Formation and stabilization of Pickering emulsions using salt-sensitive core–shell cationic nanoparticles

Pickering emulsions known for their solid emulsifiers and brilliant stability characters have attracted many researchers’ attention. The controlled stability and demulsification of emulsion are necessary in some cases such as crude oil extraction and drug release. Stimuli-responsive Pickering emulsion could provide suitable controllability and emerged in the last decade. Among various controllable factors, salt ion is known as a critical parameter, but it is rarely investigated. Here, core–shell cationic nanoparticles with a poly-(2-aminoethyl methacrylate hydrochloride) shell and a polystyrene core were used in the preparation of Pickering emulsion. The size and morphology of nanoparticles were monitored by transmission electron microscopy and dynamic light scattering. The microstructure and stability of the formed Pickering emulsion were studied via dynamic light scattering and a polarizing optical microscope under various salt ion types and concentrations. The effect of salt types (Cl⁻, ClO₄⁻, and PO₄³⁻) and salt concentrations on the Pickering emulsion was investigated. Cl⁻, ClO₄⁻, and PO₄³⁻ are in situ generated from NaCl, NaClO₄, and (NaPO₃)₆, respectively. It showed that PO₄³⁻ (100–1000 mM) was unable to form stable Pickering emulsion, while Cl⁻ and ClO₄⁻ could induce stable Pickering emulsions under optimized conditions. Furthermore, after increasing the salt concentration over a critical salt concentration, the Pickering emulsion underwent rapid demulsification. This work revealed the effects of salt on size, conformation, charge, wettability, interaction, and adsorption state of nanoparticles and proposed the stability mechanisms of the Pickering emulsion. This opened up more potential applications in the field of controlled demulsification, petroleum recovery, catalyst recovery, and so on triggered by salt ions.