Middle-phase microemulsion induced by brine in region of low cationic gemini surfactant content

The middle-phase microemulsion (Winsor III) induced by salt in the low surfactant + cosurfactant (S + A) region of n-butanol/n-octane/water/alkanediyl-α,ω(dimethydodecyl-ammonium bromide) (12-3-12) system was investigated by salinity scan. It is shown that the original water-in-oil single phase cannot form Winsor III even when the NaCl aqueous solution added reaches saturation. However, the original coexisting multi-phase and oil-in-water single phase can form Winsor III easily in a wide concentration range of salt, which is favorable to the application of Winsor III. The super low salinity for the initial formation of Winsor III (S1) attributes to the special structure of geminis. The water content, salinity, and temperature have great effect on the volume fraction of Winsor III and the temperature induces different phase transition from Winsor II to Winsor III. The condition for the formation of stable Winsor III was obtained by the interfacial tension (IFT) measurements. The multi-phase microemulsion was also studied by fluorescence, and the information, which was used to determine the microstructure of single-phase microemulsion. The results agree well with that obtained from the conductivity measurement carried out in our previous work.