Phase Inversion of Ellipsoid-Stabilized Emulsions.

The efficacy of anisotropic particles in Pickering emulsion stabilization, attributed to shape-induced capillary interactions, is well-documented in the literature. In this contribution, we show that the surface of hematite ellipsoids can be modified in situ by the addition of oleic acid to effect transitional phase inversion of Pickering emulsions. Interestingly, incorporation of oleic acid results in the formation of nonspherical emulsion drops. The phase inversion of oil-in-water to water-in-oil and the transition in shape of emulsion drops from spherical to nonspherical is observed in two different particle systems, namely, nanoellipsoids and microellipsoids. The surface of spherical emulsion drops stabilized by particles or particles along with high concentration of oleic acid is found to consist of ellipsoids arranged in a close-packed configuration with their major axis parallel to the interface. In contrast, at intermediate oleic acid concentration, the surface of nonspherical emulsion drops is observed to be covered with loosely packed particle monolayer, with the ellipsoids at the oil/water interface taking up many different orientations. Using contact angle goniometry, the change in the wettability of hematite particles due to adsorption of oleic acid is established to be the mechanism responsible for the phase inversion of Pickering emulsions.