Rheology of saturated micellar networks: Wormlike micellar solutions vs. bicontinuous micellar phases

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Physicochemical and engineering aspects, 2022-11, Vol.652, p.129927, Article 129927
• Main Authors: Stancheva, Teodora N., Georgiev, Mihail T., Radulova, Gergana M., Danov, Krassimir D., Marinova, Krastanka G.
• Format: Article
• Language: English
• Subjects: Bicontinuous micellar phases; Micelles with divalent ions; Multiconnected micelles; Rheology of complex fluid; Wormlike micelles
• ISSN: 0927-7757
• DOI: 10.1016/j.colsurfa.2022.129927

The subject of this work is to investigate the rheological behavior of mixed micellar solutions (sodium laurylethersulfate and cocamidopropyl betaine) in the presence of Mg2+ divalent counterions. With the rise of salt concentration, the viscosity of micellar solutions increases to a high maximum followed by a steep decrease because of the initial growth and entanglement of wormlike micelles and a subsequent transition to branched micelles forming of a saturated micellar network. The proposed systematic rheological measurements show considerable variations in the rheological responses of the solutions when increasing the salt concentration. Rheological behavior and data are used to distinguish the micellar phases and to study the relation to micellar structures. The wormlike micellar solutions have a typical shear thinning behavior with a well-defined zero-shear viscosity, η0, described by the Cates reptation-reaction model or the augmented Maxwell model. Our data show that the power law dependence of η0 on the surfactant concentration is stronger than that reported in the literature and it is influenced of the added electrolytes. The branched micellar structures are characterized by the lower viscosities and larger elasticities, which follow the Maxwell model up to the intermediate values of the frequency of oscillations, however peculiar deviations from the Cole-Cole plot at large frequencies are detected. The isolated bicontinuous micellar phases are Newtonian fluids with viscosity 0.4–0.7 Pa.s independent on the salt concentration up to high shear rates. The threshold salt concentration ensuring the onset of the bicontinuous micellar phase is described by a simple empirical rule. These phases are characterized by large elasticities and not negligible yield stresses. The property of the bicontinuous micellar phases to form spontaneously oil-in-water nanoemulsions could find applications in drug delivery, extraction and separation processes, pharmaceutics production, etc. [Display omitted]