Shear-induced structural and thermodynamic phase transitions in micellar systems

Abstract

In this contribution a methodology to compute and classify shear-induced structural and phase transitions in surfactant/water mixtures from rheological measurements is presented. Non-linear rheological experiments, considering variations in surfactant concentration and temperature, are analyzed. In particular, the parameters of the BMP (Bautista-Manero-Puig) model, obtained from the fitting of the shear stress versus shear rate data, which are functions of surfactant concentration and temperature, allow classifying structural and phase transition boundaries. To test this methodology, we consider the analysis of the shear-induced structural and phase transitions of two micellar systems, cetyltrimethylammonium tosylate (CTAT)/water as a function of CTAT concentrations and Pluronics P103/water as a function of temperature. We found that the CTAT/water system presents a first-order phase transition at 30 C, and around 31 to 32 wt.% from isotropic to nematic phases, whereas a 20 wt.% Pluronics P103 aqueous micellar solution has two second-order (structural) phase transitions, one from spherical to cylindrical micelles at 33.1 C, and another one from cylindrical micelles to a nematic phase at 35.8 C and one first-order phase transition around 37.9 C at high shear rates near to the cloud point previously reported. The proposed methodology is also able to identify the instability regions where the wormlike micelles are broken, producing the typical shear banding behavior.

Thin-Film Lubrication in the Water/Octyl β-d-Glucopyranoside System: Macroscopic and Nanoscopic Tribological Behavior

“…46,47 The strong shear force can induce the extension of the micelles and cause orientation along the shear direction. 48,49 It is assumed that the C8 micelles are gradually deformed, extended, ruptured, and aligned along the direction of movement caused by the strong shear force. In this way, the viscosity in shear direction becomes much lower than that perpendicular to it and the system becomes anisotropic.…”

“…The measurement of the lubricant film thickness in the friction gap and the determination of the shear rate at different mechanical loads show that the high shear rate in such samples leads to low friction (Figure ). Nilsson and He have reported that C8 micelles are present in solution in the form of a spherical-capped cylinder. , The strong shear force can induce the extension of the micelles and cause orientation along the shear direction. , It is assumed that the C8 micelles are gradually deformed, extended, ruptured, and aligned along the direction of movement caused by the strong shear force. In this way, the viscosity in shear direction becomes much lower than that perpendicular to it and the system becomes anisotropic.…”