Rheological behavior and mechanism of pH-responsive wormlike micelles based on Sodium cocoyl glycinate / cocamidopropyl hydroxypropyl sulfobetaine

Wormlike micelles (WLMs) were constructed by anionic surfactant Sodium cocoyl glycinate (SCG) and zwitterionic surfactant cocamidopropyl hydroxypropyl sulfobetaine (CHSB). The rheological behavior of the system was tested and its pH response mechanism was revealed. Results indicated that WLMs formation occurred at mass ratios of 2:8 and 3:7 in 20 wt% SCG/CHSB mixed solutions (pH=7), with rheological behavior consistent with the Maxwell model. It is worth noting that when the mass ratio of WLMs was 3:7, the mesh structure showed the highest degree of entanglement, and the maximum zero-shear viscosity was observed. The influence of pH on the rheological behavior of mixed solution was due to the electrostatic interaction between SCG and CHSB. Specifically, at higher pH, SCG mainly existed in the form of ion (R-COO-), thereby enhancing the effective area of the surfactant head group, making the packing parameter (P) in the range of 1/2 similar to 1/3 and promoting the formation of WLMs. However, at pH= 12, excessive negative charge led to the formation of a counter ion layer, resulting in a less closed micelles arrangement and hindered electrostatic attraction between SCG and CHSB.