Behavior of Smart Surfactants in Stabilizing pH-Responsive Emulsions.

Newly structured pH-responsive smart surfactants (N+ -(n)-N, n=14, 16) from alkyl trimethylammonium bromides are reported. In neutral and alkaline media N+ -(n)-N behaves as a normal cationic surfactant and stabilizes conventional emulsions alone, as well as Pickering emulsions and oil-in-dispersion emulsions together with oppositely and similarly charged nanoparticles, respectively. In acidic media N+ -(n)-N becomes a hydrophilic Bola-type surfactant, N+ -(n)-NH+ , and is an inferior emulsifier either when used alone or together with charged nanoparticles, resulting in demulsification. N+ -(n)-NH+ returns to the aqueous phase alone or together with nanoparticles after demulsification without contaminating the oil phase, and the aqueous phase can be recycled when triggered by pH change. This protocol is a green process and leads to preparation of various temporarily stable emulsions which are often used in emulsion polymerization, heterogeneous catalysis, and oil transportation.