PH and redox dual-responsive Pickering emulsion based on silica nanoparticles and novel ferrocene surfactant

Abstract

Responsive Pickering emulsions capable of responding to external stimuli have been extensively studied, however, there are few studies on double-responsive Pickering emulsions. Here in, a novel ferrocene surfactant (FcACUa) was synthesized and combined with aminated silica nanoparticles (0.5 wt%) to stabilize pH and redox sensitive smart Pickering emulsions. The carboxyl groups act as pH-sensitive parts, while ferrocene serves as the redox-responsive center. Stability and pH/redox response performance of the emulsions were investigated through various methods, such as the contact angle, macroscopic states, creaming index, microscopic images, droplet size distributions, etc. All atom molecular dynamics and dissipative particle dynamics simulations were used to probe the distribution of nanoparticles and surfactant at the water-oil interface, as well as to reveal the stabilization and pH/redox response mechanisms of the Pickering emulsions. Pickering emulsions can be reversibly switched between stable and unstable by adding HCl and NaOH or adding H2O2 and N2H4·H2O. After five pH-induced cycles, the emulsion shows no significant degradation or morphological changes. However, for the redox induced emulsification demulsification process, the size of the emulsion drops increases with the increase of the number of cycles. This work provides insights into the Pickering emulsion stabilization mechanism and the regulation mechanisms of pH and redox stimulation, which may have potential value in emulsion polymerization, biphasic catalysis, and oil and gas transportation.