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.