Ionic liquids and molten salts, from bulk and interfacial structure to excess electrons and holes

Fei Wu

doi:10.17077/etd.005196

By convention, liquid salts are often classified into two categories based on their melting temperature: Ionic liquids (ILs) that melt below 100 ℃ (although many examples exist of what we would commonly describe as ionic liquids that melt above this temperature) and molten salts which melt above 100 ℃ (often at significantly higher temperatures). My work during the past several years has explored both classes of systems, the room-temperature ionic liquids and the high-temperature inorganic molten salts. There has been a resurgence of interest in the high-temperature inorganic molten salts because of their technological potential as coolants in new prototypes of safer and cleaner nuclear reactors; inorganic salts have very low vapor pressures and high thermal stability. Ionic liquids, on the other hand, have received considerable attention because of a multitude of new applications ranging from multi-ton industrial processes, their use in solar cell devices, rechargeable batteries and biomass processing just to mention a few. For many of these applications, understanding the bulk and interfacial structure of ILs and molten salts is very important. In other cases, when these systems are exposed to extreme conditions of radiation, high temperatures or electrochemical potentials an understanding of reactivity and the fate of transient species is highly desirable. There simply is no single computational technique that can address the many questions resulting from X-ray scattering, X-ray reflectivity, and radiolysis experiments; instead during my work as a graduate student I had to use a combination of classical, semi-empirical and ab-initio molecular dynamics methods to investigate these topics. In some cases, the theoretical tools to answer certain questions needed to be created or expanded, this required old-fashioned paper and pencil work that with the help of my advisor and collaborators I successfully carried out. Such methodological advances will be useful for many complex systems beyond the ones described here. The difference between ionic liquids and molten salts is not only based on an arbitrary definition of temperature. My research explores some of the key common features and distinctions between the family of salts considered ionic liquids and those considered molten salts. For example, both ionic liquids and molten salts have what in our group has been termed a “charge alternation peak” in the structure function (S(q)). This is the hallmark feature of all salts in the liquid phase highlighting the alternation of positively and negatively charged species; such peak is absent in the scattering of common molecular liquids. As opposed to inorganic molten salts, ILs have apolar chains and the alternation of these with the charged components results in a new feature commonly called a first sharp diffraction peak or prepeak at low q values in S(q) this peak is commonly absent for molten salts. Molten salts of certain multivalent cations can also produce a prepeak and finding its structural origin can very challenging. In this thesis, I describe the case of molten MgCl2, where the prepeak appears to be because of structural correlations between same or different charge ions that belong to different but adjacent Cl--decorated Mg2+ networks. Part of this thesis deals also with the transient species that can be generated in ILs via photo-excitation, radiolysis or electrochemical processes. It is well known from radiolysis experiments that the lifetime of an excess electron in ILs can be orders of magnitude longer than in conventional solvents and its diffusion slow as that of a molecular species. According to our research, excess electrons or excess holes can localize or partially localize on cations, anions or even cavities and such species can be short- or long-lived. Each of these species has a distinct transient spectrum which we can calculate to identify the type of initial and final states connected by those transitions in the near-IR, visible and UV regions. A portion of this thesis is also devoted to the description of methodology we proposed and derived to decompose the almost featureless normalized reflectivity of ILs into rich information about subcomponents revealing the segregation of ionic and subionic components at interfaces and all the way deep into the liquid phase.

Ionic liquids and molten salts, from bulk and interfacial structure to excess electrons and holes

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