Design of a salt-tolerant Gemini viscoelastic surfactant and the study of construction of wormlike micelle structure in high-salinity aqueous environment

Herein, a zwitterionic Gemini VES (named VES-C) with double ultra-long hydrophobic tails was designed and synthesized which aimed to construct WLM net-structures in high-salinity aqueous solution. Due to the carboxylic group on the spacer, it exhibits partial carboxyl betaine properties and performs well in high salinity environment. Molecular dynamic simulation was carried out and RDF curves were analyzed to predict and explain the mechanism of salinity tolerance of VES-C. Compared with cationic Gemini VESs, a series of experiments were conducted by varying salinity, including surface tension, DLS , 1 H NMR and fluorescent probe, to explain the salt-tolerance mechanism of VES-C. The salinity cloudy point of VES-C surpasses 10 wt%, exhibiting superior salt-tolerance to those cationic Gemini VESs. Trough viscosity measurement, oscillatory measurement and microstructure observation, it proves that VES-C molecules can construct WLM netlike structures within the salinity range from 3 wt% to 10 wt% (optimal salinity: 4 wt%), thus VES-C can be applied as the clean thickener to prepared seawater or brine based fracturing fluid by constructing WLM netlike structure in high salinity aqueous environment, which leaves the preparation of clean fracturing fluid independent of freshwater resources. The carboxyl group on the spacer of VES-C interacts with one quaternary ammonium group to impart partial carboxyl betaine properties to VES-C, which attributes an extremely excellent salinity tolerance to VES-C. The molecules of VES-C can self-assemble into wormlike micelles within a wide salinity range to impart viscoelasticity to the solutions. Therefore VES-C exhibits an excellent superior to those Gemini cationic VES and owns the potential to be used as the clean thickener to prepare freshwater-independent fracture fluids.