In situ characterization of sorption and diffusion in ionic liquids

Ionic liquids possess unique physicochemical properties that make them advantageous for a broad range of applications including separation processes, homogeneous catalysis, and electrochemical devices. In these applications, the dynamics of molecular solute diffusion near interfaces plays a critical role, but is notoriously challenging to measure and model. To gain better mechanistic insight into transport in ILs, we developed a set of novel methods for characterizing solute transport across IL-fluid interfaces by measuring the spatiotemporal evolution of concentration fields using microfluidic Fabry-Perot interferometry. Here, we characterize the gradient-driven diffusion of water in methylimidazolium halide ILs. We find that the collective diffusivity is dominated by the molecular diffusivity of water, suggesting that the IL acts as an immobile matrix over time scales relevant for gradient diffusion. The results can be modeled by an activated diffusion process of water “hopping” between ion pairs, and the magnitude of the electrostatic activation barrier is consistent with the electronegativity of the anion. We anticipate that these results will help elucidate the influence of mesophase structure and concentrated ion effects on molecular transport in ILs and dense electrolytes.