A microfluidic study of liquid–liquid extraction mediated by carbon dioxide

Abstract

Liquid–liquid extraction is an important separation and purification method; however, it faces a challenge in reducing the energy consumption and the environmental impact of solvent (extractant) recovery. The reversible chemical reactions of switchable solvents (nitrogenous bases) with carbon dioxide (CO₂) can be implemented in reactive liquid–liquid extraction to significantly reduce the cost and energy requirements of solvent recovery. The development of new effective switchable solvents reacting with CO₂ and the optimization of extraction conditions rely on the ability to evaluate and screen the performance of switchable solvents in extraction processes. We report a microfluidic strategy for time- and labour-efficient studies of CO₂-mediated solvent extraction. The platform utilizes a liquid segment containing an aqueous extractant droplet and a droplet of a solution of a switchable solvent in a non-polar liquid, with gaseous CO₂ supplied to the segment from both sides. Following the reaction of the switchable solvent with CO₂, the solvent becomes hydrophilic and transfers from the non-polar solvent to the aqueous droplet. By monitoring the time-dependent variation in droplet volumes, we determined the efficiency and extraction time for the CO₂-mediated extraction of different nitrogenous bases in a broad experimental parameter space. The platform enables a significant reduction in the amount of switchable solvents used in these studies, provides accurate temporal characterization of the liquid–liquid extraction process, and offers the capability of high-throughput screening of switchable solvents.