Ultrafast structure and dynamics of ionic liquids revealed by two-dimensional infrared spectroscopy

Two-dimensional infrared spectroscopy (2D-IR) is a non-linear spectroscopy that is capable of resolving molecular dynamics with picosecond time resolution and molecular-level spatial resolution. 2D-IR has been successfully implemented in a myriad of solution systems to study the local solute and solvent dynamics. Ionic liquids are a unique type of liquid with each formula unit a pair of cation and anion. The structure grants it unique properties that are suitable for many potential applications, and the chemical structure can be modified to be “task-specific”. In this thesis, I describe our implementation of 2D-IR to understand different aspects of the solvation dynamics of ionic liquids, including solvent dynamics, ultrafast structures and dynamics of small solutes, and the dynamics of ionic liquids under confinement. A direct correlation between the microscopic solvent dynamics and the macroscopic viscosity has been observed, indicating the molecular origin of viscosity in ionic liquids is possibly the ion-cage lifetime. Some small inorganic salts form ion-pairs in the ionic liquids, and have a Gibbs free energy of ~8 kJ/mol. Some uncharged small molecules can cluster in ionic liquids, like water clusters with thiocyanate anion. The detailed structural and reorientation dynamics of ionic liquid-surfactant complexes imply a random cluster picture, rather than previously-believed reverse micelle picture; the shape of the dynamic correlation function also indicates a non-isotropic local orientation distribution that could originate from the ion-exchange between anionic surfactants and ionic liquids