Herein, we describe a novel CO2-switchable oil-in-water Pickering emulsion stabilized by functionalized silica nanoparticles with a trace amount of myristylamidopropyl amine oxide (C14PAO), which is commercially available, and readily biodegradable. C14PAO in the current system has been demonstrated to be CO2-responsive. Upon alternately bubbling CO2 and N2 under mild conditions (30 °C, 40 mL min–1), C14PAO is reversibly switched between cationic and nonionic forms, and is thereby adsorbed on or desorbed from the surface of the particles. In this way, interfacially active particles are formed and adsorbed on the surface of oil droplets, stabilizing the emulsion (CO2), or disrupted and desorbed from the surface of oil droplets, breaking the emulsion (N2). As compared to the traditional acid/base cycle, switching the current system with CO2/N2 multiple times does not lead to any evident changes in either macroscopic appearance or microscopic size. Moreover, this CO2-responsive Pickering emulsifier can be recycled when fresh oil was added after removing the original oil, and theoretically the cycling can be maintained, conforming to the principle of green and energy-saving processing. It offers a green, efficient, and recyclable container for oil product transportation, especially in high temperature area. Such a strategy is also suitable for other amine oxide-based surfactants, and does not require complicated organic synthesis.

Herein, we described for the first time a CO and redox dual responsive paraffin oil-in-water Pickering emulsion stabilized by the modified silica nanoparticles with Se-containing tertiary amine, SeTA, in which the tertiary amine serves as a CO-sensitive group, and the Se atom serves as a redox-sensitive center. The Pickering emulsion can be reversibly switched between stable and unstable states by bubbling CO and N in the reduced state, or with the addition of HO and NaSO in the absence of CO, because of the adsorption and desorption of SeTA on the silica surface. The former is mainly attributed to a CO-controllable electrostatic attraction, resulting from the transition of molecules between cationic and nonionic states; whereas, the latter is ascribed to a redox-tunable hydrogen bonding, originating from the transition of molecules between selenide and selenoxide. However, in the presence of CO, redox can only induce a change in the droplet size, not demulsification. These interesting and unique multiresponsive behaviors endow the Pickering emulsion with a capacity for intelligent control of emulsification and demulsification, as well as the droplet size, which may be an asset for a myriad of technological applications in biomedicine, microfluidics, drug delivery, and cosmetics.

No abstract

Particle-stabilized emulsions that can respond to external stimuli have attracted significant concerns due to their intelligent-controlled stability, whereas particle-stabilized Pickering emulsions responding to multistimuli but based on biomass have been rarely reported. Here, a multistimuli-responsive Pickering emulsion was developed using the modified chitosan as stabilizer. Due to electrostatic attraction, Se-containing anionic surfactant, sodium 11-(butylselenyl)­undecylsulfate (C4SeC11S), can bind with CS at an acidic pH and form CS–C4SeC11S complexes which can further self-associate to form micrometer-sized particles with the character of partially hydrophobicity. Therefore, at pH < pK a, an oil-in-water Pickering emulsion can be formed using CS–C4SeC11S particles as stabilizers and can spontaneously respond to redox, ion, and pH. First, with the addition of oxidation, the hydrophilicity of C4SeC11S was enhanced, and thus, hydrophobic association of CS–C4SeC11S decreased, leading to the disruption of CS–C4SeC11S particles. Hence, the emulsion destabilized. The demulsification process is closely related with the dosage of oxidant and the oxidation time. Second, introduction of a competitive ion (e.g., CTAB) could break the binding between C4SeC11S and CS, leading to the disruption of particle emulsifier. Thereby, demulsification occurred. Third, with sequentially increasing/decreasing pH, the emulsion can be switched from stable to unstable and then to stable again accordingly. Such a unique pH-responsive behavior has never been discovered in other pH-responsive Pickering emulsions. All of the stimuli-responsive behaviors were reversible. Upon alternately adding oxidant/reductant, CTAB/C4SeC11S, or base/acid, the current emulsion can be reversibly switched off (destabilization) and on (stabilization). Such a Pickering emulsion may be a good candidate as a vehicle of functional ingredient.

A “release and catch” method was developed by utilising the scavenging effect of a fluorous zwitterion on a homogeneous triflic acid (TfOH) catalyst in Michael addition and Rupe rearrangement. Both TfOH and the zwitterion were recycled with >90 % recovery using toluene. The zwitterions were designed by functionalising imidazole/pyridine with the perfluoroalkylsulfonylimide group. The “caught” TfOH was delivered to ethyl acetate and re‐used. The smooth delivery was primarily because of the fluorous tail of the zwitterion, the hydrophobicity of which probably weakened the ability of the zwitterion to form H bonds, so that retro‐ion‐exchange occurred towards the formation of the acid and zwitterion. The method was universal for other strong Brønsted acids such as H2SO4 and p‐MeC6H4SO3H. The method combined the significant advantages of homogeneous catalysis and heterogeneous isolation. Based on the H0 acidity function and the 31P NMR chemical shift of Et3P=O adducts, it is reasonable to deduce that the decrease of the acid strength of the formed composites of Brønsted acids and the zwitterion drove the scavenging effect.