Effect of variations in counterion to surfactant ratio on rheology and microstructures of drag reducing cationic surfactant systems

Rheology, drag reduction and cryo-TEM experiments were performed on Arquad 16-50/NaSal and Ethoquad O/12/NaSal surfactant systems at different counter ion-to-surfactant ratios and at constant low surfactant concentrations, 5 m M, appropriate for drag reduction. The molar ratio of counterion-to-surfact ant was varied from 0.6 to 2.5. All the surfactant systems described here a re viscoelastic and drag reducing. The viscoelasticity and drag reducing ef fectiveness increase with increase in counterion/surfactant ratio. Network are present in the solutions with high ratio, and they are viscoelastic. Ho wever, shear is needed to induce network formation for solutions at low rat io. Cryo-TEM images confirm the existence of thread-like micelles which for m entanglement networks, and show that the micellar network becomes denser with increasing counterion/surfactant ratio in one surfactant series. Both increase in the counterion/surfactant ratio and increase in the shear rate result in shorter relaxation times. For some of these systems, abrupt increase in viscosity is observed at cert ain sheer rates which are time effects affecting microstructure rearrangeme nts rather than formation of shear induced structures.