Habasaki J, Ngai K L
Tokyo Institute of Technology, Nagatsuta, Yokohama 226-8502, Japan.
Anal Sci. 2008 Oct;24(10):1321-7. doi: 10.2116/analsci.24.1321.
Molecular dynamics (MD) simulations have been performed to study the dynamics near the glass transition regime of molecular ions in ionic liquids. The glass transition temperature in the simulated 1-ethyl-3-methyl imidazolium nitrate (EMIM-NO(3)) system was determined by plotting density against temperatures. The dynamics at several temperatures in the liquid, supercooled liquid, and glassy states have been characterized by the diffusion coefficients, fractal dimension analysis of the trajectories, and the van-Hove functions. The diffusion coefficient approximately obeys the Vogel-Fulcher-Tammann (VFT) relation. However, two power laws or two exponentials are also good descriptions of the data. The fractal dimension of the random walks is a measure of the complexity of the trajectory, which is attributed to the geometrical correlations among successive motions. Rapid increase of the fractal dimension of the random walks on decreasing temperature is found for both cations and anions. Temperature dependence of the fractal dimension of the random walks for the long range (accelerated) motion is larger than that for short range (localized) motion. This reasonably explains the change in the slopes found in the temperature dependence of the diffusion coefficients. At around the glass transition temperature, long range motion is essentially absent during the observed times, up to several nano seconds. This feature is also confirmed by the van-Hove functions. Such slowing down of the dynamics in the fragile ionic liquids is characterized by the changes from long range motion to short range motion instead of sudden changes at around T(0) in the VFT relation.
已进行分子动力学（MD）模拟，以研究离子液体中分子离子在玻璃化转变区域附近的动力学。通过绘制密度与温度的关系曲线，确定了模拟的1-乙基-3-甲基咪唑鎓硝酸盐（EMIM-NO(3)）体系的玻璃化转变温度。通过扩散系数、轨迹的分形维数分析以及范霍夫函数，对液体、过冷液体和玻璃态在几个温度下的动力学进行了表征。扩散系数大致遵循Vogel-Fulcher-Tammann（VFT）关系。然而，两个幂律或两个指数也能很好地描述这些数据。随机游走的分形维数是轨迹复杂性的一种度量，这归因于连续运动之间的几何相关性。对于阳离子和阴离子，都发现随机游走的分形维数在温度降低时迅速增加。随机游走的分形维数对于长程（加速）运动的温度依赖性大于短程（局部）运动。这合理地解释了扩散系数温度依赖性中斜率的变化。在玻璃化转变温度附近，在长达几纳秒的观测时间内基本上不存在长程运动。范霍夫函数也证实了这一特征。在脆弱离子液体中，这种动力学减慢的特征是从长程运动变为短程运动，而不是在VFT关系中在T(0)附近突然变化。
