Synergistic Mechanism of an Ionic Liquid-Based Composite Surfactant-Stabilized Microemulsion for Effective Separation of Oily Sludge

The reduction and harmless treatment of oily sludge are challenging tasks. As green solvents, ionic liquids (ILs) have been used to treat oily sludge effectively; however, their high price limits the applications. In this paper, an imidazolium-based IL 1-hexadecyl-3-methylimidazole chloride salt ([C₁₆mim]-Cl) was synthesized and compounded with Tween20 to obtain a composite surfactant that could be used as an emulsifier to prepare a microemulsion for the treatment of oily sludge. The composite surfactant exhibited the greatest synergistic effect at a mass ratio of 4:6. Under optimal conditions, the oil removal rate reached 94.90%, and the residual oil rate was reduced to 1.67%. Moreover, the microemulsion exhibited a high removal rate for asphaltenes (81.01%). The treatment mechanism and synergism were proposed based on a combination of interfacial tension (IFT) and quartz crystal microbalance (QCM-D) measurements. Herein, the π-π stacking and hydrogen bonds between S, N, and O in asphaltenes or resins and N atoms of imidazole rings in [C₁₆mim]Cl were responsible for the effective treatment of the two components. The use of a microemulsion stabilized by a composite surfactant provided an efficient, environmentally friendly, and sustainable treatment method for oily sludge with high asphaltene and resin content.