Unusual effects of counterion to surfactant concentration ratio on viscoelasticity of a cationic surfactant drag reducer

One-to-one molar ratio sodium salicylate (5 mM)/Arquad S-50 (5 mM) solution was investigated in comparison to the sodium salicylate (12.5 mM)/Arquad S-50 (5 mM) or 2.5:1 solution, which was previously reported to be drag reducing yet non-viscoelastic [9]. Results showed a dramatic influence of the counterion to surfactant concentration ratio on the viscoelasticity of the surfactant drag reducing systems. The 1:1 solution has normal surfactant drag reducer characteristics such as viscoelastic properties and thread-like micellar networks and branches. This is the first cationic surfactant/counterion drag reducing system found, for which excess counterion makes the solution non-viscoelastic, while the microstructure of thread-like micellar networks is apparently unchanged. (C) 2000 Elsevier Science B.V. All rights reserved.