Cellulose Nanocrystals Enhance the Rheological Properties and pH-Responsiveness of Potassium Oleate Solutions

Wormlike micelles (WLMs) of surfactants with rheological properties highly responsive to pH are of growing interest for various applications. The present paper proposes an approach to enhance their rheological properties and make the pH-response more pronounced. It consists of the incorporation of a percolated network of cellulose nanocrystals (CNCs) into the solution of entangled WLMs. To provide pH-responsiveness, potassium oleate was used as a surfactant. Rheological studies demonstrated that CNCs increase the viscosity and storage modulus by one order of magnitude. This effect was attributed to the interaction of WLMs with nanocrystals and the formation of entanglements of WLMs with percolated CNCs. Moreover, added CNCs make the pH-response stronger. The lowering of pH from 10.1 to 9.7 leads to a sharp drop in viscosity by ca. 2000 Pa·s, which is much higher than the decrease in viscosity of the WLM solution without CNCs. According to SANS data, the drop in viscosity is due to the transformation of WLMs into vesicles. It occurs as a result of the protonation of surfactant carboxylic groups decreasing surface charge on the micelles. In the presence of CNCs, the transition pH shifts to an alkaline medium, indicating that CNCs promote vesicle formation. Also, CNCs cause some of the vesicles to aggregate with each other, as follows from dynamic light scattering and optical microscopy data. Both observations suggest an interaction between CNCs and vesicles, which is supported by ITC data. These findings are valuable for the research and development of high-performing surfactant-based products.