pH-Regulated surface property and pH-reversible micelle transition of a tertiary amine-based gemini surfactant in aqueous solution

Abstract

A series of tertiary amide-based gemini surfactants, 2,2′-(1,4-phenylenebis(oxy))bis(N-(3-(dimethylamino)propyl)alkylamide), abbreviated as C_m–A–C_m (m = 8; 10; 12; 14), were synthesized. The surface property and aggregation behaviors of the C_m–A–C_m aqueous solutions were studied in detail. The C_m–A–C_m exhibited high and pH-regulated surface activity at the air/water interface; i.e., the critical micelle concentration was 5.6 × 10⁻⁶ mol L⁻¹ at pH = 2.50 when m = 14 and was further regulated to 1.8 × 10⁻⁶ 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–C₁₂ aqueous 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 HCl 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. Finally, a reasonable mechanism of aggregate behavior transition was proposed from the viewpoint of the molecular states, molecular structures, and the intra- and inter-molecular interactions.