Microfluidic System Enabling Multistep Tuning of Extraction Time Periods for Kinetic Analysis of Droplet-Based Liquid-Liquid Extraction.

When analyzing the kinetics of liquid-liquid extraction (LLE), the change in the concentration of extracted target molecules over time should be monitored for a known interfacial area. Herein, we developed a microfluidic system for precisely analyzing the kinetics of LLE using droplets of a constant size even in the presence of additives. Extraction is initiated by exchanging the carrier fluid for the droplets with a target solution and then terminated by phase separation, based on the principle of hydrodynamic filtration. By using one out of several pairs of outlet/buffer inlet at a given time, the extraction time period is tuned stepwise without changing the flow rate condition. We successfully demonstrated droplet-based LLE by controlling the extraction period from ∼0.03 to ∼1.2 s and evaluated the extraction kinetics of rhodamine B from the continuous aqueous phase to droplets of 1-octanol with a diameter of ∼40 μm. In addition, the effect of additives on extraction efficiency was evaluated. The system presented in this study would be useful for determining rate constants for interfacial mass transfer in general LLE kinetic studies as well as for developing new extraction chemistries and optimizing microfluidic chemical/biochemical analysis systems that involve an LLE process.