Institute of High Performance Computing, 1 Fusionopolis Way, #16-16, Connexis, Singapore 138632, Republic of Singapore.
J Phys Chem B. 2010 Mar 4;114(8):2856-68. doi: 10.1021/jp1000557.
Whether an ionic liquid (IL) is water-miscible or immiscible depends on the particular ions that constitute it. We propose an explanation, based on molecular simulations, how ions determine the miscibility of ILs and suggest a straightforward and computationally inexpensive method to predict the miscibility of arbitrary new ILs. The influence of ions on the solvation of water is analyzed by comparing molecular dynamics simulations of water in 9 different ILs with varying cation and anion constituents. The solvation of water in ILs is found to depend primarily on the electrostatic water-ion interaction strength, which, in turn, is determined mainly by two factors: primarily, by the size of the ions and secondarily by the amount of charge on the ion surface that is coordinated with water. It is demonstrated that large ions lead to weaker interactions with water, due to the involved delocalization of the ion charge. A large charge on the ion surface, which is determined by the chemical structure of the ion, strengthens water-ion interactions. We observe that whenever the interaction strength of water with ions exceeds a certain threshold, an IL becomes water-miscible. On the basis of these findings, a simple equation is derived that estimates the water-ion interaction strength. With this equation it is possible to predict most of the observed water-miscibilities of a sample of 83 ILs correctly. A linear increase of the water saturation concentration with the estimated water-ion interaction strength is observed in water-immiscible ILs, which can be utilized to predict the water concentration in new ILs.
离子液体（IL）是否与水混溶取决于构成它的特定离子。我们提出了一种基于分子模拟的解释，说明离子如何决定 IL 的混溶性，并提出了一种简单且计算成本低廉的方法来预测任意新 IL 的混溶性。通过比较具有不同阳离子和阴离子组成的 9 种不同 IL 中水的分子动力学模拟，分析了离子对水的溶剂化的影响。发现水在 IL 中的溶剂化主要取决于静电水-离子相互作用强度，而离子相互作用强度又主要取决于两个因素：主要取决于离子的大小，其次取决于与水配位的离子表面的电荷量。结果表明，由于离子电荷的离域化，大离子导致与水的相互作用较弱。离子表面上的大电荷（由离子的化学结构决定）会增强水-离子相互作用。我们观察到，只要水与离子的相互作用强度超过一定阈值，IL 就会与水混溶。基于这些发现，推导出了一个简单的方程来估计水-离子相互作用强度。有了这个方程，就可以正确预测 83 种 IL 样品中的大部分观察到的水混溶性。在与水不混溶的 IL 中，观察到水的饱和浓度随估计的水-离子相互作用强度线性增加，这可用于预测新 IL 中的水浓度。
