Dynamic Covalent Nanoparticles for Acid-Responsive Nonaqueous Pickering Emulsions

Acid-responsive nonaqueous (glycerol in n-decane) Pickering emulsions were prepared using preferentially oil-wetted dynamic covalent silica (SiO2–pDB) nanoparticles as the Pickering emulsifiers. The acid-responsive Pickering emulsifier SiO2–pDB was prepared based on a Schiff base reaction between amino silica (SiO2–NH2) and p-decanoxybenzaldehyde (pDBA). The formation of SiO2–pDB was characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, and elemental analysis. The preferentially oil-wetted character of SiO2–pDB was indicated by contact angle measurement. Stable nonaqueous Pickering emulsions were prepared using preferentially oil-wetted SiO2–pDB as the Pickering emulsifier. However, after adjusting the nonaqueous Pickering emulsions to an acidic environment, complete phase separation occurred. In the acidic environment, preferentially oil-wetted SiO2–pDB decomposed into hydrophilic SiO2–NH2 and hydrophobic pDBA due to the decomposition of the dynamic imine bond in the SiO2–pDB. Then, the hydrophilic SiO2–NH2 and hydrophobic pDBA desorbed from the two-phase interface, resulting in complete phase separation of the initially stable nonaqueous Pickering emulsions. The acid-responsive nonaqueous Pickering emulsions show great potential in application in water sensitive systems, such as oil-based drilling fluids.