Stability, Photoinduced Destabilization, and Photoswitchability of Azo-CTAB-Based Emulsions

Area of Science:

• Colloid and Surface Science
• Materials Chemistry
• Physical Chemistry

Background:

• Stimuli-induced emulsion destabilization offers energy-efficient processing compared to bulk methods.
• Azobenzene-containing surfactants enable light-responsive interfacial properties.
• Highly stable emulsions (>45 days) are challenging to process conventionally.

Purpose of the Study:

• To demonstrate photoinduced destabilization and reconstruction of n-decane-in-water emulsions.
• To investigate the mechanism and kinetics of UV-induced demulsification.
• To explore the photoswitchability and control of emulsion stability using azobenzene surfactants.

Main Methods:

• Formulation of n-decane-in-water emulsions stabilized by azobenzene-containing cetyltrimethylammonium bromide (Azo-CTAB).
• Exposure to ultraviolet (UV) and visible light to induce and reverse phase separation.
• Kinetic analysis of droplet coalescence and phase separation.
• Systematic variation of UV light intensity, electrolyte concentration, and co-solvents.

Main Results:

• UV light illumination induced rapid phase separation via droplet coalescence, with a rate constant 5 orders of magnitude higher than ambient conditions.
• Demulsification efficiency was tunable by UV power, electrolyte addition (4-5x faster coalescence with NaCl), and volatile co-solvents.
• Reconstruction of the original emulsion was achieved by visible light exposure, demonstrating reversible photoswitchability.
• The reversible process was stable over 5-7 cycles without significant changes in droplet characteristics.

Conclusions:

• Azobenzene-containing surfactants provide a powerful tool for light-controlled emulsion destabilization and reconstruction.
• The photoswitchable nature of Azo-CTAB stabilized emulsions allows for facile and repeatable emulsion processing.
• This technology holds significant potential for applications requiring dynamic emulsion control, such as enhanced oil recovery and wastewater treatment.