Effect of Inorganic Counterions on the Self-Assembly Behavior and Rheological Properties of Ultralong-Chain Cationic...

Ultralong-chain quaternary ammonium surfactants serve as promising clean fracturing-fluid thickeners owing to their facile formation of wormlike micelles (WLMs) in salt solution. However, the ion-specific effects of common inorganic counterions on the formation of WLMs still remain unclear. In this study, we investigate the aqueous solution of docosyl(trimethyl)azanium chloride (DCTAC) with NaBr, NaNO₃, or NaI at temperatures of 60 °C and above through steady/oscillatory rheology, small-angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryo-TEM). By integrating the results of previous research, we found that the four comparable counterions, namely Cl^-, Br^-, NO₃^-, and I^-, can induce a sphere-to-wormlike transition and result in a pronounced maximum viscosity. Nevertheless, the optimal counterion-to-surfactant ratios for rheological properties vary significantly (80, 6, 3, and 1 eq. for Cl^-, Br^-, NO₃^-, and I^-, respectively). Counterion binding affinity to DCTAC plays a crucial role in determining the self-assembly behavior and rheological properties. The effects of these four counterions precisely adhere to the Hofmeister series, i.e., Cl^- < Br^- < NO₃^- < I^-. SANS and cryo-TEM verify the emergence of elongated cylindrical micelles. Meanwhile, rheology demonstrates that I^- ions generate substantially stronger micellar networks with excellent temperature resistance, which can be attributed to its strong affinity for the micellar interior. These findings establish a connection between counterion affinity, WLM growth, and thermal robustness, offering practical guidance for the design of high-temperature cationic viscoelastic surfactant fluids in complex brines.