Phase transformation of thermoresponsive surfactant triggered by its concentration and temperature

The application of switchable surfactants in oil-water emulsion systems has become more and more attractive for both academia and industry. This work synthesized a thermoresponsive copolymer surfactant, namely, poly (ethylene glycol)-b-poly (N-isopropylacrylamide) via atom transfer radical polymerization (ATRP). The transition of its surface activity and the emulsification performance stimulated by the temperature were well investigated. The cloud point temperature (Tcp) of the synthesized block copolymer was significantly higher than the lower critical solution temperature (LCST) of the PNIPAM homopolymer, which was due to the introduction of the hydrophilic PEG block. The block copolymer exhibited surface activity above Tcp, and its critical micelle concentration (CMC) was roughly 5 × 10−4 wt% at 60 °C. When the temperature was lower than its Tcp, the block copolymer became completely water soluble due to the switch of the PNIPAM block from hydrophobicity to hydrophilicity. This thermoresponsive behavior implies that this block copolymer is applicable for the emulsification at elevated temperature and demulsification at low temperature. Additionally, the morphology of the block copolymer aggregates in water was identified using the scanning electron microscope (SEM) and dynamic light scattering (DLS). It was found that the block copolymer could assemble and form colloidal particles at certain concentration through the physical crosslink in water at high temperature, which helped forming a Pickering emulsion in the oil-water system. The colloidal particles could rapidly dissociate and dissolve in the water when the temperature was lower than Tcp, resulting in fast demulsification. This emulsification/demulsification switch may provide a theoretical basis for the application of this prepared thermoresponsive surfactant in enhanced oil recovery (EOR).