Fabrication of Schiff Base Interfacial Films via Polyethyleneimine and Glutaraldehyde for pH-Responsive Oil-in-Water High Internal Phase Emulsions

Macroemulsions are thermodynamically unstable and prone to phase separation. Traditional stabilization methods using surfactants to reduce interfacial tension are limited due to the physical barrier's instability—thermal motion can cause surfactant desorption, leading to interface collapse and phase separation. Strengthening this physical barrier is therefore crucial for extending the lifespan of macroemulsions. This manuscript presents an innovative stabilization method for macroemulsions, utilizing homogeneous phase Schiff base polymerization at the oil-water interface. The Schiff base reaction between polyethyleneimine (PEI), which accumulates at the interface, and glutaraldehyde (GA) to form a robust interfacial film is exploited. This simple method prevents the phase separation of oil-in-water high internal phase emulsions (HIPEs) for over 110 days. More importantly, the acid degradation of the interfacial film, together with the protonation state of polyethyleneimine under varying pH conditions, offers a stability programmable emulsion system that leads to pH-responsive emulsions, HIPEs, and capsules.