Automated Measurement of Liquid-liquid Equilibria using Raman Spectroscopy and Single Droplet Tracking in Microfluidic Plug Flow

Abstract

Raman spectroscopy has proven to be a powerful tool for the highly efficient and automated determination of liquid-liquid equilibria (LLE) in parallel microfluidic flows. However, a stable parallel microfluidic flow regime cannot be established for numerous industrially relevant aqueous-organic LLE systems since they tend to form plug flows. These plug flows have the advantage that inner circulations in the plugs enhance the mass transfer, leading to a much faster equilibration. For moving plugs, the main challenge is that established techniques are not capable to collect sufficient Raman signal for quantification. Therefore, we developed a measurement setup for LLE in microfluidic plug flows. In our setup, a capillary is moved against the flow direction to hold one plug of either the aqueous or the organic phase in the laser focus during the Raman measurement. Full automation is established for the premixing of the components, the calculation of the plug lengths and speeds and the Raman measurements of both phases. The setup and automated measurement procedure are successfully validated by showing excellent agreement with data from the literature for the LLE of the ternary system acetone – toluene – water at t = 25 °C. The developed setup thus enables efficient access to LLE data for industrially relevant mixtures.