Highly Efficient and Reversible Inversion of a Pickering Emulsion Triggered by CO2/N2 at Ambient Conditions

Emulsion inversion has great potential in applications such as materials science, chemical reactions, and drug delivery. Therefore, developing a simple and green approach to control emulsion inversion is highly desirable. Herein an octyl- and bis(2-hydroxyethyl)-3-amino-bifunctionalized silica microsphere (SM-O-BIS) is designed, prepared, and used to fabricate a Pickering emulsion. It is found for the first time that this Pickering emulsion can be easily and reversibly inverted from water-in-oil (w/o) to oil-in-water (o/w) by alternate bubbling of CO2 and N2 at room temperature and atmospheric pressure. The molar ratio of trimethoxyoctylsilane to bis(2-hydroxyethyl)-3-aminopropyltriethoxysilane on the surface of silica is important for the emulsion inversion. This unique inversion can be recycled multiple times without any deterioration. By utilizing the emulsion inversion strategy, encapsulation and release of curcumin molecules have been actualized on demand. The possible mechanism of emulsion inversion is also investigated by measuring zeta potential, water contact angle, 13C NMR spectroscopy, and FT-IR spectroscopy. It is shown that a significant and reversible change in hydrophilicity/hydrophobicity of SM-O-BIS is the driving force for such a CO2/N2-triggered inversion.