Wu Boning, Yamashita Yuki, Endo Takatsugu, Takahashi Kenji, Castner Edward W
Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA.
Institute of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan.
J Chem Phys. 2016 Dec 28;145(24):244506. doi: 10.1063/1.4972410.
Ionic liquids with cationic organosilicon groups have been shown to have a number of useful properties, including reduced viscosities relative to the homologous cations with hydrocarbon substituents on the cations. We report structural and dynamical properties of four ionic liquids having a trimethylsilylpropyl functional group, including 1-methyl-3-trimethylsilylpropylimidazolium (Si-C-mim) cation paired with three anions: bis(fluorosulfonyl)imide (FSI), bis(trifluoromethanesulfonyl)imide (NTf), and bis(pentafluoroethanesulfonyl)imide (BETI), as well as the analogous N-methyl-N-trimethylsilylpropylpyrrolidinium (Si-C-pyrr) cation paired with NTf. This choice of ionic liquids permits us to systematically study how increasing the size and hydrophobicity of the anions affects the structural and transport properties of the liquid. Structure factors for the ionic liquids were measured using high energy X-ray diffraction and calculated from molecular dynamics simulations. The liquid structure factors reveal first sharp diffraction peaks (FSDPs) for each of the four ionic liquids studied. Interestingly, the domain size for Si-C-mim/NTf indicated by the maxima for these peaks is larger than for the more polar ionic liquid with a similar chain length, 1-pentamethyldisiloxymethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide (SiOSi-mim/NTf). For the series of Si-C-mim ionic liquids, as the size of the anion increases, the position of FSDP indicates that the intermediate range order domains decrease in size, contrary to expectation. Diffusivities for the anions and cations are compared for a series of both hydrocarbon-substituted and silicon-substituted cations. All of the anions show the same scaling with temperature, size, and viscosity, while the cations show two distinct trends-one for hydrocarbon-substituted cations and another for organosilicon-substituted cations, with the latter displaying increased friction.
带有阳离子有机硅基团的离子液体已被证明具有许多有用的性质，包括相对于阳离子上带有烃基取代基的同系阳离子而言，其粘度有所降低。我们报告了四种具有三甲基硅丙基官能团的离子液体的结构和动力学性质，包括与三种阴离子配对的1-甲基-3-三甲基硅丙基咪唑鎓（Si-C-mim）阳离子：双（氟磺酰）亚胺（FSI）、双（三氟甲磺酰）亚胺（NTf）和双（五氟乙磺酰）亚胺（BETI），以及与NTf配对的类似的N-甲基-N-三甲基硅丙基吡咯烷鎓（Si-C-pyrr）阳离子。这种离子液体的选择使我们能够系统地研究阴离子尺寸和疏水性的增加如何影响液体的结构和传输性质。使用高能X射线衍射测量离子液体的结构因子，并通过分子动力学模拟计算得出。液体结构因子揭示了所研究的四种离子液体各自的第一尖锐衍射峰（FSDPs）。有趣的是，这些峰的最大值所表明的Si-C-mim/NTf的域尺寸大于具有相似链长的极性更强的离子液体1-五甲基二硅氧甲基-3-甲基-咪唑鎓双（三氟甲磺酰）亚胺（SiOSi-mim/NTf）。对于Si-C-mim离子液体系列，随着阴离子尺寸的增加，FSDP的位置表明中间范围有序域的尺寸减小，这与预期相反。比较了一系列烃基取代和硅基取代阳离子的阴离子和阳离子扩散率。所有阴离子在温度、尺寸和粘度方面都表现出相同的标度关系，而阳离子则呈现出两种不同的趋势——一种是烃基取代阳离子的趋势，另一种是有机硅取代阳离子的趋势，后者显示出摩擦力增加。
