282126 Ionic Liquid-Based Droplet Microreactors
We have implemented a set of reactions using ionic liquid solvent systems in microfluidic reactors. Ionic liquids are low volatility, nonflammable, recyclable solvents that can replace traditional organic solvents in a host of synthetic reactions. Microfluidic devices allow for on-demand production with minimal reagent waste and minimization of storage and transport hazards. When combined, these two green chemistry technologies bring about synergistic advantages. Ionic liquids are also uniquely well suited to replace organic solvents for reactions performed in poly(dimethylsiloxane) (PDMS) microfluidic devices since, unlike organic solvents, they do not partition into the PDMS matrix.
We have shown that while ionic liquid-based lab-scale batch reactions can produce high-quality metal nanoparticles, these particles are improved in terms of shape, monodispersity, and size when the reactions are performed in microfluidic droplet flows. This is likely due to the improved mixing brought about by convection in microdroplets, which rapidly homogenizes the reactants and constrains the particle nucleation burst. Forming microdroplets using conventional ionic liquids such as BMIM bistriflimide is a challenge, however, due to the low contact angle of the ionic liquid with a PDMS surface. By using a gas-phase method for coating the interior of PDMS channels with a fluorocarbon polymer, we have lowered the surface energy of the PDMS and successfully formed ionic liquid droplets over a wide range of flow conditions. We characterized the droplet formation behavior of ionic liquids in terms of the capillary number of the system, generating broadly applicable design rules for droplet flows of these unusually viscous liquids. Using these droplet systems, we have generated small, uniform, gold and silver nanoparticles in a continuous flow microreactor.
We have also adapted ionic liquid solvents to combinatorial microreactors for synthesizing drug-like molecules via a trimolecular condensation reaction.