Pickering emulsions synergistically stabilized by oligomeric amines and silica nanoparticles

Pickering emulsions have been widely applied in various fields. In this work, oligomeric amines (2N-C n -2N, n = 6, 8, 10) with two secondary and two tertiary amine groups were successfully synthesized. Pickering emulsions were stabilized by using silica nanoparticles and oligomeric amines with an oil-water ratio of 1:1 (v/v). At a fixed silica particle concentration of 0.2 wt.%, the minimum concentrations of oligomeric amines 2N-C n -2N (n = 6, 8, 10) required to stabilize the emulsion are 0.3, 0.3, and 0.06 mM, respectively, which decrease significantly with increasing spacer chain length. Contact angle experiments revealed that the in situ hydrophobization of nanoparticles was caused by the surface adsorption of amine molecules. Different with the previously reported silica particles modified by oppositely charged surfactants, the major interactions between the oligomeric amines and silica nanoparticles are hydrogen bonds instead of electrostatic attractions. The Pickering emulsion remained stable at 80 °C for at least 72 hours without phase separation. In addition, the system exhibits pH responsiveness, and the emulsion can be cycled between demulsification and emulsification state for over 5 times by alternating addition of HCl and NaOH. This work provides a novel approach for the in situ hydrophobization of nanoparticles through hydrogen bonding and can be widely applied in the fields of medicine and cosmetics. Pickering emulsions can be well stabilized by low concentrations of silica nanoparticles together with newly synthesized oligomeric amines through hydrogen bonds.