Impact of Salt Co- and Counterions on Rheological Properties and Structure of Wormlike Micellar Solutions.

Rheological properties of aqueous solutions of long-tailed cationic surfactant erucyl bis-(hydroxyethyl)methylammonium chloride (EHAC) were examined as a function of concentration C of different inorganic salts (KCl, CaCl, and LaCl) at a fixed surfactant concentration of 0.6 wt %. The structural evolution of micelles was followed by small-angle neutron scattering and cryogenic transmission electron microscopy. It was observed that, upon addition of salt, the zero-shear viscosity η of semidilute surfactant solutions goes through a maximum by passing the following three regimes: η ∼ C (regime I), η ∼ C (regime II), and η ∼ C (regime III). In regime I, the micelles grow in length; in regime II, the linear growth of micelles proceeds simultaneously with their branching; and in regime III, the branching becomes dominating. With increase in the salt valence, the viscosity curves shift to a lower salt content, indicating that these salts are more effective in inducing micellar elongation and branching, as they contain a larger amount of anionic species Cl screening the repulsion between cationic surfactant heads. Diverse roles of salt co- and counterions (i.e., salt ions that are similar and oppositely charged with respect to surfactant head groups) at different salt concentrations were demonstrated. It was shown that at low salt concentrations corresponding to the rising branch of the viscosity curve (regimes I and II), salt counterions (Cl) fully determine the rheological behavior of the system. At high salt concentrations, when the electrostatic repulsions between micelles and salt co-ions are essentially screened, the co-ions start affecting the rheological properties. Under these conditions, monovalent co-ions (K) provide much lower viscosity of surfactant solutions than the multivalent ones (Ca, La), which is consistent with theoretical predictions that suggest the penetration of K inside the micellar corona increasing the charge of the micelles and therefore hindering their growth.