Non-classical diffusion in ionic liquids

Abstract

In this study the diffusion coefficient of neutral and cationic ferrocenyl-derivatives have been characterised in a range of 1-alkyl-3-methylimidazolium ionic liquids of the general form [C_nC₁Im]⁺[X]⁻. The electrochemistry of ferrocene, 1-ferrocenylmethylimidazole (FcC₁Im), 1-ferrocenylmethylimidazolium bis(trifluoromethanesulfonyl)imide ([FcC₁C₁Im][Tf₂N]) and N,N,N,N-trimethylferrocenyl-methylammonium bis(trifluoromethanesulfonyl)imide ([FcC₁NMe₃][Tf₂N]), in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C₂C₁Im][Tf₂N]) was investigated. It was shown that the diffusion coefficients of each were not significantly affected by the presence and location of a positive charge on the ferrocenyl-derivative, suggesting that coulombic solvent–solute interactions did not hinder motion of these species in ionic liquids. The diffusion coefficients for [FcC₁C₁Im][Tf₂N] in five [C_nC₁Im][X] ionic liquids were determined as a function of temperature and the data shown to disobey the Stokes–Einstein equation. This observation is consistent with the fact that ionic liquids are glass formers, systems in which non-Stokesian behaviour is well documented. Measured diffusion coefficient data was used to determine correlation length in the ionic liquid and was found to correlate with the average size of holes, or voids, within the ionic liquid. This interpretation suggests that a model by which a migrating species can jump between voids or holes within the liquid is highly appropriate and is consistent with the observed behaviour measured across a range of temperatures.