Highly efficient oil-fouling and foam removal achieved by surfactant mixed systems

Excessive usage of surfactants in daily life and industry and their undesirable high foamability have caused serious environmental pollution and economic loss. Improving cleaning efficiency and reducing foam stability concurrently is a delicate strategy but a challenging task. Herein, we mixed the most widely used surfactant sodium dodecyl sulfate (SDS) with cyclic amines (CnN, n = 6, 8, 12), by which the self-assembly ability of SDS at the air/water interface and in bulk is significantly enhanced, while spherical micelles, vesicles and wormlike micelles are formed at appropriate total surfactant concentration (CT) and molar fraction of SDS (XSDS). Especially around XSDS = 0.50 and above critical micellar concentration (CMC), the stronger self-assembly ability leads to a higher contact angle of machine oil on stainless-steel plates and lower oil–water interfacial tension in CnN/SDS solution, thus the oil-fouling removal efficiency of CnN/SDS solutions is remarkably improved. Meanwhile, the foamability and foam stability dramatically decline at smaller XSDS and slightly above CMC, attributed to the rapid molecular migration from liquid film of foams to the bulk between the films when the limited surfactant molecules in the films prefer to aggregate in bulk. As a result, C8N/SDS exhibits the best oil cleaning and lowest foaming simultaneously at low XSDS and just above the CMC. This study opens an efficient avenue to eliminate the contradiction between cleaning ability and foamability, thereby obtaining a high-efficiency and low-foam detergent.