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.