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

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, NaNO 3, 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 –, NO 3 –, 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 –, NO 3 –, 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 – < NO 3 – < 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.