Nanoconfinement between Graphene Walls Suppresses the Near-Wall Diffusion of the Ionic Liquid BMIMPF6

C Shao and WL Ong and J Shiomi and AJH McGaughey, JOURNAL OF PHYSICAL CHEMISTRY B, 125, 4527-4535 (2021).

DOI: 10.1021/acs.jpcb.1c02562

We identify two distinct regimes for the diffusion of the ionic liquid BMIMPF6 confined between parallel graphene walls using molecular dynamics simulations. Within 2 nm of the wall, the cations and anions form a well-defined layered structure. In this region, the in-plane diffusion coefficients are suppressed when compared to their bulk values and increase monotonically with the distance away from the wall. Beyond 2 nm from the wall, the density profile and in-plane diffusion coefficients recover their bulk values. The channel-averaged in-plane diffusion coefficients increase monotonically with wall separation and recover the bulk values at a separation of 15 nm. A simple semianalytical model is proposed that mirrors this trend. The results also highlight the importance of applying a finite-size correction to molecular dynamics-predicted diffusion coefficients of confined liquids, which may otherwise be unusually larger than their bulk values.

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