pH-Responsive Viscoelastic Fluids of a C-Tailed Surfactant Induced by Trivalent Metal Ions.

pH-responsive viscoelastic fluids are often achieved by adding hydrotropes into surfactant solutions. However, the use of metal salts to prepare pH-responsive viscoelastic fluids has been less documented. Herein, a pH-responsive viscoelastic fluid was developed by blending an ultra-long-chain tertiary amine, -erucamidopropyl--dimethylamine (UCAMPM), with metal salts (i.e., AlCl, CrCl, and FeCl). The effects of the surfactant/metal salt mixing ratio and the type of metal ions on the viscoelasticity and phase behavior of fluids were systematically examined by appearance observation and rheometry. To elucidate the role of metal ions, the rheological properties between AlCl- and HCl-UCAMPM systems were compared. Results showed the above metal salt evoked the low-viscosity UCAMPM dispersions to form viscoelastic solutions. Similar to HCl, AlCl could also protonate the UCAMPM into a cationic surfactant, forming wormlike micelles (WLMs). Notably, much stronger viscoelastic behavior was evidenced in the UCAMPM-AlCl systems because the Al as metal chelators coordinated with WLMs, promoting the increment of viscosity. By tuning the pH, the macroscopic appearance of the UCAMPM-AlCl system switched between transparent solutions and milky dispersion, concomitant with a viscosity variation of one order of magnitude. Importantly, the UCAMPM-AlCl systems showed a constant viscosity of 40 mPa·s at 80 °C and 170 s for 120 min, indicative of good heat and shear resistances. The metal-containing viscoelastic fluids are expected to be good candidates for high-temperature reservoir hydraulic fracturing.