Despite that the heterogeneous structure of ionic liquids (ILs) has been established, the consequences of the nanoscopic segregation on the dynamics of ILs has remained elusive. In particular, the spatial heterogeneity in ILs may result in inherently different local solute environments when using ILs as solvents. Such different local interactions will affect the ability to re-structure and dissipate excess energy during a chemical reaction, which determines the performance as a solvent.

We study these solvent dynamics using femtosecond infrared spectroscopy for a series of alkylammonium ionic liquids [1, 2]. In these experiments vibrational excitation of the ND or NH stretching vibrations results in an ultrafast, local temperature jump and we monitor the subsequent equilibration as a response to the dissipated energy. Owing to the intrinsic molecular specificity of infrared vibrations we can thus probe specifically the different moieties of the ionic liquid’s ions, providing access to the intrinsic solvation dynamics in the absence of any probe molecules. Our results show that ionic liquids are not only structurally heterogeneous, but exhibit a substantial dynamic heterogeneity, with the equilibration within the hydrophobic domains being two-fold faster than in the ionic domains.

The excitation anisotropy, which provides information on the local rotational mobility of the ionic fragments of the cations, is rather insensitive to the size of the cations: Our results even suggest that increasing the hydrophobic domain size goes along with an acceleration of the restricted rotational motion of the ammonium group of the cationic fragments of the IL’s ions. Such accelerated dynamics are indicative of reduced directional interionic interactions, which in turn signify a more fragile three-dimensional structure as the size of the hydrophobic fragments increases.

Our study shows that local dynamics in these ionic liquids are decoupled from macroscopic properties, like e.g. viscosity. Thus, our findings underline that the structural heterogeneity of ILs makes local dynamics heterogeneous, with substantially different solvation properties within the hydrophobic and the ionic sub-domains.

[1] J. Hunger, T. Sonnleitner, L Liu, R. Buchner, M. Bonn and H.J. Bakker, „Hydrogen-Bond Dynamics in a Protic Ionic Liquid: Evidence of Large-Angle Jumps“ J. Phys. Chem. Lett. 3, 3034 (2012).

[2] Z.P. Zheng, W.H. Fan, S. Roy, K. Mazur, A. Nazet, R. Buchner, M. Bonn, and J. Hunger “Ionic Liquids: Not only Structurally but also Dynamically Heterogeneous”, Angew. Chem. Int. Ed. 54, 687 (2015).