Rheo-optics of cationic surfactant micellar solutions with mixed aromatic counterions

The effects of different ratios of mixed aromatic counterions of sodium salicylate (NaSal) and sodium 4-ethylbenzenesulfonate (NaEBS) at a total counterion concentration of 0.125 M on the rheo-optical behaviors of 0.05 M cetyltrimethylammonium chloride (CTAC) cationic surfactant solutions were studied. The introduction of NaEBS into CTAC–NaSal solution led to a significant deviation from the Cox–Merz rule. Mixing of the two aromatic counterions lowered the CTAC solutions’ zero shear viscosities, increased their plateau shear modulus, and decreased their relaxation time. In addition, the persistence lengths of the threadlike micelles and the mesh size of the micellar network were also significantly reduced. These effects are believed to be due to increased degree of branching and denser meshes of the micellar networks in the mixed counterion systems.