Phase and rheological behavior of the hexadecyl(trimethyl)azanium; 2-hydroxybenzoate/water system

The phase and rheological behavior of hexadecyl(trimethyl)azanium; 2-hydroxybenzoate (CTAS), and water as a function of surfactant concentration and temperature are investigated here. The critical micellization concentration (cmc1) and the concentration at which the structure of aggregates changes (cmc2) as well as the Krafft temperature (TK) are reported. A large micellar solution region exhibiting high viscosity, as well as hexagonal- and lamellar-phase regions were identified. In the dilute region, below the overlapping entanglement concentration (C∗), the micellar solutions exhibit shear thickening. Above C∗, wormlike micelles form and the solutions show strong viscoelasticity with Maxwell behavior in the linear regime and shear banding flow in the non-linear regime.The linear viscoelastic regime was analyzed with the Granek–Cates model, showing that the relaxation is controlled by the kinetics of reformation-and-scission of the micelles. The steady response in the non-linear regime is compared with the predictions of the Bautista–Manero–Puig (BMP) and the Giesekus models.

Sound velocity and density allow a precise determination of the critical concentrations cmc1, cmc2 and the micellar growth concentration (mgc).Figure optionsDownload high-quality image (93 K)Download as PowerPoint slideResearch highlights
► Hexadecyl(trimethyl)azanium; 2-hydroxybenzoate forms wormlike micelles in water in very diluted solutions due to the strong hydrophobicity of its counterion.
► 2-Hydroxybenzoate hydrophobicity determines the self-assembling properties.
► CTAS/water system exhibits shear thickening behavior at very low concentrations.