Dynamic Covalent Silica Nanoparticles for pH-Switchable Pickering Emulsions.

Dynamic covalent surfactants have been recently reported for preparation of pH-switchable emulsions [ Sun , D. Langmuir , 2017 , 33 , 3040 ]. In this study, dynamic covalent silica (SiO2-B) nanoparticles of switchable wettability were fabricated by a pH-responsive dynamic (covalent) imine bond between hydrophilic amino silica (SiO2-NH2) nanoparticles and hydrophobic benzaldehyde molecules. The properties of SiO2-B were characterized by Fourier transform infrared spectroscopy, elemental analysis, contact angle measurement, and ζ potential measurement. The hydrophilicity and hydrophobicity of SiO2-B were shown to be readily switchable by adjusting pH between 7.8 and 3.5. At pH 7.8, SiO2-B was partially hydrophobic and adsorbed at oil-water interface to stabilize O/W Pickering emulsions, which were characterized by electrical conductivity, optical microscopy, and confocal laser scanning microscopy. Upon lowering the pH to 3.5, the dynamic covalent bond is dissociated to convert partially hydrophobic SiO2-B into highly hydrophilic SiO2-NH2 and surface-inactive benzaldehyde. Both of them desorb from oil-water interface, resulting in a rapid oil-water separation of the Pickering emulsions. Alternating stabilization and phase separation of the Pickering emulsions over 3 cycles were demonstrated by adjusting the pH. The pH-switchable Pickering emulsions show great potential in application to effective oil-water separation of emulsions.