Studies On The Nanostructure Rheology And Drag Reduction Characteristics Of Drag Reducing Cationic Surfactant Solutions

Abstract: At concentrations above CMC (critical micellization concentration) or temperatures above CMT (critical micellization temperature) surfactant molecules dissolved in aqueous solution self-assemble into colloidal aggregates such as micelles or vesicles. These colloidal aggregates vary in shape and size depending on a number of system conditions such as surfactant molecular structure, surfactant concentration, salt concentration, temperature, etc. Among the variety of micellar structures in solution, wormlike micelles resembling the long chain molecules of high polymers may reduce friction energy loss in turbulent flow by up to 90% at relatively low surfactant concentrations under appropriate flow and temperature conditions. This phenomenon is termed drag reduction (by surfactant additives) and it has significant potential impacts on fluid transport and on the environment. Among surfactant drag reducing additives, cationic surfactants with organic counterions have received the most attention in the past two decades mainly because of their excellent drag reducing ability, broad availability, low concentration requirements and general insensitivity to ionic metal impurities. Typical cationic surfactants studied for drag reduction are quaternary ammonium salts with one long alkyl chain (carbon number from 14 to 22) and methyl or hydroxyethyl groups in the other positions. They are, however, mildly toxic with poor biodegradability, so there is a need to develop more environmentally friendly surfactant drag reducing additives. Other types of surfactants such as anionics, zwitterionics and nonionics have also been studied. To obtain desired drag reducing properties, previous research has been focused on utilizing synergistic effects that may arise when two surfactant species are mixed. Mixed surfactant systems studied for drag reduction included cationic surfactants of mixed alkyl chain lengths, cationic/anionic, nonionic/nonionic, nonionic/anionic and zwitterionic/anionic surfactant mixtures in aqueous solutions and in water/co-solvent systems. Organic counterions added to dilute cationic surfactant aqueous solutions are effective in inducing and stabilizing wormlike micelle formation at relatively low counterion to surfactant molar ratios, thereby promoting their drag reducing effectiveness. The interactions of the cationic surfactant and organic counterion can be enhanced by tuning either or both of them, structurally and/or by concentration and molar ratio, to tailor-make highly efficient drag reducing systems suitable for different applications. Understanding the important role of organic counterions in the dynamics of the formation of cationic surfactant wormlike micelles and their networks is important. In this work, investigations have been conducted in how changes in the organic counterion chemical structure of a series of p-halobenzoates and counterion to surfactant ratio affect zeta potential, nanostructure, drag reduction and rheological properties. Also, certain mixed aromatic counterion systems were studied which showed excellent synergistic effects on promoting wormlike micellar branched networks and enhancing drag reducing effectiveness. In this work, an enclosed rotating disk apparatus was designed and constructed for screening novel surfactant species synthesized in chemistry laboratories. After correlating its drag reducing results with those obtained through the conventional pipe flow test system, this small scale apparatus is capable of testing materials for drag reduction effectiveness independently. A long range goal of this research is to develop effective low concentration surfactant systems with good drag reduction effectiveness. Guided by the correlations and understandings obtained in the past research, in this work, a number of new surfactants or counterions were selected or synthesized for exploratory drag reduction tests.