4 years ago

From Nanoscale to Microscale: Crossover in the Diffusion Dynamics within Two Pyrrolidinium-Based Ionic Liquids

From Nanoscale to Microscale: Crossover in the Diffusion Dynamics within Two Pyrrolidinium-Based Ionic Liquids
Stefano Passerini, Mosè Casalegno, Giovanni Battista Appetecchi, Andrea Mele, Franca Castiglione, Guido Raos
Knowledge of the ion motion in room temperature ionic liquids (RTILs) is critical for their applications in a number of fields, from lithium batteries to dye-sensitized solar cells. Experiments on a limited number of RTILs have shown that on macroscopic time scales the ions typically undergo conventional, Gaussian diffusion. On shorter time scales, however, non-Gaussian behavior has been observed, similar to supercooled fluids, concentrated colloidal suspensions, and more complex systems. Here we characterize the diffusive motion of ionic liquids based on the N-butyl-N-methylpyrrolidinium (PYR14) cation and bis(trifluoro methanesulfonyl)imide (TFSI) or bis(fluorosulfonyl)imide (FSI) anions. A combination of pulsed gradient spin–echo (PGSE) NMR experiments and molecular dynamics (MD) simulations demonstrates a crossover from subdiffusive behavior to conventional Gaussian diffusion at ∼10 ns. The deconvolution of molecular displacements into a continuous spectrum of diffusivities shows that the short-time behavior is related to the effects of molecular caging. For PYR14FSI, we identify the change of short-range ion–counterion associations as one possible mechanism triggering long-range displacements.

Publisher URL: http://dx.doi.org/10.1021/acs.jpclett.7b02431

DOI: 10.1021/acs.jpclett.7b02431

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