Thermally induced structural transitions from fluids to hydrogels with pH-switchable anionic wormlike micelles

Abstract

A novel pH-switchable anionic wormlike micellar system has been developed by simply changing pH value of natural erucic acid solutions without needing specialized organic synthesis or addition of hydrotropes. Through rheology, optical transmittance, and cryo-TEM observation, it was found that at 60°C, the 100mM erucic acid solutions transform from low viscous emulsion-like fluids to viscoelastic hydrogels when the pH is increased from 8.03 to 12.35, and when the pH is cycled between 9.02 and 12.35, the viscosity of the solutions varies correspondingly between 2 and 200,000mPas, five orders of magnitude in viscosity range. Such a reversible switch can be easily cycled more than four times without any deterioration. In addition, when the pH is fixed between 9.0 and 12.35, the solutions shift to highly elastic solid-like gels with decreasing temperature. These pH-switchable and temperature-sensitive rheological properties are attributed to the presence of carboxylic groups in the erucic acid molecules. With increasing pH, erucic acid is converted into an anionic surfactant, sodium erucate, which self-assembles into aggregates evolving from spherical micelles to wormlike micelles; when temperature is decreased, these disordered micellar structures become highly ordered under the effect of crystallization of hydrophobic erucate chains, forming opaque solid-like hydrogels.