Effect of Pressure on the Transport Properties of Ionic Liquids: 1-Alkyl-3-methylimidazolium Salts

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 (kappa) 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 approximately fij(eta). The Nernst-Einstein deviation parameter, Delta, is nearly constant for each salt under the conditions examined. It is emphasized that nonzero values of Delta are not necessarily due to ion pairing but result from differences between the like-ion and unlike-ion VCCs, because Delta is proportional to (f+ + + f- - - 2 f+ -). The diffusion and molar conductivity (Lambda) data are found to fit fractional forms of the Stokes-Einstein relationship, (LambdaT) proportional, variant (T/eta)(t) and Di proportional, variant (T/eta)(t), with t=(0.90+/-0.05) for all these ionic liquids, independent of both temperature and pressure within the ranges studied.

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

“…30 There is very good agreement [ Figure S1(b)] for the combined data sets for samples A and B with the 31 and that of Koller et al 32 The data of Neves et al 33 deviate slightly at temperatures between 30 and 70°C; those of Seki et al 34 lie below those of the other data sets by about 8 × 10 −3 g·cm −3 and, thus, can be disregarded. Good agreement [ Figure S1(c)] was found for [EMIM]-[CF 3 SO 3 ] with the vibrating-tube results of Freire et al 35 and the buoyancy results of Klomfar et al, 36 though the scatter for the latter is somewhat large; those of García-Miaja et al 37 lie slightly below our results. The vibrating-tube results of Rodiguez and Brennecke, 38 of Vuksanovic et al, 39 and of Seki et al 40 are somewhat lower, more than the combined uncertainties, and those of Gardas et al 41 and of Mbondo Tsamba et al 42 are very much lower still.…”

“…Figure 1 shows expansivities, α p = −(∂ρ/∂T) p /ρ, for the ionic liquids studied here: the temperature dependences are negligible or very slightly negative. 3 SO 3 ]. The experimental viscosities, conductivities, and average ion self-diffusion coefficients are listed in Tables S2(a), S3, and S4(a).…”

“…20 A comparison of the experimental values of T g determined from differential scanning calorimetry and those estimated with eq 12 is also given in Table 3 for [EMIM]-[CH 3 SO 3 ] and the other ionic liquids discussed below. As shown in Figure S3(a), the conductivity measurements of Stark et al 49 (conductivity meter) and of Watanabe and Tokuda 50 (impedance analyzer) lie significantly below the results of this work.…”

Influence of ion pairing in ionic liquids on electrical double layer structures and surface force using classical density functional approach

“…Thus ion paring per se may more properly be seen as a manifestation of strong correlations, which slow diffusion over an intermediate time scale, rather than as very long-lived entities. In this context, Harris et al have pointed out that an ionicity value less than unity can be attributed to correlations in ion velocities [26,27].…”

Chronoamperometric determination of diffusion coefficients of ferrocene in ionic liquids mixed with CO2 at high pressures

“…4, where the viscosities of neat ILs were used [32,39]. Similar trend was observed for ionic diffusions in neat 1-alkyl-3-methylimidazolium hexafluorophosphates and tetrafluoroborates; the fractional form of the StokesEinstein relation, D / (T/g) t , holds over a wide range of temperature and pressure with t % 0.9 [40,41].…”