Banerjee Swarup, Ghorai Pradip Kr, Das Suman, Rajbangshi Juriti, Biswas Ranjit
Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER), Kolkata, India.
Department of Chemical, Biological and Macromolecular Sciences, S.N. Bose National Centre for Basic Sciences, Kolkata, India.
J Chem Phys. 2020 Dec 21;153(23):234502. doi: 10.1063/5.0024355.
Heterogeneous relaxation dynamics often characterizes deep eutectic solvents. Extensive and molecular dynamics simulations have been carried out in the temperature range, 303 ≤ T/K ≤ 370, for studying the anion and temperature dependencies of heterogeneous dynamics of three different ionic acetamide deep eutectics: acetamide + LiX, X being bromide (Br), nitrate (NO ), and perchlorate (ClO ). These systems are chosen because the fractional viscosity dependence of average relaxation rates reported by various measurements has been attributed to the heterogeneous dynamics of these systems. Simulations performed here attempt to characterize the heterogeneous relaxation dynamics in terms of correlated time and length scales and understand the solution inhomogeneity in microscopic terms. Additionally, simulation studies for pure molten acetamide have been performed to understand the impact of ions on motional features of acetamide in these ionic deep eutectic systems. The computed radial distribution functions suggest microheterogeneous solution structure and dependence upon anion identity and temperature. A significant plateau in the simulated time dependent mean squared displacements indicates pronounced cage-rattling and inhomogeneity in relaxation dynamics. Simulated diffusion coefficients for acetamide and ions show decoupling from the simulated viscosities of these deep eutectics. Calculated two- and four-point correlation functions reveal the presence of dynamic heterogeneity even at ∼180 K above the measured thermodynamic glass transition temperature (T). Further analyses reveal the existence of multiple timescales that respond strongly to the rise in solution temperature. The simulated dynamic structure factor and overlap function relaxations show strong stretched exponential relaxations. The simulation results support the experimental observation that the bromide system is the most dynamically heterogeneous among these three systems. Correlated length scales show much weaker anion and temperature dependencies with an estimated length of ∼1 nm, suggesting formation of clusters at the local level as the origin for the micro-heterogeneous nature of these ionic deep eutectics.
非均匀弛豫动力学通常是深层低共熔溶剂的特征。在303 ≤ T/K ≤ 370的温度范围内进行了广泛的分子动力学模拟，以研究三种不同离子型乙酰胺深层低共熔物：乙酰胺 + LiX（X为溴化物（Br）、硝酸盐（NO₃）和高氯酸盐（ClO₄））的非均匀动力学对阴离子和温度的依赖性。选择这些体系是因为各种测量报告的平均弛豫速率的分数粘度依赖性归因于这些体系的非均匀动力学。此处进行的模拟试图根据相关的时间和长度尺度来表征非均匀弛豫动力学，并从微观角度理解溶液的不均匀性。此外，还对纯熔融乙酰胺进行了模拟研究，以了解离子对这些离子型深层低共熔体系中乙酰胺运动特征的影响。计算得到的径向分布函数表明存在微观非均匀溶液结构，且依赖于阴离子特性和温度。模拟的随时间变化的均方位移中出现的显著平台表明弛豫动力学中存在明显的笼内振动和不均匀性。乙酰胺和离子的模拟扩散系数与这些深层低共熔物的模拟粘度解耦。计算得到的两点和四点相关函数表明，即使在比测量的热力学玻璃化转变温度（Tg）高约180 K时，也存在动态非均匀性。进一步分析揭示了存在多个对溶液温度升高有强烈响应的时间尺度。模拟的动态结构因子和重叠函数弛豫显示出强烈的拉伸指数弛豫。模拟结果支持了实验观察结果，即在这三个体系中，溴化物体系的动态非均匀性最强。相关长度尺度显示出较弱的阴离子和温度依赖性，估计长度约为1 nm，表明在局部水平形成簇是这些离子型深层低共熔物微观非均匀性质的起源。
