Effect of ionic environment on the rheology of wormlike micelle solutions of mixtures of surfactants with opposite charge.

This work reports on the rheological characterization of aqueous solutions of mixtures of a cationic surfactant (hexadecyltrimethylammonium p-toluenesulfonate, CTAT), capable of forming wormlike micelles, and an anionic surfactant (sodium dodecylsulfate, SDS) when the ionic environment of the solution is altered by the addition of electrolytes. Previous work showed that mixtures CTAT/SDS exhibit a strong rheological synergy in shear flows caused by promotion of interaction between wormlike micelles by the presence of the dodecylsulfate anion. In this work, we explore the CTAT/SDS synergy in the presence of electrolytes. The effect of the added electrolyte varies with wormlike micelle concentration range and type of flow. In simple shear flows and relatively low wormlike micelle concentrations (dilute regime), electrolyte addition inhibits the shear thickening effect observed in CTAT/SDS solution. In porous media flows, which have an important elongational component, electrolyte addition results in an appreciable increase in apparent viscosity at low electrolyte concentrations. In the semi-dilute regime, electrolyte addition at low concentrations strengthens the synergy between CTAT and SDS, leading to higher shear viscosities, especially at low-shear rates. An important consequence of these results is the potential for manipulation of the rheology of solutions of wormlike micelles by induction of intermicelle associations and/or promotion of conformational changes by electrolyte addition.