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

Area of Science:

• Colloid and Surface Chemistry
• Materials Science
• Rheology

Background:

• Developing switchable materials is crucial for advanced applications.
• Natural surfactants offer sustainable alternatives to synthetic ones.
• Understanding self-assembly mechanisms is key to designing functional soft materials.

Purpose of the Study:

• To create a novel pH-switchable anionic wormlike micellar system.
• To investigate the rheological properties of erucic acid solutions under varying pH and temperature.
• To explore the self-assembly behavior of erucic acid into wormlike micelles and hydrogels.

Main Methods:

• Rheological measurements to determine viscosity and viscoelasticity.
• Optical transmittance to monitor phase transitions.
• Cryogenic transmission electron microscopy (cryo-TEM) for structural visualization.
• pH and temperature control experiments.

Main Results:

• Erucic acid solutions (100 mM) at 60 °C transformed from emulsion-like fluids to viscoelastic hydrogels upon increasing pH from 8.03 to 12.35.
• Viscosity reversibly switched over five orders of magnitude (2 to 200,000 mPas) by cycling pH between 9.02 and 12.35.
• Solutions formed solid-like gels at fixed pH (9.0-12.35) with decreasing temperature due to hydrophobic chain crystallization.

Conclusions:

• A simple, pH-switchable anionic wormlike micellar system was developed from natural erucic acid without complex synthesis.
• The pH- and temperature-dependent rheological behavior is attributed to the carboxylic groups of erucic acid, enabling tunable self-assembly.
• This system demonstrates potential for applications requiring reversible changes in viscosity and material state.