Studies on physicochemical properties of three Gemini surfactants with different spacer groups

Three Gemini surfactants with the same hydrophobic chain but different spacer groups, including two cationic Gemini surfactants (CGS and CGS-O) and a zwitterionic Gemini surfactant (ZGS-S), were synthesized, and their chemical structures were characterized by FT-IR and 1H NMR. The effect of spacer group on surface/interface activity, interfacial tension at the oil-water interface, wetting ability, electrolytes tolerance and thermal stability was determined. The results showed the structure of spacer group has a certain influence on the surface/interface activity. Among the three Gemini surfactants, ZGS-S has the strongest ability to reduce surface tension, followed by CGS-O. Three Gemini surfactants can reduce the oil-water interfacial tension to ultra-low value, but the time required to reach the minimum value is different, indicating that the surfactant molecules move at different adsorption rates from solution to the interface. This phenomenon is also confirmed by dynamic surface tension and dynamic contact angle tests. Compared with CGS, CGS-O molecules move slower in the solution, while the adsorption rate of ZGS-S on the interface surpasses that of CGS near the critical micelle concentration. Among the three Gemini surfactants, the electrolytes tolerance of ZGS-S is the strongest, while CGS-O is the lowest. The introduction of sulfate group in spacer group reduces the thermal stability of Gemini surfactant.