CO2-induced smart viscoelastic fluids based on mixtures of sodium erucate and triethylamine

Simple "green" triggers such as CO2 for switching viscoelastic surfactant solutions between an "on" and "off" state without compromising their inherent properties have been a key focus of research in smart soft matter over the past decade. Here we report a CO2-induced anionic wormlike micellar fluid by introducing triethylamine (TEA) into natural anionic surfactant sodium erucate (NaOEr) at a molar ratio (CNaOEr:CTEA) of 3:10. When CO2 is bubbled, pH of the solution decreases from 12.3 to 10.0, and TEA is protonated into a quaternary ammonium salt, which facilitates the growth of micelles as a hydrotrope by screening electrostatic repulsion between the anionic headgroups in NaOEr molecules, resulting in the formation of wormlike micelles and viscoelasticity buildup; upon removal of CO2, the quaternized TEA is deprotonated back into a non-ionic tertiary amine, thus electrostatic repulsion strengthens and the viscoelastic fluid converting it back to the initial low viscosity spherical micellar solution. Such a reversible sphere-to-worm transition could be repeated several cycles without any loss of response to CO2 in the pH range of 12.3-10.0, but when further bubbling excess CO2 to pH<9.50, the solution transforms into a cloudy dispersion with low viscosity due to the de-neutralization of NaOEr, and this process is irreversible. This study offers a facile way to fabricate smart viscoelastic fluids by incorporating a low-molecular weight CO2-sensitive hydrotrope with conventional long-chain surfactants.