Understanding the Droplet Diffusion in Ionic Liquid Microemulsions

Ionic liquids (ILs) as polar components in nonaqueous microemulsions are complex formulations that have interesting transport and structural properties, and offer broader applicability of ILs in areas such as drug delivery and cleaning technology. The phase behavior, electrical conductivity, and nanostructures of these formulations have been investigated for quite some time, but the characteristics of the diffusion of nanodroplets were rarely explored- and hence little understood. This work investigates the droplet diffusion processes in a series of IL-microemulsions containing 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim]BF4) by means of viscosity and depolarized dynamic light scattering (DDLS) measurements. The intensity correlation functions are strikingly similar to polymeric solutions in nonaqueous media and aqueous microemulsions containing block copolymers-having bimodal relaxations that are separated by three to four decades of correlation delay times. The "faster" diffusion process is likely a collective process characterizing the correlated motions of droplets in droplet clusters. The collective diffusion coefficient D col values are quite comparable to aqueous microemulsions. The "slower" diffusion is likely due to the "caging" effect caused by nearby clusters and/or bulk solvent-this mode may be linked to the microemulsion bulk viscosity. Interestingly, the D col variations on increasing [C4mim]BF4 concentration are strongly correlated to the microemulsion viscosity changes as well as locations of these compositions on the microemulsion phase diagram.