Recyclable and re-usable smart surfactant for stabilization of various multi-responsive emulsions alone or with nanoparticles.

A novel multi-responsive surfactant (abbreviated as N + -8P8-N) was synthesized, in which one octyl trimethylamine group (quaternary ammonium) and one octyl dimethylamine group are connected to a benzene ring through ether bonds. This novel surfactant can stabilize conventional oil-in-water (O/W) emulsions alone, and O/W Pickering emulsions and novel oil-in-dispersion emulsions together with oppositely and similarly charged nanoparticles, respectively. In all cases rapid demulsification can be achieved through either pH or CO 2 /N 2 triggers, by which the surfactant is reversibly converted between a normal cationic surfactant form (N + -8P8-N) and a strongly hydrophilic and surface-inactive bola form (N + -8P8-NH + ). Notably, the bola form N + -8P8-NH + dissolves in the aqueous phase alone or together with nanoparticles after demulsification without contamination of the oil phase, and the aqueous phase can be recycled many times triggered by pH or CO 2 /N 2 in accordance with the principle of green chemistry. This newly designed re-usable smart surfactant is significant for the development of various temporarily stable emulsions, which are extensively applied in emulsion polymerization, new material synthesis, heterogeneous catalysis and oil transportation.