Dhungana Kamal B, Margulis Claudio J
Department of Chemistry, University of Iowa , Iowa City, Iowa 52242, United States.
J Phys Chem B. 2017 Jul 20;121(28):6890-6897. doi: 10.1021/acs.jpcb.7b04038. Epub 2017 Jul 5.
The response of ionic liquids to external perturbations including elevated pressure is a topic of current interest for applications such as tribology. Ionic liquids come in many classes, including those that are amphiphilic and some that are mostly polar having either cationic or anionic tails that are functionalized. The current study compares the effect of elevated pressure on two ionic liquids with different types of cationic tail. 1-Decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (Cmim/NTf) is amphiphilic whereas isoelectronic ether-functionalized 1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (COmim/NTf) has cationic tails that are more polar and conformationally different. We find that the response to elevated pressure for these two systems is quite unsimilar. Intramolecular conformational changes as well as changes in the structure of liquid nanodomains appear to be significantly more prominent in the case of Cmim/NTf. Whereas both the density of Cmim/NTf and COmim/NTf change at elevated pressure, the change is more dramatic for Cmim/NTf. The very different response for each of these two types of ionic liquids can be clearly gleaned from distribution functions in real space and the partial subcomponents of the X-ray structure function, S(q), in reciprocal space. Liquid structure in the case of COmim/NTf, and the intramolecular conformational structure of COmim in particular, appear to be more resilient to pressure changes than those in the isoelectronic amphiphilic analogue.
离子液体对包括高压在内的外部扰动的响应是摩擦学等应用领域当前关注的一个话题。离子液体有许多种类，包括两亲性的以及一些大多为极性的，其阳离子或阴离子尾部带有官能团。当前的研究比较了高压对两种具有不同类型阳离子尾部的离子液体的影响。1-癸基-3-甲基咪唑双（三氟甲基磺酰）酰胺（Cmim/NTf）是两亲性的，而异电子醚官能化的1-(2-(2-(2-甲氧基乙氧基)乙氧基)乙基)-3-甲基咪唑双（三氟甲基磺酰）酰胺（COmim/NTf）具有极性更强且构象不同的阳离子尾部。我们发现这两个体系对高压的响应非常不同。在Cmim/NTf的情况下，分子内构象变化以及液体纳米域结构的变化似乎更为显著。虽然Cmim/NTf和COmim/NTf的密度在高压下都会发生变化，但Cmim/NTf的变化更为剧烈。从实空间的分布函数以及倒易空间中X射线结构函数S(q)的部分子分量可以清楚地看出这两种类型离子液体各自非常不同的响应。与等电子两亲类似物相比，COmim/NTf的液体结构，特别是COmim的分子内构象结构，似乎对压力变化更具弹性。
