151, no. 19 (November 21, 2019): 194503, https://doi.org/10.1063/1.5128420.
  • Sebastian Kloth et al., “Coarse-Grained Model of a Nanoscale-Segregated Ionic Liquid for Simulations of Low-Temperature Structure and Dynamics,” Journal of Physics: Condensed Matter 33, no. 20 (May 19, 2021): 204002, https://doi.org/10.1088/1361-648X/abe606.
  • Tamisra Pal and Michael Vogel, “Role of Dynamic Heterogeneities in Ionic Liquids: Insights from All-Atom and Coarse-Grained Molecular Dynamics Simulation Studies,” ChemPhysChem 18, no. 16 (August 18, 2017): 2233–42, https://doi.org/10.1002/cphc.201700504.