Influence of pH and Salt Concentration on Pickering Emulsions Stabilized by Colloidal Peanuts

Solid-stabilized emulsions commonly known as Pickering emulsions offer unique benefits such as superior stability and controlled permeability compared to conventional surfactant stabilized emulsions. In this article, the effect of pH, the electrolyte and particle concentration, homogenization speed, and volume fraction of oil on the formation, stability, and the microstructure of emulsion droplets stabilized by micron-size peanut-shaped hematite particles are investigated. The influence of surface charge of particles on emulsification is studied by varying the pH of the dispersing medium, the addition of an electrolyte or a combination of both. Stable O/W emulsions are formed only when the aqueous dispersions at intermediate pH between 4 and 11, and decane (2:1 volume ratio) are vigorously mixed. However, emulsions are not formed when the particles are highly charged that is, at pH 2 and 12. The presence of monovalent salt or high-speed homogenization assists the emulsion formation at pH 3, whereas their combination helps in emulsification at pH 2. However, neither the addition of an electrolyte nor the high-speed homogenization or their combination facilitates the formation of emulsions at pH 12. We show that the image-charge repulsion and the surface charge induced wettability change can explain the influence of both pH and salt concentrations on the formation of Pickering emulsions. Although oil-in-water emulsions typically cream because of the density difference, microscopy observations revealed the presence of a large number of small particle-covered oil droplets in the sediments of the emulsified samples. These drops are observed to be entrapped in dense-particle networks. This leads to a considerable reduction in the number of particles available for the stabilization of floating emulsion droplets and thus influences their size and surface coverage. The possibility of tailoring the stability, droplet size and, the surface coverage discussed in this article can play a crucial role in situations that demand controlled release of active components.