CO2/pH-Controllable Viscoelastic Nanostructured Fluid Based on Stearic Acid Soap and Bola-Type Quaternary Ammonium Salt.

Abstract

Fatty acid soaps such as sodium stearate (NaOSA) represent a class of cheap, environmentally friendly surfactants; however, their poor solubility seriously challenges their application in various fields. Herein, we describe a CO2/pH-controllable viscoelastic nanostructured fluid, which was developed by simple mixing of the commodity soap NaOSA with a bola-type quaternary ammonium salt (Bola2be) in a 2:1 molar ratio without the need for complex organic synthesis. The introduction of Bola2be increased NaOSA solubility and promoted micelle growth by forming a noncovalent pseudo-Gemini structure, 2NaOSA-Bola2be. Long aggregates are formed with increases in concentration, and these become entangled into a three-dimensional network at 10 times that of the critical micelle concentration (0.057 mM), showing strong thickening ability. Micellar branching occurs above 22.38 mM, as deduced by rheology and verified by cryo-transmission electron microscopy. The worm-based fluid formed from the noncovalent pseudo-Gemini surfactant is highly thermosensitive, and features a higher flow activation energy of 399.76 kJ·mol(-1) compared with common worm systems. Because of the pH-sensitivity of NaOSA, the viscoelastic fluid can respond to common pH stimuli or green CO2 gas, and shows a transition between a gel-like wormlike micellar network and a water-like dispersion with precipitate. However, the CO2-responsive behavior is irreversible.