| Abstract
| - The self-diffusion coefficients (D) of the cation and anion in the ionic liquid 1-butyl-3-methylimidazoliumhexafluorophosphate ([BMIM]PF6) have been determined together with the electrical conductivity (κ) underhigh pressure. All three quantities strongly decrease with increasing pressure to ∼20% of their atmosphericpressure values at 200 MPa. D(PF6-) is always less than D([BMIM]+), despite the larger van der Waalsvolume of the cation. The pressure effect on the transport coefficients is discussed in terms of velocitycorrelation coefficients (VCCs or fij), the Nernst−Einstein equation (ionic diffusivity−conductivity), and thefractional form of the Stokes−Einstein relation (viscosity−conductivity and viscosity−diffusivity). It wasfound that the VCCs for the cation−cation, anion−anion, and cation−anion pairs are all negative and stronglypressure-dependent, increasing (becoming less negative) with increasing pressure. However, when the valuesof the VCCs for a given isotherm are normalized relative to the corresponding atmospheric pressure values,they collapse onto a single curve, as might be expected because the pressure should affect the interionicvelocity correlations in the same way for each type of interaction. These isothermal curves can be representedby the form exp(αp + βp2). The Nernst−Einstein deviation parameter, Δ, which depends on the differencesbetween the like−like ion and unlike ion VCCs (f++ + f-- − 2f+-), is very nearly constant under the conditionsexamined. The diffusion and molar conductivity (Λ) data are found to fit fractional forms of the Stokes−Einstein relationship with the viscosity, (ΛT) ∝ (T/η)t and Di ∝ (T/η)t , with t = (0.92 ± 0.05), independentof both temperature and pressure within the ranges studied and common to the three independently determinedproperties.
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