Harris Kenneth R, Kanakubo Mitsuhiro, Tsuchihashi Noriaki, Ibuki Kazuyasu, Ueno Masakatsu
School of Physical, Environmental, and Mathematical Sciences, University College, University of New South Wales, Australian Defence Force Academy, Canberra, ACT 2600, Australia.
J Phys Chem B. 2008 Aug 14;112(32):9830-40. doi: 10.1021/jp8021375. Epub 2008 Jul 19.
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.
已测定了离子液体1-己基-3-甲基咪唑鎓六氟磷酸盐（[HMIM]PF6）、1-辛基-3-甲基咪唑鎓六氟磷酸盐（[OMIM]PF6）、1-丁基-3-甲基咪唑鎓四氟硼酸盐（[BMIM]BF4）和1-辛基-3-甲基咪唑鎓四氟硼酸盐（[OMIM]BF4）中阳离子和阴离子的自扩散系数（D），以及[HMIM]PF6和[BMIM]BF4在高压下的电导率（κ）。根据速度交叉相关系数（VCCs或fij）、能斯特 - 爱因斯坦方程（离子扩散率 - 电导率）以及斯托克斯 - 爱因斯坦关系的分数形式（粘度 - 电导率和粘度 - 扩散率），讨论了压力对传输系数的影响。阳离子 - 阳离子、阴离子 - 阴离子和阳离子 - 阴离子对的（质量固定参考系）VCCs均为负值，且强烈依赖于压力，随压力增加而增大（负值减小）。对于更强的离子速度相关性，VCCs更正；因此，在每种情况下f + - 最不呈负性。一般来说，f - - 比f + + 更不呈负性，这表明不同阳离子的速度相关性小于不同阴离子的速度相关性。然而，对于[OMIM]PF6，同类离子的fii彼此非常相似。给定离子 - 离子相关性的VCCs与流动性（粘度倒数）的关系图表明，当温度或压力变化时,fij与粘度密切相关，即fij近似于fij(η)。在所研究的条件下，每种盐的能斯特 - 爱因斯坦偏差参数Δ几乎是恒定的。需要强调的是，Δ的非零值不一定是由于离子对引起的，而是由于同类离子和异类离子VCCs之间的差异导致的，因为Δ与(f + + + f - - - 2 f + -)成正比。发现扩散和摩尔电导率（Λ）数据符合斯托克斯 - 爱因斯坦关系的分数形式，(ΛT) 与 (T/η)(t)成正比，Di与 (T/η)(t)成正比，对于所有这些离子液体，t =(0.90±0.05)，在所研究的温度和压力范围内与温度和压力均无关。
