Understanding viscosity reduction of a long-tail sulfobetaine viscoelastic surfactant by organic compounds

Though the transition from cylindrical micelles to spherical micelles of the anionic surfactant potassium oleate in the presence of oils has been studied, these changes have not been studied for long-tail zwitterionic surfactants. The effects of n-decane, crude oil (CO), extra virgin olive oil (EVOO) and polyglycolic acid (PGA) on the zero-shear viscosity of an aqueous solution of a sulfobetaine surfactant system were investigated at 30 degrees C and 60 degrees C. The main surfactant in the system was erucamidopropyl hydroxypropyl sulfobetaine. The methods employed were rheology and cryo-TEM. The solution with 3.96 wt% surfactant system and 6.2 wt% CaCl2 was viscoelastic at both test temperatures due to the formation of entangled cylindrical micelle networks. n-Decane induced the following regimes of zeroshear viscosity change at both temperatures: (i) the high viscosity regime (HVR), (ii) the transition regime (TR), and (iii) the low viscosity regime (LVR). The HVR was characterized by high zero-shear viscosities. The TR was characterized by a sharp drop in zero-shear viscosity due to the formation of untangled micelles. The LVR was due to the formation of microemulsions. The formation of these regimes depended on the balance between micellization and oil solubilization. We reveal for the first time that the number of regimes depends on the type of oil: both CO and EVOO induced only one and two regimes at 30 degrees C and 60 degrees C, respectively. PGA did not significantly affect the solution at either temperature with increasing concentration, meaning the solution was resistant to decreasing pH even at higher temperatures.