Co-solvent effects on drag reduction, rheological properties and micelle microstructures of cationic surfactants

Some quaternary cationic surfactants, when mixed with a counterion, are known to self-assemble into threadlike micelles in water. Such behavior causes drastic changes in rheological properties of even very dilute solutions, allowing them to be used as drag reducing agents (DRA) in turbulent pipe flow circulating systems, such as district cooling/heating systems. Surfactant self-assembly is a physicochemical phenomenon whose character depends on surfactant nature and concentration, nature of the solvent, temperature and type and concentration of counterions. This study investigates drag reduction (DR) and rheological properties of two cationic surfactants, Ethoquad O/12 (oleyl bis(hydroxyethyl)methylammonium chloride) and Ethoquad O/13 (oleyl tris(hydroxyethyl) ammonium acetate), with excess salicylate counterion (NaSal), in mixed solvents containing 0 to 28 wt% ethylene glycol (EG) and water. The addition of EG to the solvent had greater effects on solutions' DR ability, shear viscosity, apparent extensional viscosity and viscoelasticity at 25 °C than at ∼0 °C. Cryo-TEM images show threadlike micelle in these systems. DR at low temperatures in solutions containing moderate amount of EG can be utilized in a new approach to energy saving in district cooling systems using EG–water based mixtures as the cooling fluids.