Habasaki J, Ngai K L
Tokyo Institute of Technology, Nagatsuta 4259, Yokohama 226-8502, Japan.
J Chem Phys. 2008 Nov 21;129(19):194501. doi: 10.1063/1.3005372.
Molecular dynamics simulations have been performed to study the complex and heterogeneous dynamics of ions in ionic liquids. The dynamics of cations and anions in 1-ethyl-3-methyl imidazolium nitrate (EMIM-NO(3)) are characterized by van Hove functions and the corresponding intermediate scattering functions F(s)(k,t) and elucidated by the trajectories augmented by the use of singular spectrum analysis (SSA). Several time regions are found in the mean squared displacement of the ions. Change in the slope in a plot of the diffusion coefficient against temperature is found at around 410 K in the simulation. Heterogeneous dynamics with the presence of both localized ions and fast ions capable of successive jumps were observed at long time scales in the self-part of the van Hove functions and in the trajectories. Non-Gaussian dynamics are evidenced by the self-part of the van Hove functions and wave number dependence of F(s)(k,t) and characterized as Levy flights. Successive motion of some ions can continue even after several nanoseconds at 370 K, which is longer than the onset time of diffusive motion, t(dif). Structure of the long time dynamics of fast ions is clarified by the phase space plot of the successive motion using the denoised data by SSA. The continual dynamics are shown to have a long term memory, and therefore local structure is not enough to explain the heterogeneity. The motion connecting localized regions at about 370 K is jumplike, but there is no typical one due to local structural changes during jump motion. With the local motion, mutual diffusion between cation and anion occurs. On decreasing temperature, mutual diffusion is suppressed, which results in slowing down of the dynamics. This "mixing effect of cation and anion" is compared with the "mixed alkali effect" found in the ionics in the ionically conducting glasses, where the interception of paths by different alkali metal ions causes the large reduction in the dynamics [J. Habasaki and K. L. Ngai, Phys. Chem. Chem. Phys. 9, 4673 (2007), and references herein]. Although a similar mechanism of the slowing down is observed, strong coupling of the motion of cation and anion prevents complete interception unless deeply supercooled, and this explains the wide temperature region of the existence of the liquid and supercooled liquid states in the ionic liquid.
已进行分子动力学模拟来研究离子液体中离子的复杂且非均相动力学。通过范霍夫函数以及相应的中间散射函数(F(s)(k,t))对1-乙基-3-甲基咪唑鎓硝酸盐（EMIM-NO(3)）中阳离子和阴离子的动力学进行了表征，并利用奇异谱分析（SSA）增强的轨迹对其进行了阐释。在离子的均方位移中发现了几个时间区域。在模拟中，扩散系数与温度关系图中的斜率变化在约410 K处被发现。在范霍夫函数的自部分以及轨迹的长时间尺度上，观察到了存在局部化离子和能够连续跳跃的快速离子的非均相动力学。范霍夫函数的自部分以及(F(s)(k,t))的波数依赖性证明了非高斯动力学，并将其表征为列维飞行。即使在370 K下经过几纳秒后，一些离子的连续运动仍可继续，这比扩散运动的起始时间(t(dif))更长。通过使用SSA去噪数据的连续运动相空间图，阐明了快速离子长时间动力学的结构。连续动力学显示具有长期记忆，因此局部结构不足以解释非均质性。在约370 K下连接局部区域的运动是跳跃式的，但由于跳跃运动期间的局部结构变化，不存在典型的跳跃。随着局部运动，阳离子和阴离子之间发生相互扩散。随着温度降低，相互扩散受到抑制，这导致动力学减慢。将这种“阳离子和阴离子的混合效应”与在离子导电玻璃中的离子学中发现的“混合碱效应”进行了比较，在那里不同碱金属离子对路径的拦截导致动力学大幅降低[J. Habasaki和K. L. Ngai，《物理化学化学物理》9，4673（2007），以及此处的参考文献]。尽管观察到了类似的减慢机制，但阳离子和阴离子运动的强耦合阻止了完全拦截，除非深度过冷，这解释了离子液体中液体和过冷液体状态存在的宽温度区域。
