4. Multigranular modeling of ionic liquids

Abstract

Ionic liquids (ILs) are a special category of molten salts with melting points near ambient temperatures (or by convention below 100°C). Owing to their numerous valuable physicochemical properties as bulk liquids, solvents, at surfaces and in confined environments, ILs have attracted increasing attention in both academic and industrial communities in a variety of application areas involving physics, chemistry, material science and engineering. Due to their nearly limitless number of combinations of cation-anion pairs and mixtures with cosolvents, a molecular level understanding of their hierarchical structures and dynamics, requiring strategies to connect several length and time scales, is of crucial significance for rational design of ILs with desired properties, and thereafter refining their functional performance in applications. As an invaluable compliment to experiments from synthesis to characterization, computational modeling and simulations have significantly increased our understanding on how physicochemical and structural properties of ILs can be controlled by their underlying chemical and molecular structures. In this chapter, we will give examples from our own modeling work based on selected IL systems, with focus on imidazolium-based and tetraalkylphosphonium-orthoborate ILs, studied at several spatio-temporal scales in different environments and with particular attention to applications of high technological interest. We start by describing studies performed using ab initio methods on force field development for tetraalkylphosphonium-orthoborate ILs, and computational studies on thermal decomposition of these ILs. The delicate interplay between hydrogen bonding and π-type interactions in an imidazolium-orthoborate IL was studied by performing ab initio molecular dynamics simulations. On the atomistic level, atomistic simulations were performed with constructed force field parameters to study intrinsic molecular interactions between residual water molecules and tetraalkylphosphoniumorthoborate ionic species. For a typical trihexyltetradecylphosphonium bis(oxalato) borate IL at varied concentrations, microstructures and dynamics were systematically analyzed as water concentration increases. The liquid viscosities of typical trihexyltetradecylphosphonium-based ILs were estimated through equilibrium atomistic simulations using Green-Kubo relation with charge scaling factors on ionic species.Additionally, atomistic simulations were combined with X-ray scattering experiments to investigate phase behavior and ionic structures in IL mixtures with varied concentrations. Peculiar features of X-ray scattering spectra of IL mixtures with organic solvent are also discussed with an example dealing with ethyl ammonium nitrate and acetonitrile mixtures. On the mesoscopic level, a united-atom model for trihexyltetradecylphosphonium cation and coarse-grained models for butylmethy-limidazolium hexafluorophosphate were proposed, and effective interactions between coarse-grained beads were validated against experimental...

Microstructural and Dynamical Heterogeneities in Ionic Liquids

“…By a careful selection of cation–anion combinations, it is possible to prepare a large variety of ILs. These pioneering works and significant breakthroughs in IL communities opened up avenues for a surge of research on ILs and initiated paramount research activities across areas of physics, chemistry, biology, materials science and engineering, and environmental science, which are subjects of numerous reviews, − special themed issues in prestigious journals, − and book chapters. − …”

Effects of Nitridation and Vinylation of Imidazolium Rings on Hydrogen Bonding Interactions, π–π-Stacking Structures, and Dynamical Heterogeneities in Imidazolium and Triazolium Ionic Liquids

“…Room-temperature ionic liquids (ILs) are fascinating molten salts composed solely of ion species with distinct molecular symmetry and charge delocalization, having their melting points below 100 °C. − Recent years have seen a great enthusiasm for ILs regarding their utilities as facilitating functional materials in diverse applications including material synthesis and catalysis, , microlubrication and nanotribology, , gas adsorption and separation, , and electrochemical devices for energy storage and harvesting. ,,, Among the whole IL community, imidazolium ILs hold peculiar significance because of their multifaceted physicochemical characteristics, such as low volatility and flammability, reasonable viscosity–conductivity features, high thermal- and electrochemical-oxidative stabilities, wide electrochemical windows, and exceptional affinities to molecular compounds having varied polarities. − ,, These remarkable properties of imidazolium ILs are intrinsically correlated with inherent ion structures and delicate interactions among constituent ions at the nanoscopic level. ,,,, Therefore, the microstructural and dynamical quantities of ion species, the self-assembled mesoscopic liquid morphologies, and the macroscopic functionalities of imidazolium ILs can be widely tuned via a judicious selection of ion moieties, an introduction of a controllable amount of molecular solvents (solutes) into IL matrices, and a specific mutation of constituent ions with atoms and molecular groups to meet specific requirements in diverse applications. − …”