Redox and Doubly pH-Switchable Pickering Emulsion.

In this work, a novel Pickering emulsion is stabilized by silica nanoparticles functioned with a redox and pH-responsive surfactant FA-DMDA-Ox that is prepared simply by direct neutralization of ferrocenecarboxylic acid (FA) and ,-dimethyldodecylamine (DMDA) and exhibits redox and doubly pH-switchable behavior. Here, the Pickering emulsion can be stabilized easily by combining hydrophilic silica nanoparticles with less than 0.1 wt % FA-DMDA-Ox. After adding NaSO and HO alternately, the demulsification and emulsification of this Pickering emulsion are controlled reversibly. Moreover, the emulsion is switched "off" upon the addition of HCl and switched "on" upon the addition of NaOH and is also switched off upon the addition of NaOH and switched on upon the addition of HCl, which demonstrate the doubly pH-switchable behavior. Based on the analysis of ζ-potential, contact angle, and adsorbed amount of silica nanoparticles, the pH and redox-switchable mechanism of the Pickering emulsion are analyzed. Here, the redox-switchable behavior is induced by the reversible adsorption and desorption of FA-DMDA-Ox on the surface of silica nanoparticles. The pH-switchable behavior is driven by the controllable dispersion systems of silica nanoparticles and FA-DMDA-Ox because of the doubly pH switchability of FA-DMDA-Ox. More importantly, upon adding fresh oil after removing the original oil, the Pickering emulsion is recycled three times. Hence, the multiswitchable Pickering emulsion can be expected to be treated as a multifunctional material in practical applications, such as oil or wax removal in the petroleum industry.