Tokyo Institute of Technology, Nagatsuta 4259, Yokohama 226-8502, Japan.
J Phys Chem B. 2010 Mar 25;114(11):3902-11. doi: 10.1021/jp911157k.
Experimentally, superpositioning of dynamic properties such as viscosity, relaxation times, or diffusion coefficients under different conditions of temperature T, pressure P, and volume V by the scaling variable TV(gamma) (where gamma is a material constant) has been reported as a general feature of many kinds of glass-forming materials. In the present work, molecular dynamics (MD) simulations have been performed to study the scaling of dynamics near the glass-transition regime of ionic liquids. Scaling in the simulated 1-ethyl-3-methylimidazolium nitrate (EMIM-NO(3)) system has been tested over wide ranges of temperatures and pressures. TV(gamma) scaling of the dynamics is well described by master curves with gamma = 4.0 +/- 0.2 and 3.8 +/- 0.2 for cation and anion, respectively. Structures and Coulombic terms of the corresponding states are found to be quite similar. The temperature and pressure dependence of the pair correlation function show similar trends and therefore can be superpositioned onto the master curve. Although the behaviors with gamma = 4 might be expected from the relation, gamma = n/3, for the dynamics with the soft-core-type potential U = epsilon(sigma/r)(n), with n = 12, pair potentials used in the MD simulation have a more complex form, and not all the repulsive terms can play their roles in the heterogeneous structures determined by ion-ion interactions. Scaling is related to the common part of effective potentials related to the pair correlation functions, including the many-body effect in real space.
实验上，通过缩放变量 TV(gamma)（其中 gamma 是材料常数）将不同温度 T、压力 P 和体积 V 条件下的动态特性（如粘度、弛豫时间或扩散系数）叠加，已被报道为许多种玻璃形成材料的一般特征。在本工作中，通过分子动力学（MD）模拟研究了离子液体玻璃化转变区附近动力学的标度。在很宽的温度和压力范围内测试了模拟的 1-乙基-3-甲基咪唑硝酸盐（EMIM-NO(3)）系统的标度。用 gamma = 4.0 +/- 0.2 和 3.8 +/- 0.2（分别对应阳离子和阴离子）的主曲线很好地描述了动力学的 TV(gamma)标度。发现相应状态的结构和库仑项非常相似。对关联函数的温度和压力依赖性显示出相似的趋势，因此可以叠加到主曲线上。尽管对于具有软核型势 U = epsilon(sigma/r)(n)的动力学，gamma = n/3 的关系可能预期行为为 gamma = 4，但在 MD 模拟中使用的对势能具有更复杂的形式，并非所有的排斥项都能在由离子-离子相互作用决定的不均匀结构中发挥作用。标度与与对关联函数相关的有效势的共同部分有关，包括实空间中的多体效应。
