Quantum design of ionic liquids for extreme chemical inertness and a new theory of the glass transition
Quantum design of ionic liquids for extreme chemical inertness and a new theory of the glass transition
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DOI：
10.1007/s10008-012-1974-2
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发表时间：
2013-01
影响因子：
2.5
通讯作者：
S. Fletcher;V. J. Black;I. Kirkpatrick;Thomas S. Varley
中科院分区：
文献类型：
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作者：
S. Fletcher;V. J. Black;I. Kirkpatrick;Thomas S. Varley
在许多现代技术（如电池和超级电容器）中，强烈需要氧化还原稳定的离子液体。在实验上，离子液体的稳定性可以通过不发生电子隧穿的电压范围来量化，但到目前为止，量子理论还没有系统地应用于这个问题。在这里，我们报告的电化学还原的一系列季铵阳离子的存在下，双（三氟甲基磺酰基）酰亚胺（TFSI）阴离子和使用非绝热电子转移理论来解释的结果。我们发现，增加链长的烷基基团赋予改善的化学惰性在所有可访问的温度。同时，降低季铵阳离子的对称性降低了相应离子液体的熔点，在两种情况下，在室温下产生高度惰性的溶剂。这些被称为己基三乙基铵TFSI（HTE-TFSI）和丁基三甲基铵TFSI（BTM-TFSI）。事实上，后者是电化学历史上氧化还原最稳定的两种溶剂。为了深入了解它们的特性，在+20 °C至+190 °C范围内进行了非常高精度的电导率测量。在这两种情况下，数据符合Vogel-Tammann-Fulcher（VTF）方程，具有“六个九”精度（R2> 0.999999）。电导率开始的临界温度与玻璃化转变温度Tg相一致。这是令人信服的证据，证明离子液体实际上是软化的玻璃。最后，通过关注离子液体的分子自由度和它们的体积电导率之间先前未被怀疑的联系，我们能够提出一个新的玻璃化转变理论。这应该具有远远超出离子液体的实用性，在玻璃金属和聚合物科学等不同领域。
In many modern technologies (such as batteries and supercapacitors), there is a strong need for redox-stable ionic liquids. Experimentally, the stability of ionic liquids can be quantified by the voltage range over which electron tunneling does not occur, but so far, quantum theory has not been applied systematically to this problem. Here, we report the electrochemical reduction of a series of quaternary ammonium cations in the presence of bis(trifluoromethylsulfonyl)imide (TFSI) anions and use nonadiabatic electron transfer theory to explicate the results. We find that increasing the chain length of the alkyl groups confers improved chemical inertness at all accessible temperatures. Simultaneously, decreasing the symmetry of the quaternary ammonium cations lowers the melting points of the corresponding ionic liquids, in two cases yielding highly inert solvents at room temperature. These are called hexyltriethylammonium TFSI (HTE-TFSI) and butyltrimethylammonium TFSI (BTM-TFSI). Indeed, the latter are two of the most redox-stable solvents in the history of electrochemistry. To gain insight into their properties, very high precision electrical conductivity measurements have been carried out in the range +20 °C to +190 °C. In both cases, the data conform to the Vogel-Tammann-Fulcher (VTF) equation with “six nines” precision (R2> 0.999999). The critical temperature for the onset of conductivity coincides with the glass transition temperatureTg. This is compelling evidence that ionic liquids are, in fact, softened glasses. Finally, by focusing on the previously unsuspected connection between the molecular degrees of freedom of ionic liquids and their bulk conductivities, we are able to propose a new theory of the glass transition. This should have utility far beyond ionic liquids, in areas as diverse as glassy metals and polymer science.