Dual-Responsive Microemulsion Based on Hydrophobic Deep Eutectic Solvents for Tuning Highly Efficient Knoevenagel Reaction

The Knoevenagel reaction, crucial for synthesizing α, β-unsaturated compounds, faces challenges like harsh conditions and separation difficulties. In this study, a novel class of temperature and CO₂ stimuli-responsive deep eutectic solvent (DES)-based microemulsions was designed, composed of DES, Triton X-100, and H₂O. The variations in microstructure and particle size distribution of these microemulsions, triggered by CO₂/N₂ or temperature changes as environmentally friendly stimuli, were investigated through electrical conductivity, dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS). The results indicate that these microemulsions could be reversibly switched from a W/O single phase to a state of complete oil-water separation by an alternating process of bubbling and removal of CO₂ or changing the temperature. In addition, UV-vis spectroscopy, Fourier transform infrared spectrometry (FTIR), and nuclear magnetic resonance (NMR) were utilized to understand the reversible phase separation of DES-based microemulsions driven by both temperature and CO₂. The outcomes indicate that the reversible phase separation mechanism results from variations in the oil-water interface of the microemulsion caused by the insertion of hydrophobic CO₂ molecules into the surfactant interface or the changes in the force between the Triton X-100 and DES or H₂O, respectively. This dual-responsive DES-based microemulsion was applied in Knoevenagel reactions, achieving the efficient reuse of microemulsions, separation of products, and integration of reactions.