58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Ionic liquids (ILs) have emerged as an important class of environmentally friendly solvents that show tremendous promise for applications involving the production, storage, and efficient utilization of energy. Through selective chemical functionalization of the constituent molecules, the physical properties of ILs can be tuned for specific applications. For example, designer ILs have been developed to remove selectively harmful CO2 from flue gas mixtures, thus allowing for a cleaner utilization of fossil fuels. It is important that the fundamental structure and dynamics of ILs be understood to aid in the design of new ILs for unique applications. Unlike conventional, single-component solvents, ILs are known to exhibit heterogeneous structure and dynamics that have profound implications for their physical properties. This project focuses on the use of molecular dynamics (MD) simulations to understand solvation dynamics in ILs. Solvation responses in ILs occur over seven decades in time (from ~10 fs to ~10 ns) and with complex kinetic profiles. While solvation responses have been measured experimentally in a broad range of ILs, the molecular-level factors responsible for their complex kinetic profiles have not yet been elucidated. The objective of the proposed simulations is to compute the solvation responses over the full range of time scales for a series of ILs using all-atom force fields. A direct comparison to experiment provides a unique validation of both the simulation models and the methods used to compute the responses, and the proposed decomposition schemes will resolve the molecular mechanisms of solvation dynamics in ILs.

2. Summary of Results

We have successfully calculated the solvation response of coumarin 153 (C153) in the ionic liquid [emim][BF₄] (Figure 1). This calculation required an extensive (2000 ns) molecular dynamics simulation of C153 in [emim][BF₄], as well as quantum chemical calculations to model the ground and excited state charge distributions of the C153 molecule. The agreement with experiment is impressive, which serves to validate the simulations. The complex, multi-exponential kinetics of the response over five orders of magnitude of time, including the interesting "plateau" behavior around 1 ps, are captured by the simulations.

We have determined the mechanism for the solvation response of  coumarin 153 (C153) in the ionic liquid [emim][BF₄] based on four observations from the validated simulations: (1) the [BF₄] anion dominates the solvation response, (2) the [BF₄] part of the response is dominated by translational motions of molecule, (3) the C153 molecule is preferentially solvated by [emim] molecules, and (4) the kinetics for the translational motion of [BF₄] into and out of the first solvation shell of C153 mimic the total response. Based on these observations, we can state the mechanisms of solvation response in ionic liquids succinctly as originating from the motion of the anions into and out of the first solvation shell of the dye molecule. This mechanism is exceedingly different than in conventional polar solvents where the rotational motion of molecules in near proximity to the dye dominate the response.      

Lastly, the solvation response of coumarin 153 has been computed in a series of four different imidazolium-based ionic liquids, including [emim][BF₄], [bmim][BF₄], [bmim][PF₆], and [emim][TfO]. The longest timescale for solvation dynamics was extracted with a single exponential fit and is shown, compared to experiment, in Figure 2. The trend in the longest timescale between the different ionic liquids is reproduced fairly well. The mechanism of solvation dynamics was found to be identical in the four liquids studied, which confirms the generality of the mechanism.

3. Future Directions

An important question remains for future studies: how general is this anion-translation mechanism? However, as one deviates from the structural motifs of the C153/[imidazolium][BF₄] system, new solvation dynamics mechanisms could emerge. This presents both challenges and opportunities for the theoretical and experimental communities interested in designing ILs with properties selectively tuned for specific applications.

References

(1)         Zhang, X.-X.; Liang, M.; Ernsting, N. P.; Maroncelli, M. Complete Solvation Response of Coumarin 153 in Ionic Liquids. J. Phys. Chem. B 2013, 117, 4291–4304.

(2)         Maroncelli, M.; Zhang, X.-X.; Liang, M.; Roy, D.; Ernsting, N. P. Measurements of the Complete Solvation Response of Coumarin 153 in Ionic Liquids and the Accuracy of Simple Dielectric Continuum Predictions. Faraday Discuss. 2012, 154, 409–424.

Figure 1. Calculated (black) and experimental^1,2 (green) solvation response functions.

Figure 2. Comparison between experiment and theory of the longest timescale for solvation dynamics in a series of imidazolium-based ILs.