CO2-induced smart viscoelastic fluids based on mixtures of sodium erucate and triethylamine
2015
Zhang, Yongmin | Feng, Yujun
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.
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