Rotational and Translational Diffusion of Ionic Liquids in Silica Nanopores

Abstract

Diffusion in ionic liquids (ILs) contained in silica nanopores is investigated in a wide frequency and temperature range by a combination of Broadband Dielectric Spectroscopy (BDS) and Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR). By applying the Einstein-Smoluchowski relations to the dielectric spectra, diffusion coefficients are obtained in quantitative agreement with independent PFG NMR. More than tenfold systematic decrease in the effective diffusion coefficient (for [HMIM] [PF$_{6}$]) from the bulk value is observed in the silica nanopores. A model assuming a reduced mobility at the IL/porous matrix is proposed and shown to provide quantitative explanation for the remarkable decrease of effective transport quantities (such as diffusion coefficient, DC conductivity and consequently, the dielectric loss) of the IL in bare porous silica membranes. This approach is supported by the observation that silanization of silica nanopores results in significant increase of the effective diffusion coefficient, which approaches the value for the bulk liquid. For a different IL ([BMIM] [BF$_{4}$]), it is observed that ionic mobility at lower temperatures is enhanced by more than two decades under nanoconfinement in comparison to the bulk value. This increase in the diffusivity is attributed to reduced packing density of the ions in the nanopores. In summary, the resultant macroscopic transport properties of glass-forming ILs in confining space are determined by a subtle interplay between surface- and confinement-effects.