How do (fluorescent) surfactants affect particle-stabilized emulsions?

Previous investigations into particle-stabilized, or Pickering-Ramsden emulsions (panel(b)), have primarily focused on emulsions solely stabilized by colloidal particles. However, it has been reported that emulsions with both particulate and molecular emulsifiers can display superior shelf life compared to those stabilized by particles or surfactants alone. Three mechanisms have been proposed to explain this synergy. Firstly, particles may facilitate surfactant adsorption at the liquid-liquid interface, which reduces interfacial tension. Secondly, surfactant adsorption may affect the particles' contact angle, thereby inhibiting particle desorption from the liquid-liquid interface. Finally, surfactants can promote particle flocculation at the liquid-liquid interface, which is likely to enhance interfacial viscosity. Though plausible, these hypotheses have not yet been backed by direct observations.

Here, we consider synergistic effects between particulate and molecular emulsifiers in water-in-oil emulsions using a combination of confocal fluorescence microscopy, macroscopic observations and contact-angle measurements. Micron-sized PMMA spheres were labeled with the fluorescent dye NBD , which had been covalently linked to the PMMA during particle synthesis. The water phase was labeled by dissolving a fluorescent dye with surfactant-like properties, i.e. one promoting foam formation in water. Our results confirm that the formation of particle-stabilized emulsions can strongly be enhanced through the addition of molecular surfactants, with surfactant adsorption onto the liquid-liquid interface and onto the particles playing a crucial role.