Ultrafast Aqueous Dynamics in Concentrated Electrolytic Solutions of Lithium Salt and Ionic Liquid

In the last few decades, aqueous electrolytes based on ionic liquids (ILs) have been attractive because of their various uses in chemical sciences and potential applications in Li-ion batteries. The presence of water molecules influences the molecular structure and complex transformations occurring at ultrafast timescales because of the interaction between the water and the ionic entities of ILs. In this study, we investigate water-IL interactions that correlate the vibrational dynamics of the associated infrared probe using a fast and accurate computational approach. The obtained results from our approach are directly compared with those of the ultrafast two-dimensional infrared spectroscopy experiments. We investigated the dynamics of ion-water interactions of aqueous lithium bis(trifluoromethylsulfonyl)imide, (LiNTf₂), and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, ([EMIm] [NTf₂]), with the help of vibrational dynamics of the deuteroxyl probe of water molecules using wavelet analysis of trajectories of classical molecular dynamics simulations. The normalized OD stretch frequency distribution of aqueous [EMIm][NTf₂] shows a heterogeneous bimodal line shape associated with both water and ion hydrogen-bonded OD populations. However, H₂O/LiNTf₂ solution depicts a broad spectral band without a distinct spectral feature. We observe faster timescales of frequency fluctuations, time-dependent hole propagation, and reorientation dynamics in aqueous [EMIm][NTf₂] solution. At a time-lapse of 0.1 ps, the 2D IR correlation spectrum in aqueous LiNTf₂ appears elongated along with the frequency diagonal. The spectral signature in the water-[EMIm][NTf₂] mixture is relatively symmetric in shape, indicating faster dynamics. Our results show the characteristic behavior of water molecules present in the aqueous ionic mixtures that distinguish two mixtures dynamically. While examining water interactions in aqueous ionic solutions, we predict that the additional water in the aqueous LiNTf₂ solution might be engaged in the structural arrangement of the restricted IL environment. In contrast, water in aqueous [EMIm][NTf₂] enhances the dynamics. Hence, we observe that a change in the cationic constituent of the IL solution results in a substantial change in the overall system dynamics.