Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560 064, India.
Chemphyschem. 2010 Jun 21;11(9):2001-10. doi: 10.1002/cphc.201000111.
The complex dynamics of a room-temperature ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]), is studied using equilibrium classical molecular dynamics simulations in the temperature range of 250-450 K. The activation energies for the self-diffusion of ions are around 30-34 kJ mol(-1), with that of the anion a little higher than that for the cation. The electrical conductivity of the liquid is calculated and good agreement with experiments is obtained. Structural relaxation is studied through the decay of coherent (total density-density correlation) and incoherent (self part of density-density correlation) intermediate scattering functions over a range of temperatures and wave vectors relevant to the system. The relaxation data are used to identify and characterize two processes, alpha and beta. The dependence of the two relaxation times on temperature and wave vector is obtained. The dynamical heterogeneity of the ions determined through the non-Gaussian parameter indicates the motion of the cation to be more heterogeneous than that of the anion. The faster ones among the cations are coordinated to faster anions, while slower cations are surrounded predominantly by slower anions. Thus, the dynamical heterogeneity in this ionic liquid is shown to have structural signatures.
使用 250-450 K 温度范围内的平衡经典分子动力学模拟研究了室温离子液体 1-丁基-3-甲基咪唑六氟磷酸盐([bmim][PF(6)])的复杂动力学。离子自扩散的活化能约为 30-34 kJ mol(-1)，其中阴离子的活化能略高于阳离子。计算了液体的电导率，并与实验结果吻合良好。通过相干（总密度-密度相关）和非相干（密度-密度相关的自部分）中间散射函数在与系统相关的温度和波矢范围内的衰减，研究了结构弛豫。弛豫数据用于识别和表征两个过程，α和β。获得了两个弛豫时间随温度和波矢的依赖性。通过非高斯参数确定的离子动力学异质性表明阳离子的运动比阴离子更不均匀。阳离子中的较快者与较快的阴离子配位，而较慢的阳离子主要被较慢的阴离子包围。因此，表明这种离子液体的动力学异质性具有结构特征。
