Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
J Phys Chem B. 2021 Nov 11;125(44):12274-12291. doi: 10.1021/acs.jpcb.1c07698. Epub 2021 Nov 2.
Because of the amphiphilic nature of ethanol in the aqueous solution, ions cause an interesting microheterogeneity where the water molecules and the hydroxy groups of ethanol preferentially solvate the ions, while the ethyl groups tend to occupy the intervening space. Using computer simulations, we study the dynamics of rigid monovalent cations (Li, Na, K, and Cs) in aqueous ethanol solutions with chloride as the counterion. We vary both the size of the ions and the composition of the mixture to explore size- and composition-dependent ion diffusion. The relative stability of enhanced microheterogeneous configurations makes ion diffusion slower than what would be surmised by using the bulk properties of the mixture, using the Stokes-Einstein relation. We study the structure through partial radial distribution functions and the stability through coordination number fluctuations. The ion diffusion coefficient exhibits sharp re-entrant behavior when plotted against viscosity varied by composition. Our studies reveal multiple anomalous features of ion motion in this mixture. We formulate a mode-coupling theory (MCT) that takes into account the interaction between different dynamical components; MCT can incorporate the effects of heterogeneous dynamics and nonlinearity in composition dependence that arise from the feedback between mutually dependent ion-solvent dynamics.
由于乙醇在水溶液中具有两亲性，离子会导致有趣的微不均匀性，其中水分子和乙醇的羟基优先溶剂化离子，而乙基基团则倾向于占据中间空间。我们使用计算机模拟研究了刚性单价阳离子（Li、Na、K 和 Cs）在氯化物作为抗衡离子的水-乙醇溶液中的动力学。我们改变了离子的大小和混合物的组成，以探索尺寸和组成依赖性的离子扩散。增强的微观不均匀构型的相对稳定性使得离子扩散比使用混合物的体相性质（根据 Stokes-Einstein 关系）所推测的要慢。我们通过部分径向分布函数研究结构，通过配位数波动研究稳定性。当将离子扩散系数绘制为与组成变化的粘度的关系时，它表现出明显的再进入行为。我们的研究揭示了该混合物中离子运动的多种异常特征。我们提出了一种模式耦合理论（MCT），该理论考虑了不同动力学分量之间的相互作用；MCT 可以将不均匀动力学的影响以及由相互依赖的离子-溶剂动力学之间的反馈引起的组成依赖性中的非线性纳入考虑。
