Development and Application of Molecular Simulation Methods to Compute Bulk and Interfacial Properties of Ionic Liquids

纽约州立大学布法罗分校的杰弗里·埃林顿（Jeffrey Errington）获得了化学学部化学理论、模型和计算方法项目的一项奖励，他开展了分子模拟方法的开发和应用，以计算界面上离子流体的性质，例如，在遇到另一种流体或容器壁的地方。与其他远离接口的地方相比，这些地方特别难以进行精确计算。许多现有的和新兴的技术，如超级电容器（在小范围内储存能量）和太阳能电池（将阳光转化为电能）都含有离子流体，例如，离子在室温下以液体形式运动的盐。在许多情况下，这种装置的工作效果取决于流体在多个地方的性质，即靠近和远离容器壁的地方。对离子流体在存在表面时的行为有更深入的基本理解，将对科学家和工程师提高设备性能、设计新技术和利用自然系统独特特征的能力产生深远的影响。例如，这种类型的信息可以提供洞察如何调整离子液体或表面的化学性质，以优化超级电容器在能量存储中的性能，改进放置在各种材料上的涂层，或促进透镜的开发，其焦点和其他特性可以随意改变。研究生和本科生都在表面热力学、统计力学和计算机模拟领域接受训练。首席研究员还参加了当地一所中学的外展活动。该小组研究了两个基本问题：(1)离子流体（如室温离子液体（RTILs）、熔盐和带电胶体）的界面性质（如接触角）对温度的依赖关系；(2)小分子（如水和二氧化碳）对RTILs界面性质的影响。界面性质的温度依赖性得到了相对较少的关注。最近对模型离子流体的模拟结果表明，这些流体表现出高度可调的温度依赖性，这可以在技术装置的设计中加以利用。关于水如何影响RTILs的表面张力，实验提供了相互矛盾的观点。在目前的工作中，分子模拟被用来解决这些差异，并提供了一个更好的理解宏观界面性质和分子在这些不均匀系统中分配的方式之间的联系。为了促进对这些问题的研究，该小组追求两种方法上的进步：(1)界面势方法的等温等压版本和(2)离子液体混合物采样策略。界面势方法关注的是与衬底接触的薄流体膜的表面多余自由能。该研究小组已经成功地采用了这种方法的一个大规范版本好几年了。然而，这种技术在复杂系统中的应用受到大正则系综中所需的分子转移的阻碍。研究小组通过开发一种在等温-等压集成中实现的方法来解决这一限制。这一进步使人们既可以解决复杂的系统，又可以利用流行的分子动力学软件来完成计算。

Jeffrey Errington of the State University of New York at Buffalo is supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry Division to carry out the development and application of molecular simulation methods to compute properties of ionic fluids at interfaces, e.g., where another fluid or the walls of a container is encountered. These places are particularly difficult for accurate computations compared to others away from interfaces. Numerous current and emerging technologies such as supercapacitors (which store energy in small areas) and solar cells (which convert sunlight to electricity) contain ionic fluids, e.g., salts in which the ions move as liquids at room temperature. In many cases, how well such devices perform their work depends on properties of the fluid in more than one place, i.e., both close to and far from containing walls. Development of a deeper fundamental understanding of the behavior of ionic fluids in the presence of surfaces would have a profound influence on the ability of scientists and engineers to improve device performance, design novel technologies, and harness the unique features of natural systems. For example, information of this type may provide insight into how the chemistry of an ionic liquid or surface could be tuned to optimize the performance of a supercapacitor in energy storage, improve a coating placed on various materials, or facilitate development of a lens whose focus and other properties can be changed at will. Both graduate and undergraduate students are being trained in the areas of surface thermodynamics, statistical mechanics, and computer simulation. The principal investigator also participates in outreach at a local middle school.The group studies two fundamental issues: (1) the temperature dependence of the interfacial properties (e.g., contact angle) of ionic fluids such as room temperature ionic liquids (RTILs), molten salts and charged colloids, and (2) the impact of small molecules, such as water and carbon dioxide, on the interfacial properties of RTILs. The temperature dependence of interfacial properties has received relatively little attention. Recent simulation results for model ionic fluids suggest that these fluids exhibit highly tunable temperature dependence, which could be exploited in the design of technological devices. Experiments provide contradictory views regarding how water impacts the surface tension of RTILs. In the present work, molecular simulation is used to resolve these discrepancies and provide a better understanding of the link between macroscopic interfacial properties and the manner in which molecules partition within these inhomogeneous systems. To facilitate the investigation of these issues, the group pursues two methodological advances: (1) an isothermal-isobaric version of the interface potential approach and (2) strategies for sampling mixtures of ionic liquids. The interface potential approach focuses on the surface excess free energy of a thin fluid film in contact with a substrate. The investigator's group has employed a grand canonical version of this approach for several years with success. However, application of this technique to complex systems is hindered by molecule transfers required within a grand canonical ensemble. The research team addresses this limitation via development of a version of the method that is implemented within the isothermal-isobaric ensemble. This advance allows one to both address complex systems and leverage popular molecular dynamics software to complete calculations.