Phase and Rheological Behavior of Cetyldimethylbenzylammonium Salicylate (CDBAS) and Water

The temperature–composition phase diagram in the diluted region of the cationic surfactant cetyldimethylbenzylammonium salicylate/water system was studied with a battery of techniques. The Krafft temperature ( T k = 33 ± 1 °C) was measured by differential scanning calorimetry, polarizing microscopy, conductimetry, viscosimetry, and rheometry. The critical vesicle concentration (cvc, ~0.002 wt%) and a vesicle–micellar transition (cvm, ~0.005 wt%) was detected at a temperature of 35 °C. Below T k and concentrations ≤2 wt%, a transparent solution is formed (I). Above 2–8.5 wt%, a lamellar (L 1 ) phase forms. At higher concentrations and up to 12 wt%, a second lamellar phase (L 2 ) is detected. From 12.4 to 15.5 wt%, an emulsion phase (E) is formed. Rheological dynamic measurements for the I phase indicate that the system exhibits a predominantly viscous behavior ( G ′ < G ″) for concentrations lower than the overlap or entanglement concentration ( C e , ~0.75 wt%). At higher concentrations, wormlike micelles form and the elastic behavior predominates ( G ′ > G ″). The elastic ( G ′) modulus collapses in a concentration–time master curve in the whole reduced frequencies range ωτ c examined, whereas the viscous modulus ( G ″) collapses only at reduced frequencies lower than 0.1. Reduced stress plotted as a function of the reduced shear rate yields a good superposition of the curves at the different concentrations up to the onset of the non-linear behavior.