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A ferrocene surfactant can be switched between single and double head form (FcN+C12/Fc+N+C12) triggered by redox reaction. FcN+C12 can neither stabilize an O/W emulsion alone nor an oil‐in‐dispersion emulsion in combination with alumina nanoparticles due to the steric hindrance of the ferrocene group. However, such steric hindrance can be overcome by increasing the charge density in Fc+N+C12, so that oil‐in‐dispersion emulsions can be co‐stabilized by Fc+N+C12 and alumina nanoparticles at very low concentrations (1×10−7 M (≈50 ppb) and 0.001 wt %, respectively). Not only can reversible formation/destabilization of oil‐in‐dispersion emulsions be achieved by redox reaction, but also reversible transformation between oil‐in‐dispersion emulsions and Pickering emulsions can be obtained through reversing the charge of alumina particles by adjusting the pH. The results provide a new protocol for the design of surfactants for stabilization of smart oil‐in‐dispersion emulsions. [ABSTRACT FROM AUTHOR]

A novel charge‐reversible surfactant, (CH3)2N‐(CH2)10COONa, was designed and synthesized, which together with silica nanoparticles can stabilize a smart n‐octane‐in‐water emulsion responsive to pH. At high pH (9.3) the surfactant is anionic carboxylate, which together with the negatively charged silica nanoparticles co‐stabilize flowable oil‐in‐dispersion emulsions, whereas at low pH (4.1) it is turned to cationic form by forming amine salt which can hydrophobize in situ the negatively charged silica nanoparticles to stabilize viscous oil‐in‐water (O/W) Pickering emulsions. At neutral pH (7.5), however, this surfactant is converted to zwitterionic form, which only weakly hydrophobises the silica particles to stabilize O/W Pickering emulsions of large droplet size. Moreover, demulsification can be achieved rapidly triggered by pH. With this strategy particles can be controlled either dispersed in water or adsorbed at the oil‐water interface endowing emulsions with the capacity for intelligent and precise control of stability as well as viscosity and droplet size. [ABSTRACT FROM AUTHOR]

Separation of surfactants from oil or water is essential for many research and industrial applications, such as in emulsions for biphasic catalysis and oil pipeline transportation. However, it is difficult to efficiently remove or recycle surfactants from the above systems due to their amphiphilic nature. Here, we report on redox-responsive Pickering emulsions stabilized by alumina nanoparticles and a selenium-containing surfactant at concentrations as low as 0.1 wt% and 1 × 10−6 M (≈0.001 cmc), respectively. Rapid demulsification can be achieved after the surfactant is converted from bola-type to trimeric form by a redox reaction, and the oil phase can be separated with minimal surfactant contamination (<0.005 ppb). In addition, the alumina nanoparticles together with the surfactant return to the aqueous phase after demulsification and are fully recoverable and re-usable. [ABSTRACT FROM AUTHOR]