(591f) Measuring Ion Dynamics at Ionic Liquid-Fluid Interfaces

The dynamics of ion and molecular transport at ionic liquid-fluid interfaces are important in a variety of applications, including ion exchange (IL-IL), liquid-liquid extraction (IL-solution), and water sorption (IL-H₂O). The self-assembled nanostructure and concentrated ion effects present in ILs complicates ion mobility, and often precludes the use of standard analytical electrolyte transport models. Here, we report a novel method for characterizing transport in ILs by measuring the spatio-temporal evolution of IL concentration and structural fields after IL-fluid contact using microfluidic Fabry-Perot interferometry and birefringence visualization. In these experiments, two fluids are contacted within a semireflective microfluidic device. Light passed through the device forms Fringes of Equal Chromatic Order (FECO), which are determined by the local refractive index near the interface. Tracking changes in FECO with time allows for the calculation of changes in the refractive index (and subsequently the local concentration), while birefringence allows for tracking development of liquid crystalline nanostructure. In this work, we apply the method to study the contact of methylimidazolium-based ILs with other ILs that form different structural mesophases and with water. Measuring the dynamic concentration profiles provides insight into the concentration-dependent macroscopic diffusivity of ions within binary IL mixtures, and also gives rapid mapping of phase behavior relative to other methods. We anticipate that the method will provide measures of ion diffusivity and structure that are complementary to alternative techniques (e.g. conductivity, pulsed-field NMR, QENS), and help elucidate the influence of mesophase structure and concentrated ion effects on ion transport.