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

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.