Comparison of the effects of methyl- and chloro-substituted salicylate counterions on drag reduction and rheological behavior and micellar formation of a cationic surfactant

We studied drag reduction, rheological behavior, and cryogenic transmission electron microscopy (cryo-TEM) imaging of aqueous solutions of a cationic surfactant, Arquad 16-50 (5 mM) (commercial cetyltrimethylammonium chloride, CTAC), with two series of isomer counterions at 5 mM: 3-, 4-, and 5-methylsalicylate, and 3-, 4-, and 5-chlorosalicylate. All these systems demonstrated viscoelastic behavior at room temperature and showed effective drag reduction in the temperature range of 20-70 A degrees C. The additional Cl- or CH3- group on the salicylate extended the upper effective drag reduction temperature limit 5-10 A degrees C higher than that of sodium salicylate solution. The 4-methylsalicylate, and 3- and 5-chlorosalicylate solutions are effective drag reducers down to 5 A degrees C, the lowest temperature tested in the flow loop. All of the methyl isomers had very similar rheological behaviors, while the chloro isomers exhibited dramatic differences. The 5-chlorosalicylate system had near zero N (1), but did show viscoelastic behavior by showing recoil after swirling and overshoot at shear startup. Cryo-TEM images show that four of the isomer systems formed threadlike micellar nanostructures, but 3-methylsalicylate aggregated into vesicles and open vesicles. The 3-chloro and 5-chlorosalicylate showed vesicles with a few spherical micelles and threadlike micelles. It is postulated that the vesicles were transformed into threadlike micelles under shear and that this structure made them effective drag reducers.