Effect of pressure on the transport properties of ionic liquids: 1-alkyl-3-methylimidazolium salts
Effect of pressure on the transport properties of ionic liquids: 1-alkyl-3-methylimidazolium salts
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DOI：
10.1021/jp8021375
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发表时间：
2008-08-14
影响因子：
3.3
通讯作者：
Ueno, Masakatsu
中科院分区：
文献类型：
--
作者：
Harris, Kenneth R.;Kanakubo, Mitsuhiro;Ueno, Masakatsu
离子液体1-己基-3-甲基咪唑和1-辛基-3-甲基咪唑六氟磷酸盐中阳离子和阴离子的自扩散系数D（[HMIM] PF 6和[OMIM] PF 6）和1-丁基-3-甲基咪唑鎓和1-辛基-3-甲基咪唑鎓四氟硼酸盐测定了[BMIM] BF_4和[OMIM] BF_4的高压电导率K值，并与[HMIM] PF_6和[BMIM] BF_4的高压电导率K值进行了比较。压力对传输系数的影响进行了讨论的速度互相关系数（VCC或f（ij）），能斯特-爱因斯坦方程（离子扩散率-电导率），和分数形式的斯托克斯-爱因斯坦关系（粘度-电导率和粘度-扩散率）。阳离子-阳离子、阴离子-阴离子和阳离子-阴离子对的（质量固定参照系）VCC都是负的，并且强烈依赖于压力，随着压力的增加而增加（变得不那么负）。离子速度相关性越强，VCC越正;因此，在每种情况下，f（+-）的负性最小。一般来说，f（--）比f（++）负得少，表明不同阳离子的速度相关性比不同阴离子的速度相关性小。然而，对于[OMIM] PF 6，类似离子f（ii）彼此非常相似。对于给定的离子-离子相关性对流动性（倒数粘度）的VCC曲线图显示，当温度或压力变化时，f（ij）与粘度强烈相关，即f（ij）近似于f（jj）（η）。能斯特-爱因斯坦偏差参数，德尔塔，几乎是恒定的每种盐的条件下检查。需要强调的是，Delta的非零值不一定是由于离子配对，而是由相似离子和不相似离子VCC之间的差异引起的，因为Delta与（f（++）+ f（--）- 2f（+-））成比例。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，（λ T）与（T/eta）（t）成比例，D-i与（T/eta）（t）成比例，对于所有这些离子液体，t =（0.90 +/-0.05），在所研究的范围内与温度和压力无关。
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 (K) 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 f(ij)), 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 f(ii) are very similar to one another. Plots of the VCCs for a given ion-ion correlation against fluidity (reciprocal viscosity) show the f(ij) to be strongly correlated with the viscosity as either temperature or pressure are varied, that is, f(ij) approximate to f(jj)(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(--) - 2f(+-)). The diffusion and molar conductivity (Lambda) data are found to fit fractional forms of the Stokes-Einstein relationship, (Lambda T) proportional to (T/eta)(t) and D-i proportional to (T/eta)(t), with t = (0.90 +/- 0.05) for all these ionic liquids, independent of both temperature and pressure within the ranges studied.