文摘
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 °C. The additional Cl- or CH3- group on the salicylate extended the upper effective drag reduction temperature limit 5–10 °C higher than that of sodium salicylate solution. The 4-methylsalicylate, and 3- and 5-chlorosalicylate solutions are effective drag reducers down to 5 °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. Keywords Drag reduction Rheology Cationic surfactant Counterion First normal stress difference (N1) Cryogenic transmission electron microscopy (Cryo-TEM)