Institute of New Energy Material Chemistry and Department of Material Chemistry, Nankai University, Tianjin 300071, China.
J Phys Chem B. 2010 May 27;114(20):6886-904. doi: 10.1021/jp908914d.
An electronically polarizable model has been developed for the ionic liquid (IL) 1-ethyl-3-methyl-imidazolium nitrate (EMIM(+)/NO(3)(-)) (Yan et al. J. Phys. Chem. B DOI:10.1021/jp9089112). Molecular dynamics simulations were then performed with both the polarizable and nonpolarizable models. Both models exhibited certain properties that are similar to a supercooled liquid behavior even though the simulations were run at 400 K (89 K above the melting point of EMIM(+)/NO(3)(-)). The ionic mean-squared displacement and transverse current correlation function of both models were well represented by a memory function with a fast Gaussian initial relaxation followed by the two-step exponential functions for beta- and alpha- structural relaxations. Another feature shared by both models is the dynamic heterogeneity, which highlights the complex dynamic behavior of ILs. Apart from the overall slow dynamics, the relaxation of the H-atoms attached to the methyl group demonstrates a "free rotor" type of motion. Also, the ethyl group shows the fastest overall relaxation, due to the weak electrostatic interactions on it. Such flexibility enhances the entropic effect and thus favors the liquid state at room temperature. For the dynamical properties reported in this paper, the polarizable model consistently exhibited faster relaxations (including translational and reorientational motions), higher self-diffusion and ionic conductivity, and lower shear viscosity than the nonpolarizable model. The faster relaxations of the polarizable model result from attenuated long-range electrostatic interactions caused by enhanced screening from the polarization effect. Therefore, simulations based on the polarizable model may be analogous to simulations with the nonpolarizable model at higher temperatures. On the other hand, the enhanced intermolecular interactions for the polarizable model at short-range due to the additional charge-dipole and dipole-dipole interactions result in a red shift of the intramolecular C-H stretch spectrum and a higher degree of ion association, leading to a spectrum with enhanced conductivity across the whole frequency range. The vibrational motion associated with the intermolecular hydrogen bonding is highly IR active, highlighting the importance of hydrogen bond dynamics in ILs.
已经为离子液体 1-乙基-3-甲基咪唑硝酸盐（EMIM(+) / NO3(-)）（Yan 等人，J. Phys. Chem. B DOI：10.1021/jp9089112）开发了一个可极化模型。然后，使用可极化和不可极化模型进行了分子动力学模拟。尽管在 400 K（EMIM(+) / NO3(-)的熔点以上 89 K）下进行了模拟，但两种模型均表现出某些类似于过冷液体行为的特性。两种模型的离子均方位移和横向电流相关函数均由具有快速高斯初始松弛的记忆函数很好地表示，然后是β和α结构松弛的两步指数函数。两种模型的另一个共同特征是动态异质性，这突出了离子液体的复杂动态行为。除了整体缓慢的动力学之外，与甲基相连的 H 原子的弛豫表现出“自由转子”类型的运动。此外，由于其较弱的静电相互作用，乙基表现出最快的整体弛豫。这种灵活性增强了熵效应，从而有利于室温下的液态。对于本文报道的动力学性质，可极化模型始终表现出更快的弛豫（包括平移和重取向运动），更高的自扩散和离子电导率，以及更低的剪切粘度，而非极化模型。可极化模型更快的弛豫是由于极化效应增强屏蔽导致的长程静电相互作用减弱所致。因此，基于可极化模型的模拟可能类似于在较高温度下进行的非极化模型模拟。另一方面，由于附加的电荷偶极子和偶极子偶极子相互作用，可极化模型在短程内增强了分子间相互作用，导致分子内 C-H 伸缩光谱的红移和更高程度的离子缔合，从而导致整个频率范围内增强的电导率谱。与分子间氢键相关的振动运动具有很高的 IR 活性，突出了氢键动力学在离子液体中的重要性。
