Effects of Added Surfactant On Particle Stabilized Emulsion Droplets

It is well known that colloidal particles can strongly adsorb to an oil-water interface and stabilize emulsion droplets. The properties of any particle stabilized liquid-liquid interface can be further tuned with surfactant additives, and the effects of added surfactant can be surprisingly varied and complex. Surfactant molecules can adsorb to the liquid interface, thereby lowering the interfacial tension and the adsorption strength of the particles. Alternatively, the surfactant can adsorb to the particle surface and enhance or lower the particles' stabilizing effect by altering their wetting behavior. Depending on its composition and architecture, the surfactant can also induce and electric surface charge at the particle-oil interface; and reversed surfactant micelles in the bulk oil phase can provide electrostatic screening of such particle surface charge. We report experimental evidence for each of these scenarios, and discuss the delicate interplay of competing effects when surfactants meet a particle-stabilized emulsion droplet.