Study Of PH-responsive Gemini Surfactants

pH stimuli-responsive materials whose structure and properties vary with the change of the pH, which arose great changes in performance. Especially responsive worm-like micelles have advantages such as controllable viscosity, and the surfactants in system can be recycled. These particularities can make it have huge potential in the field of oil and gas development.In this dissertation, a series of tertiary amide-based gemini surfactants, abbreviated as Cm-A-Cm (m=8;10;12;14), were synthesized, which based on long chain fatty acid, hydroquinone and N,N'-Dimethyl-1,3-propanediamine according to three steps. And the structure of these products were verified by IR and 1H NMR subsequently. The surface property and aggregation behaviors of the Cm-A-Cm aqueous solutions were studied in detail. The results showed that the Cm-A-Cm exhibited high and pH-regulated surface activity at the air/water interface; i.e., the critical micelle concentration was 5.6106 mol L-1 at pH= 2.50 when m= 14 and was further regulated to 1.8106 mol L-1 by altering the pH to 6.50. When the pH was tuned from 2.0 to 12.0, the appearance of the C12-A-C12aqueous solution (35 mM) underwent 5 states:transparent water-like solution, viscous fluid, gel-like fluid, turbid liquid and dispersion system with white precipitate. The results of rheology, cryogenic transmission electron microscopy, and dynamic light scattering characterization revealed that the transition from water-like to viscous or gel-like liquid was actually due to aggregate microstructure transition from spherical to worm-like micelles. This transition was completely reversible between pH= 2.50 and 6.81, tuned by adding HC1 and NaOH solutions for at least 4 cycles. Similar micellar transitions regulated by pH were also found for m=8 and 10, whereas only worm-like micelles were formed for m=14 at both acidic and nearly neutral conditions,In addition, a series of amphoteric gemini surfactants Cm-B-Cm (m=10 and 12) were synthesized by Cm-B-Cm and sodium chloroacetate according to quaternization, and the structure of these products were verified by IR and 1H NMR subsequently. The surface property and aggregation behaviors of the Cm-B-Cm aqueous solutions were studied in detail. The results showed that the Cm-B-Cm exhibited high surface activity at the air/water interface at acidic, neutral, and basic conditions, respectively, and the cmc also increased with the increase of pH. The viscosity of Cm-B-Cm solution increased sharply when the pH of the aqueous solution was increased from 5.6 to 8.5. The dramatical change in viscosity was actually due to aggregate microstructure transition from spherical to worm-like micelles, which was further confirmed by rheology, cryogenic transmission electron microscopy, and dynamic light scattering characterization. This transition could be tuned for at least 4 cycles between pH=2.50 and 6.81.