Mesoscopic Organization and Dynamics in Structured Liquids

An increasing number of liquids of both natural and technological importance are known to exhibit spatial and dynamic heterogeneity at the mesoscale due to specific non-covalent intermolecular interactions such as hydrogen-bonding, coulombic interactions, or solvophobic exclusion. However, there is little understanding as to how the organization and dynamics at the mesoscale influence the physical and chemical properties of the bulk liquids.In this dissertation, two classes of materials, ionic liquids and imidazoles, are selected as case studies and investigated by a combination of experimental techniques which provide insight into the interplay of mesoscale organization, dynamics, and physicochemical properties. The mesoscale organization in these materials originates primarily from two different types of non-covalent interactions. For ionic liquids (ILs), this interaction is the solvophobic exclusion of extended aliphatic chains substituted on the cation from regions occupied by the polar ions. Here, new experimental signatures of mesoscale solvophobic aggregate dynamics are identified in the dielectric and dynamic-mechanical spectra. Using these signatures, it is found for instance, in phosphonium-based ionic liquids, that the formation of long-lived aggregates depends not only on the volume fraction of aliphatic groups, but also on the formation of a well-defined polar phase through strong coulombic interactions of the cation and anion charge centers. Finally, the ability to tune physicochemical properties, notably the static dielectric permittivity, by composition-dependent control of mesoscale aggregate morphology and dynamics in binary IL mixtures, is demonstrated. In imidazoles, the organization is driven by intermolecular hydrogen-bonds resulting in supramolecular chains of imidazole molecules. The existence of these chains is commonly believed to promote proton conductivity by a fast intermolecular proton transfer mechanism. A detailed analysis of neat 2-ethyl-4-methylimidazole and mixtures with minute amounts of levulinic acid and butyramide reveal an inverse relation between the average hydrogen-bonded chain length and conductivity with no direct correlation between the static dielectric permittivity and proton conductivity. In addition, an unusual temperature dependence of static dielectric permittivity is attributed to the formation of antiparallel alignment of neighboring hydrogen-bonded chains, a degree of previously unrecognized mesoscale organization.