Structural Dynamics and Charge Transport in Room Temperature Ionic Liquids

Room temperature ionic liquids are an important class of materials due to their chemical tunability and numerous advantageous physicochemical properties. As a result, ionic liquids are currently being investigated for use in a wide array of chemical and electrochemical applications. Despite their great potential, however, the relationship between the chemical structure and physicochemical properties of ionic liquids is not well understood.

To this end, this dissertation presents experimental studies of the reorientational structural dynamics and charge transport properties of a variety of room temperature ionic liquids using quasielastic light scattering spectroscopy and broadband dielectric spectroscopy.

Studies of a series of 1-butyl-3-methylimidazolium based ionic liquids, in which the anion was systematically varied, have revealed that the anion chemical structure has a significant impact on the structural dynamics and charge transport properties of ionic liquids. As the anion becomes larger and more asymmetrical, ionic mobility increases strongly as a result of an increased fluidity. Surprisingly, the mole fraction of free ions remains unaffected by the anion chemical structure.

The influence of cation chemical structure on the transport properties of ionic liquids was explored in a homologous series of tetra-alkylammonium based ionic liquids. It was found that pronounced chemical heterogeneity causes ionic liquids to exhibit complex and strongly heterogeneous molecular dynamics. Furthermore, when aliphatic side-groups of cations occupy a large liquid volume fraction, both the ion mobility as well as the mole fraction of free ions were found to decrease strongly as a consequence of pronounced nanophase segregation.

The transport properties of a novel carboxylic acid-tertiary amine based protic ionic liquid were also studied in this dissertation. The structural dynamics and charge transport mechanism in this protic ionic liquid are surprisingly very similar to the aprotic ionic liquids, despite the fundamental differences in the intrinsic charge carrier.

Through these comparative studies, this dissertation not only provides a fundamental understanding of the unique dynamical properties of room temperature ionic liquids, but it also elucidates the complex interrelationship between key chemical structure variations and molecular transport properties of these important materials.