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 (L₁) phase forms. At higher concentrations and up to 12 wt%, a second lamellar phase (L₂) 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.