pH-Induced Re-entrant Microstructural Transitions in Cationic Surfactant-Hydrotrope Mixtures

Area of Science:

• Physical Chemistry
• Colloid Science
• Materials Science

Background:

• Cationic surfactant-hydrotrope mixtures exhibit complex phase behavior.
• Understanding structural transitions is crucial for applications in formulation and materials science.

Purpose of the Study:

• To investigate the pH-dependent structural transitions of aqueous CTAB-NaSal mixtures.
• To elucidate the role of temperature on these microstructural changes.

Main Methods:

• Dynamic Light Scattering (DLS) for micelle size and shape analysis.
• Small-Angle Neutron Scattering (SANS) for detailed structural characterization.
• Systematic variation of pH and temperature.

Main Results:

• Observed a pH-induced transition from rigid cylindrical micelles (neutral pH) to spherical micelles (∼pH 2).
• An unexpected reversion to flexible cylindrical micelles occurred at lower pH values.
• These microstructural transitions were found to be highly temperature-sensitive.

Conclusions:

• The study reveals complex pH- and temperature-dependent structural transitions in CTAB-NaSal mixtures.
• Beyond electrostatic and hydrophobic interactions, cation-π and hydrogen bonding interactions likely govern these pH-induced changes.
• Findings contribute to the fundamental understanding of surfactant-hydrotrope systems.