Rheological Behavior and Temperature-Responsive Properties in pH-Responsive Worm-Like Micelles System Induced by Additives

The stimuli-responsive worm-like micelles (WLM) constructed by mixing organic acids and surfactants have broad applications in industries. However, it is unknown that how the additives with different carboxyl group number affect the rheological behaviors and temperature-responsive properties of WLM. In this paper, three pH-responsive WLM systems were constructed as clean fracturing fluids by mixing oleamidopropyl dimethylamine (OA) and benzoic acid (BA), p-phthalic acid (PA), and 1,3,5-benzenetricarboxylic acid (TA), respectively. The steady shear viscosity, dynamic shear viscoelastic and zero-shear viscosity were researched by using rheometer; the macroscopic phase behavior were observed by bottle tests; the microscopic aggregates structures and hydrodynamic diameter were measured by Cryo-TEM and DLS; the chemical shifts of protons in aromatic acids at different pH were measured by 1H NMR to explore the inserted positions of BA, PA and TA; furthermore, the thickening mechanism was analysed by using the packing parameter theory and the changed chemical environment of aromatic acids. The results show that the different carboxyl number leads to different pH-responsive ranges. The viscosity of OABA and OAPA systems increases as the pH increases from about 1.99 to 8.99. However, the viscosity of OATA solutions changes in a narrow pH range of 2.01 - 4.60. The three additives show the different degrees of ionization and hydrophilicity due to the different numbers of carboxyl groups. Therefore, BA− and PA2− insert into the polar shell, but TA3− enter into the water and stay on the surface of micelles. And the Gemini-like and Trimer-like surfactants are built in the OAPA and OATA systems, respectively. As a result, they show the different thickening abilities in the order of OATA > OAPA > OABA. The solubility and hydrophilicity of three additives can be improved as the temperature increases, resulting in the BA− and PA2− transfer to the surface of micelles from the polar shell. Finally, the OABA and OAPA solutions exhibit that the viscosity increases as the temperature increases. Contrarily, TA3− will be separated from the surface of micelles as the temperature increases, resulting in the viscosity of OATA solutions always decreasing. This research provides deeper insights into the temperature- and pH-responsive mechanism of clean fracturing fluids and is beneficial in constructing the novel temperature-resistant fracturing fluids.