Protic Ionic Liquids with Unusually High Dielectric Permittivities
Protic Ionic Liquids with Unusually High Dielectric Permittivities
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DOI：
10.1002/cphc.200800523
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发表时间：
2008-10-24
期刊：
影响因子：
2.9
通讯作者：
Weingaertner, Hermann
中科院分区：
文献类型：
--
作者：
Huang, Mian-Mian;Weingaertner, Hermann
离子液体（ILs）作为普通分子溶剂的替代品的应用正在迅速扩大为了评估它们的溶剂行为，极性是关键的兴趣。[2-4]极性描述了溶剂化能力，但不同的实验探讨了不同的方面。测量极性的一个关键量是相对介电常数εS（介电常数）。il的电导率使常规的εS测量变得不可能，但微波介电光谱[5-10]可以分离介电和导电响应，从而可以测定εS。结果表明，以1,3 -二烷基咪唑盐为主的广泛应用的il具有显著的特征。首先，在258C时，εS= 10-15的值与分子溶剂所覆盖的大范围值相比变化不大。这种对离子化学性质的不敏感，与离子离子的许多其他性质形成了鲜明对比，这些性质可以在很宽的范围内变化。其次，εS显著低于普通极性溶剂，这与带电体系形成高极性环境的预期相矛盾。人们可以想到许多需要更高介电常数的il的情况。有一个紧迫的问题是，这些特征在多大程度上是ILs的通用特征。在分子液体中，在氢键结构中经常发现高介电常数，其中偶极子之间的强取向相关性可以增强介电极化。[11,12]因此，研究“质子离子液体”的介电行为似乎是值得的。质子il是由一种硼离子酸和一种硼离子碱结合形成的。近年来对其性质和应用进行了综述一个著名的代表是硝酸乙胺，[14]，可以形成一个扩展的氢键网络其介电常数εS= 26.2在258c[5]的介电常数确实高于普通非质子介电常数。其他探针，例如基于溶剂变色位移的探针，也表明了质子il的高极性对于导电液体，εS的测量依赖于测定频率相关的介电色散曲线ε0ðνÞ在微波波段，这里是30MHz ν 20 GHz，在这里介电响应可以与电导响应分离。[6] εS由色散曲线的零频极限给出[Eq.(1)] εS μ m ν！ε0ðνÞ。ð1Þ
The use of ionic liquids (ILs) as alternatives to common molecular solvents is rapidly expanding.[1] For assessing their solvent behavior, the polarity is of key interest.[2–4] Polarity describes the solvation capability, but different experiments probe different facets. A key quantity for gauging the polarity is the relative dielectric permittivity, εS (‘dielectric constant’). The electrical conductivity of ILs renders conventional measurements of εS impossible, but microwave dielectric spectroscopy [5–10] allows to separate the dielectric and conductive responses, which enables the determination of εS. The results for widely used ILs, mainly 1, 3-dialkylimidazolium salts, indicate remarkable features. First, the values of εS= 10–15 at 258C vary little compared to the wide range of values covered by molecular solvents. This insensitivity to the chemical nature of the ions contrasts the behavior of many other properties of ILs, which can be varied over wide ranges. Second, εS is substantially lower than observed for common polar solvents, which contradicts the expectation that a charged system forms an environment of high polarity. One can think of many scenarios where ILs of higher dielectric constant are desirable. There is the pressing question to which extent these features are generic for ILs. In molecular liquids, high dielectric constants are often found in hydrogen-bonded structures, where strong orientational correlations between dipoles can enhance dielectric polarization.[11, 12] It seems therefore worthwhile to study the dielectric behavior of “protic ionic liquids”. Protic ILs are formed by a combination of a Brønsted acid and a Brønsted base. Their properties and applications have recently been reviewed.[13] A well-known representative is ethylammonium nitrate,[14] which can form an extended hydrogenbonded network.[13] Its dielectric constant of εS= 26.2 at 258C [5] is indeed higher than those of common aprotic ILs. A high polarity of protic ILs is also indicated by other probes, for example based on solvatochromic shifts.[13] For conductive liquids, measurement of εS resorts to the determination of the frequency-dependent dielectric dispersion curve ε0ðνÞ in the microwave regime, here 30MHz ν 20 GHz, where the dielectric response can be separated from the conductance response.[6] εS is given by the zero-frequency limit of the dispersion curve [Eq.(1)] εS ¼ lim ν! 0 ε0ðνÞ. ð1Þ