  The impact of mesoscale organization on dynamics and ion transport in binary
ionic liquid mixtures is investigated by broadband dielectric spectroscopy,
dynamic-mechanical spectroscopy, x-ray scattering, and molecular dynamics
simulations. The mixtures are found to form distinct liquids with macroscopic
properties that significantly deviate from weighted contributions of the neat
components. For instance, it is shown that the mesoscale morphologies in ionic
liquids can be tuned by mixing to enhance the static dielectric permittivity of
the resulting liquid by as high as 100$\%$ relative to the neat ionic liquid
components. This enhancement is attributed to the intricate role of interfacial
dynamics associated with the changes in the mesoscopic aggregate morphologies
in these systems. These results demonstrate the potential to design the
physicochemical properties of ionic liquids through control of solvophobic
aggregation
