| Abstract
| - The self-diffusion coefficients (D) of the cation and anion in the ionic liquids 1-hexyl-3-methylimidazolium and 1-octyl-3-methylimidazolium hexafluorophosphates ([HMIM]PF6 and [OMIM]PF6) and 1-butyl-3-methylimidazolium and 1-octyl-3-methylimidazolium tetrafluoroborates ([BMIM]BF4) and ([OMIM]BF4) have been determined together with the electrical conductivities (κ) of [HMIM]PF6 and [BMIM]BF4 under high pressure. The pressure effect on the transport coefficients is discussed in terms of velocity cross-correlation coefficients (VCCs or fij), the Nernst−Einstein equation (ionic diffusivity−conductivity), and the fractional form of the Stokes−Einstein relation (viscosity−conductivity and viscosity−diffusivity). The (mass-fixed frame of reference) VCCs for the cation−cation, anion−anion, and cation−anion pairs are all negative and strongly pressure dependent, increasing (becoming less negative) with increasing pressure. VCCs are the more positive for the stronger ion-velocity correlations; therefore, f+ − is least negative in each case. In general, f− − is less negative than f+ +, indicating a smaller correlation of velocities of distinct cations than that for distinct anions. However, for [OMIM]PF6, the like-ion fii are very similar to one another. Plots of the VCCs for a given ion−ion correlation against fluidity (reciprocal viscosity) show the fij to be strongly correlated with the viscosity as either temperature or pressure are varied, that is, fij ≈ fij(η). The Nernst−Einstein deviation parameter, Δ, is nearly constant for each salt under the conditions examined. It is emphasized that nonzero values of Δ are not necessarily due to ion pairing but result from differences between the like-ion and unlike-ion VCCs, because Δ is proportional to (f+ + + f− − − 2f+ −). The diffusion and molar conductivity (Λ) data are found to fit fractional forms of the Stokes−Einstein relationship, (ΛT) ∝ (T/η)t and Di ∝ (T/η)t, with t = (0.90 ± 0.05) for all these ionic liquids, independent of both temperature and pressure within the ranges studied.
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