Counterion chemical structure and counterion to cationic surfactant molar ratio,, control counterion binding, micelle nanostructures, drag reduction (DR) effectiveness and rheological behavior of quaternary ammonium surfactant systems. The effects of chemical structures of four sodium para-halobenzoate (F, Cl, Br, l) counterions with different values on these properties were compared for dilute solutions of cetyltrimethyl-ammonium chloride (CTAC). Counterion binding was determined by zeta-potential and H-1 NMR measurements. Nanostructures were determined by H-1 NMR and cryo-TEM imaging. Nanostructures, drag reduction effectiveness measured over a range of temperatures and Reynolds numbers, shear viscosities and first normal stress differences NI were related to the chemical structures of the four counterions and their molar ratios to CTAC. Micelle nanostructures of the four systems were compared at 30 degrees C. The p-fluorobenzoate system at 0.6 showed no band broadening in 114 NMR spectra and only short threadlike micelle (TLM) structures in cryo-TEM images, was not drag reducing and had zero N1. The other halobenzoates had larger TLMs, were DR and viscoclastic at xi = 0.6. The p-bromo and p-iodobenzoate counterions showed H-1 NMR band broadening at xi = 0.6, while the p-chlorobenzoate showed band broadening at xi = 1.5. At xi = 4.0, the p-fluoro system had small TLMs and no band broadening, was also not drag reducing and had zero N1 while the p-chloro, p-bromo and p-iodo systems which contain TLMs and showed band broadening are all DR and have appreciable N1. The p-fluoro system also showed only small zeta-potential reductions at all ratios while the other three showed moderate to large decreases in zeta-potential with increase in, crossing from positive to negative values. The ratios at the crossing points followed the order of the molecular size and hydrophobicity: l, Br and Cl. (c) 2008 Elsevier B.V. All rights reserved.