Microfluidics with: In situ Raman spectroscopy for the characterization of non-polar/aqueous interfaces

The design of microfluidics with in situ Raman spectroscopy is reported in the present work for the investigation of immiscible non-polar/aqueous interactions. A dynamic aqueous phase in contact with a static hydrophobic phase (semi-flow) was engineered on-chip by performing a force balance, and it was validated using high-resolution in situ Raman spectroscopy. The semi-flow microfluidic device isolated non-polar solvents in a series of micro-reservoirs that communicated with aqueous laminar flows via molecular diffusion. As a consequence, the inspection of concentration and density profiles from the bulk-to-bulk of toluene-, diethyl ether-, and xylenes-water were made possible. The Schlieren pattern was observed for all solvent pairs, and the bulk-to-bulk region was characterized by water rarefaction (17.5 to 23.2 μm), mixing (3.4 to 6.4 μm), and hydrophobic shockwaves (34.9 to 38.8 μm). Understanding the position, thickness, and profiles of each zone revealed new insight on the chemistry at the interface, which could someday be used to validate molecular simulations, reduce the quantities of solvents wasted during separations/purifications of compounds, and improve kinetic models for mixed mass-transfer-reaction-rate-limited and mass-transfer-limited scenarios in continuous-flow processes.