EFFECT OF CHEMICAL-STRUCTURE ON VISCOELASTICITY AND EXTENSIONAL VISCOSITY OF DRAG-REDUCING CATIONIC SURFACTANT SOLUTIONS

Drag reduction, shear and extensional rheometry, and cryogenic transmi ssion electron microscopy (cryo-TEM) were used to study aqueous soluti ons of one cationic surfactant, Arquad 16-50 (5 mM), with three isomer ic counterions, 2-, 3-, or 4-Cl-benzoate at 12.5 mM. Each isomer showe d different; types of rheological and drag reduction behavior and diff erent micellar structures. The 4-Cl system showed good drag reduction, high apparent extensional viscosity, and a thread-like micellar netwo rk, while the 2-Cl system showed no drag reduction, low apparent exten sional viscosity, and only spherical micelles. The 3-Cl system was dra g reducing and had high extensional viscosities at 30 degrees C. Howev er, at 20 degrees C, the 3-Cl solution precipitated at high shear or e xtensional rates, leading to loss of drag reduction and low apparent e xtensional viscosity. The 3-Cl at 20 degrees C showed threadlike struc tures in some pictures and vesicles in others, presumably because of v ariations in the level of shear the samples were subjected to during s ample preparation. The differences in behavior are explained by the po sition of the chlorine group on the benzoate ring. The hydrophobic chl orine in the 2-Cl system must reside in an unfavorable position in the aqueous phase and hence only spherical micelles are formed, which lea ds to no drag reduction and very low apparent extensional viscosity. T he chlorine groups in the 3-Cl and 4-Cl counterions reside in the nonp olar hydrocarbon core of the micelles and, hence, stable elongated mic elles can form. A schematic micellar phase diagram is proposed to summ arize the transformations of surfactant molecular aggregates at differ ent temperatures and external forces.