Comparison of the effects of dimethyl and dichloro benzoate counterions ondrag reduction, rheological behaviors, and microstructures of a cationic surfactant

Arquad 16-50 (commercial CTAC, cetyltrimethylammonium chloride) (5 mM) with the counterions 3, 4-dichlorobenzoate (5 and 10 mM), 3, 4-dimethylbenzoate (5 and 10 mM) and 3, 5-dichlorosalicylate (5 mM) were studied to compare t he effect of concentration of the counterion and its ratio to surfactant co ncentration (5) on drag reduction, theological behavior, and microstructure s. The first four solutions are good drag reducers at different temperature ranges. The 3, 4-dimethylbenzoate system (xi = 1) is effective at 5-40 deg reesC and the 3, 4-dichlorobenzoate system (xi = 1) at 20-70 degreesC. Incr easing the concentration ratio to xi = 2 increased the upper temperature li mit 10 degreesC for each system. The counterion concentration changes affec t microstructures in the quiescent state in different ways. The viscoelasti c xi = 1 solution of 3, 4-dimethylbenzoate has a microstructure of both ves icles and threads in the quiescent state which probably transform to a thre adlike micellar network under sheat However, its ratio of apparent extensio nal viscosity to sheat viscosity is very low, unusual for a surfactant drag reducer, its xi = 2 solution has typical surfactant drag reducer propertie s, viscoelastic, high extensional viscosity, and threadlike micellar networ ks. The xi = 1 solution of 3, 4-Cl-benzoate is like the xi = 2 solution of 3, 4-CH3-benzoate while its xi = 2 solution is nonviscoelastic with vesicle s and spherical micelle microstructures in the quiescent state which also p robably transform to a network structure in strong shear fields. The only s ystem tested with a counterion having four-substituent groups, 3, 5-dichlor osalicylate, is not effective as a drag reducer, has water-like behavior, a nd contains very large vesicles. (C) 2001 The Society of Rheology.