Self-Diffusion Coefficients and Related Transport Properties for a Number of Fragile Ionic Liquids

Ion self-diffusion coefficients (DSi) for [EMIM][CF3SO3] and [chol][Tf2N] and cation self-diffusion coefficients for [CH3SO3]-, [TCB]-, and [FAP]- salts of [EMIM]+ are reported together with ancillary data allowing calculation of velocity cross-correlation (VCC or fij), distinct diffusion (Ddij) and Laity resistance coefficients (rij). Comparison of ion Stokes-Einstein-Sutherland (SES) exponents is a useful test of experimental consistency. New analysis shows the viscosity (η) and molar conductivity (Λ) of [EMIM][CH3SO3] and [EMIM][TCB] do not show the dynamic crossovers previously reported. Nor do DS and η for the comparison substance propylene carbonate. For all the ionic liquids, DS+ > DS-. Data show fractional SES (DSi-η, Ddij-η) and Walden (Λ-η) plots. The Nernst-Einstein parameter, Δ, for [EMIM][FAP] is approximately zero, implying the relative velocity of any ion pair selected randomly is uncorrelated with those of other distinct pairs. For the other ionic liquids, the relative pair velocities are anticorrelated. VCC values for the [EMIM]+ salts except [EMIM][FAP] lie in the order |f+-| < |f++| < |f-|; that is, the anticorrelation is smallest for the unlike ion interactions: correspondingly, r++ < r- < r+-. For [EMIM][FAP], |f++| < |f+-| < |f-|, with f+- ∼ (f++ + f-)/2 and r+-2 ∼r++ r-, consistent with Δ ∼ 0. © 2016 American Chemical Society.