(709c) Visualizing Sorption and Anomalous Solute Diffusion in Ionic Liquids and Ionogels

Dynamic diffusion of molecular solutes near ionic liquid interfaces plays a critical role in many emerging applications, including liquid-liquid extraction (IL-solute), vapor scrubbing (IL-volatile solute), and carbon capture (IL-CO₂). In these applications, sorption is notoriously challenging to measure and model, since the self-assembled nanostructure complicates ion mobility and often precludes the use of standard analytical transport models. To gain better mechanistic insight into transport in ILs, we developed a novel method, microfluidic Fabry-Perot interferometry, that enables direct, label-free visualization of solute concentration fields as they evolve spatially and temporally. We use this technique to measure the gradient-driven diffusion of solutes as they traverse the IL-vapor interface in three systems: (1) CO₂ physisorption and chemisorption; (2) H₂O sorption by methylimidazolium halide ILs; and (3) H₂O sorption by PEG methylimidazolium ionogels. In system (1), the measured concentration profiles directly reveal dramatic viscosity increases in select amine-functionalized ILs as they chemisorb CO₂. In systems (2) and (3), the concentration profiles enable composition-dependent diffusivities to be extracted over a wide range of compositions from a single experiment, facilitating the subsequent development of mechanistically distinct, quantitatively accurate transport models. Specifically, diffusion is modeled by an activated process of water hopping between ion pairs, in which the magnitude of the electrostatic activation barrier follows the strength of hydrogen bonding with the methylimidazolium head group. We anticipate that the new conceptual frameworks developed using this visualization technique will help elucidate the influence of mesophase structure on solute transport, and ultimately enable rational design of task-specific ILs.