Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, SE-10691 Stockholm, Sweden.
Soft Matter. 2018 May 30;14(21):4252-4267. doi: 10.1039/c8sm00387d.
Computer simulations provide a unique insight into the microscopic details, molecular interactions and dynamic behavior responsible for many distinct physicochemical properties of ionic liquids. Due to the sluggish and heterogeneous dynamics and the long-ranged nanostructured nature of ionic liquids, coarse-grained meso-scale simulations provide an indispensable complement to detailed first-principles calculations and atomistic simulations allowing studies over extended length and time scales with a modest computational cost. Here, we present extensive coarse-grained simulations on a series of ionic liquids of the 1-alkyl-3-methylimidazolium (alkyl = butyl, heptyl-, and decyl-) family with Cl, [BF4], and [PF6] counterions. Liquid densities, microstructures, translational diffusion coefficients, and re-orientational motion of these model ionic liquid systems have been systematically studied over a wide temperature range. The addition of neutral beads in cationic models leads to a transition of liquid morphologies from dispersed apolar beads in a polar framework to that characterized by bi-continuous sponge-like interpenetrating networks in liquid matrices. Translational diffusion coefficients of both cations and anions decrease upon lengthening of the neutral chains in the cationic models and by enlarging molecular sizes of the anionic groups. Similar features are observed in re-orientational motion and time scales of different cationic models within the studied temperature range. The comparison of the liquid properties of the ionic systems with their neutral counterparts indicates that the distinctive microstructures and dynamical quantities of the model ionic liquid systems are intrinsically related to Coulombic interactions. Finally, we compared the computational efficiencies of three linearly scaling O(N log N) Ewald summation methods, the particle-particle particle-mesh method, the particle-mesh Ewald summation method, and the Ewald summation method based on a non-uniform fast Fourier transform technique, to calculate electrostatic interactions. Coarse-grained simulations were performed using the GALAMOST and the GROMACS packages and hardware efficiently utilizing graphics processing units on a set of extended [1-decyl-3-methylimidazolium][BF4] ionic liquid systems of up to 131 072 ion pairs.
计算机模拟为理解离子液体的许多独特物理化学性质提供了微观细节、分子相互作用和动态行为方面的独特见解。由于离子液体的动力学缓慢且不均匀，以及具有长程纳米结构的性质，粗粒介观模拟为详细的第一性原理计算和原子模拟提供了不可或缺的补充，允许在适度的计算成本下研究扩展的长度和时间尺度。在这里，我们对一系列 1-烷基-3-甲基咪唑鎓（烷基=丁基、庚基和癸基）家族的带有 Cl、[BF4] 和 [PF6] 反离子的离子液体进行了广泛的粗粒模拟。系统地研究了这些模型离子液体体系在很宽的温度范围内的液体密度、微观结构、平移扩散系数和重取向运动。在阳离子模型中添加中性珠粒会导致液体形态从极性骨架中分散的非极性珠粒转变为液体基质中具有双连续海绵状互穿网络的形态。阳离子模型中中性链的长度增加以及阴离子基团的分子尺寸增大，导致阳离子和阴离子的平移扩散系数降低。在所研究的温度范围内，不同阳离子模型的重取向运动和时间尺度也表现出类似的特征。离子体系的液体性质与中性体系的比较表明，模型离子液体体系的独特微观结构和动力学性质与库仑相互作用密切相关。最后，我们比较了三种线性标度 O（N log N）Ewald 求和方法，即粒子-粒子-格子方法、格子 Ewald 求和方法和基于非均匀快速傅里叶变换技术的 Ewald 求和方法，以计算静电相互作用。使用 GALAMOST 和 GROMACS 软件包进行粗粒模拟，并在一组扩展的[1-癸基-3-甲基咪唑][BF4]离子液体体系上高效地利用图形处理单元进行计算，离子对数高达 131072。
