General destabilization mechanism of pH-responsive Pickering emulsions.

The superior stability of Pickering emulsions can be detrimental in several applications. There is therefore growing interest in using stimuli responsive particle emulsifiers that can considerably reduce the cost of demulsification. Among the possible triggers for the remote control of emulsion stability, pH sensitive systems are advantageous because of the simplicity and ease of implementation. Herein, the destabilization mechanism of pH-switchable emulsions stabilized solely by particles of different shapes and surface properties - namely - hematite, silica and polystyrene - are discussed. The surface activity of all these particles can be tuned by controlling the surface charge density via dispersing them in aqueous solutions of different pH. Hematite particles of cuboidal, spherocylindrical, peanut and ellipsoidal shapes stabilized O/W emulsions at pH 6.5, but they were completely destabilized when the pH of the continuous phase was adjusted to either 2 or 12. Similarly, the O/W emulsions stabilized by silica rods and spheres at pH 2 were destabilized upon adjusting the pH to 6.5 and 12. The detachment of particles from the droplet surfaces resulted in partially covered drops that coalesced, leading to the destabilization of emulsions as confirmed by direct visualization via optical microscopy. The destabilization mechanism of the Pickering emulsions stabilized solely by pH responsive particles was observed to be general, and was mediated by the detachment of particles because of the pH induced wettability change. Furthermore, the pH responsive behaviour was found to be reversible. Since stable emulsions are formed in several particle-water-oil systems, the possibility of conveniently changing the pH of the continuous phase can be exploited to tune the particle wettability and thus the destabilization in applications that demand controlled demulsification.