Microstructure and dynamics of wormlike micellar solutions formed by mixing cationic and anionic surfactants

Small-angle neutron scattering (SANS) and rheology are used to probe the wormlike micelles formed in mixtures of a cationic (cetyl trimethylammonium tosylate, CTAT) and an anionic surfactant (sodium dodecyl benzene sulfonate, SDBS). For a fixed composition of 97/3 CTAT/SDBS, the zero-shear viscosity eta (0) initially increases rapidly with surfactant concentration, but decreases beyond an intermediate concentration phi (max). The solutions show a scattering peak in SANS and the height of the scattering peak also exhibits a maximum around phi (max). These results are interpreted in terms of a maximum in the linear micellar contour length (L) over bar at phi (max), and suggest that the hydrodynamic and electrostatic correlation lengths reach an optimal ratio at this point. For a fixed total surfactant concentration, the viscosity eta (0) also reaches a maximum at an intermediate SDBS fraction. The decrease in eta (0) at high SDBS fractions is interpreted in terms of the polyelectrolyte nature of the micelles and the increased chain flexibility caused by the rising ionic strength of the solutions. An alternate possibility may involve a progression from linear to branched micelles with increasing SDBS content.