Revised and Extended Values for Self-Diffusion Coefficients of 1-Alkyl-3-methylimidazolium Tetrafluoroborates and Hexafluorophosphates: Relations between the Transport Properties

Earlier measurements of the self-diffusion coefficients of 1-alkyl-3-methylimidazolium (or RMIM, R = alkyl) tetrafluoroborates and hexafluorophosphates have been revised and extended to 90 °C. The main changes are to DS+ and DS- for HMIMPF6 (C6C1ImPF6) and DS- for OMIMBF4 (C8C1ImBF4). New atmospheric pressure self-diffusion, density, and conductivity data are provided for HMIMBF4 (C6C1ImBF4). Velocity cross-correlation, distinct diffusion, and Laity resistance coefficients have been calculated. There is no evidence for ion association. A new relation between the Nernst-Einstein deviation parameter (Δ) and resistance coefficients (rij) is derived; Δ tends toward 0.5 when the like-ion rii are much smaller than the unlike-ion rij, i.e., when the counterion interactions dominate. OMIM+ ion salts approach this limit. Stokes-Einstein-Sutherland and Walden plots overlap almost quantitatively for BF4-, PF6-, and Cl- salts with a common RMIM+ cation. That is, in thermodynamic states that have the same viscosity, the salt molar conductivities and hence ionic electrical mobilities of, say, BMIMBF4 and BMIMPF6 are almost equal, as are the corresponding Brownian or diffusive mobilities, (DSi/RT), for the cation, and also for these three small anions.