Surfactant-enhanced liquid–liquid extraction in microfluidic channels with inline electric-field enhanced coalescence

Abstract

Continuous microfluidic liquid–liquid extraction is realized in a microfluidic device by generating emulsions with large interfacial areas for mass transfer, and subsequently breaking these emulsions using electric fields into easily separated segments of immiscible liquids (plugs). The microfluidic device employs insulated electrodes in a potassium hydroxide-etched channel to create large electric fields (100 kV m⁻¹) that drive coalescence of the emulsion phase. The result is a transition from disperse to slug flow that can then readily be separated by gravity. Extractions of phenol and p-nitrophenol from an aqueous to hexane–surfactant solution serve as model systems. In addition to the increased surface area in the emulsion, extraction efficiency is enhanced by reverse micelles resulting from the presence of surfactants. The surfactant concentration is varied ∼1–10 wt% and a general two-parameter model is developed to quantify the extraction behavior and demonstrate the effectiveness of reverse micelle enhanced extraction.