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

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journal contribution
posted on 30.11.2016, 00:00 by Kenneth R. Harris, Mitsuhiro Kanakubo
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 [HMIM]­[PF6] ([C6C1Im]­[PF6]) and DS– for [OMIM]­[BF4] ([C8C1Im]­[BF4]). New atmospheric pressure self-diffusion, density, and conductivity data are provided for [HMIM]­[BF4] ([C6C1Im]­[BF4]). 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, [BMIM]­[BF4] and [BMIM]­[PF6] are almost equal, as are the corresponding Brownian or diffusive mobilities, (DSi/RT), for the cation, and also for these three small anions.