Orientational Dynamics of Room Temperature Ionic Liquid/Water Mixtures: Water-Induced Structure
Orientational Dynamics of Room Temperature Ionic Liquid/Water Mixtures: Water-Induced Structure
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DOI：
10.1021/jp209942r
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发表时间：
2012-02-16
影响因子：
3.3
通讯作者：
Fayer, Michael D.
中科院分区：
文献类型：
--
作者：
Sturlaugson, Adam L.;Fruchey, Kendall S.;Fayer, Michael D.
本文报道了一系列1-烷基-3-甲基咪唑四氟硼酸盐室温离子液体（RTIL）的光外差探测光克尔效应（OHD-OKE）随链长和水浓度的变化。纯RTIL重定向动力学的形式是相同的其他分子液体研究OHD-OKE（两个幂律其次是一个单一的指数衰减在很长的时间），但在室温下慢得多。与此相反，水的加入到较长的烷基链RTIL的原因出现了一个长时间的双指数取向各向异性衰减。这种明显的双指数衰减没有看到以前在OHD-OKE实验中的任何类型的液体，并在这里使用一个摆动的圆锥模型进行分析。长链RTILs的慢组分不服从Debye-Stokes-Einstein（DSE）方程的范围内的解决方案，因此，我们将其归因于缓慢的阳离子重定向扩散所造成的硬化的阳离子烷基尾尾协会。衰减的快速分量被分配给拴系咪唑鎓头基的运动（摆动）。圆锥体摆动分析提供了在长时间尺度完全取向随机化之前由咪唑头基采样的角度范围的估计。这里观察到的非均相动力学和非DSE行为应该对RTIL/水共溶剂混合物中的反应速率有显著影响。
Optical heterodyne detected optical Kerr effect (OHD-OKE) measurements on a series of 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) as a function of chain length and water concentration are presented. The pure RTIL reorientational dynamics are identical in form to those of other molecular liquids studied previously by OHD-OKE (two power laws followed by a single exponential decay at long times), but are much slower at room temperature. In contrast, the addition of water to the longer alkyl chain RTILs causes the emergence of a long time biexponential orientational anisotropy decay. Such distinctly biexponential decays have not been seen previously in OHD-OKE experiments on any type of liquid and are analyzed here using a wobbling-in-a-cone model. The slow component for the longer chain RTILs does not obey the Debye-Stokes-Einstein (DSE) equation across the range of solutions, and thus we attribute it to slow cation reorientational diffusion caused by a stiffening of cation alkyl tail tail associations. The fast component of the decay is assigned to the motions (wobbling) of the tethered imidazolium head groups. The wobbling-in-a-cone analysis provides estimates of the range of angles sampled by the imidazolium head group prior to the long time scale complete orientational randomization. The heterogeneous dynamics and non-DSE behavior observed here should have a significant effect on reaction rates in RTIL/water cosolvent mixtures.