Recyclable surfactant containing a dynamic covalent bond and relevant smart emulsions

Abstract

A novel pH-responsive Bola type surfactant FA–AA was constructed by formation of the CN dynamic covalent bond using 4-formylbenzoic acid (FA) and 12-aminolauric acid (AA). In acidic aqueous media (pH < 4.1) the CN bond is broken and the H⁺AA released behaves as a cationic surfactant which can stabilize conventional emulsions alone and co-stabilize Pickering emulsions and oil-in-dispersion emulsions (oil-in-water with n-decane) with negatively charged silica nanoparticles and positively charged alumina nanoparticles, respectively. In alkaline aqueous media (pH > 10) FA–AA behaves as an inferior emulsifier resulting in demulsification of all three types of emulsion. Smart Pickering emulsions and oil-in-dispersion emulsions triggered by pH can then be obtained with low concentrations of FA–AA and nanoparticles. Due to the strong hydrophilicity, FA–AA together with the charged nanoparticles return to the aqueous phase after demulsification without contaminating the oil phase, and the whole aqueous phase is recyclable and re-usable many times. This novel protocol of constructing a recyclable and re-usable surfactant by means of a dynamic covalent bond avoids complicated synthesis, which is in good accordance with green chemistry. The smart surfactant may have wide applications in multi-phase catalysis, oil product transport and synthesis of new materials.