Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, USA.
J Chem Phys. 2011 Sep 28;135(12):124507. doi: 10.1063/1.3643124.
Molecular dynamics simulations have been performed to investigate the structure and dynamics of the ionic liquid, 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C(4)mim][Tf(2)N]) in the temperature range of 283-460 K. Extensive analysis was carried out to characterize a number of structural and dynamic features. Transport properties were computed using a variety of equilibrium methods that employed the Green-Kubo and Einstein formulations. Nonequilibrium techniques were also used. In general, different methods mostly yielded consistent results, although some differences were observed. Computed self-diffusivities and ionic conductivities tended to be slightly lower than experimental values, while computed viscosities were significantly higher than experiment. Computed thermal conductivities agreed reasonably well with experimental data. Despite these discrepancies, the simulations capture the experimental temperature-dependent trends for all these transport properties. Single ion dynamics were studied by examining diffusional anisotropy, the self-part of the van Hove function, non-Gaussian parameters, and incoherent intermediate scattering functions. It is found that cations diffuse faster than anions and are more dynamically heterogeneous. A clear anisotropy is revealed in cation displacement, with the motion normal to the imidazolium ring plane being the most hindered and the motion along the alkyl chain in the plane of the ring being the most facile. Cations structurally relax faster than anions but they rotationally relax slower than anions. There is a pronounced temperature dependence to the rotational anisotropy of the cations, but only a weak temperature dependence for the anions. The ionic conductivity deviates from the Nernst-Einstein relation due to the correlated motion of cations and anions. The results suggest that the dynamical behavior of this and related ionic liquids is extremely complex and consists of many different modes with widely varying timescales, making the prediction of dynamical trends extremely difficult.
已进行分子动力学模拟，以研究离子液体 1-丁基-3-甲基咪唑双（三氟甲烷磺酰基）亚胺 ([C(4)mim][Tf(2)N]) 在 283-460 K 温度范围内的结构和动力学。进行了广泛的分析以表征许多结构和动态特征。使用各种平衡方法计算输运性质，这些方法采用格林-库伯和爱因斯坦公式。还使用了非平衡技术。一般来说，不同的方法主要产生一致的结果，尽管观察到一些差异。计算的自扩散率和离子电导率略低于实验值，而计算的粘度明显高于实验值。计算的热导率与实验数据相当吻合。尽管存在这些差异，但模拟捕获了所有这些输运性质的实验温度依赖性趋势。通过检查扩散各向异性、van Hove 函数的自部分、非高斯参数和非相干中间散射函数来研究单离子动力学。发现阳离子的扩散速度比阴离子快，并且动态异质性更大。阳离子位移呈现明显的各向异性，垂直于咪唑环平面的运动受到最大阻碍，而沿环平面的烷基链的运动最容易。阳离子的结构弛豫速度比阴离子快，但旋转弛豫速度比阴离子慢。阳离子的旋转各向异性对温度有明显的依赖性，但阴离子的依赖性很弱。由于阳离子和阴离子的相关运动，离子电导率偏离了能斯特-爱因斯坦关系。结果表明，这种和相关离子液体的动力学行为极其复杂，由具有广泛变化时间尺度的许多不同模式组成，使得预测动力学趋势变得非常困难。
