Temperature- and pH-Responsive Pickering Emulsions Stabilized by Chitosan−Metal Nanogels for Esterification Catalysis

Abstract

In recent years, Pickering emulsions have been successfully applied in interfacial reaction systems due to their unique interfacial stability. The effective separation of products and the efficient recovery of catalysts and emulsions are key considerations in the design and development of Pickering emulsion systems. Here, we synthesized chitosan-based amphiphilic microgels whose functional groups can bind to metals through coordination. The poly(N-isopropyl acrylamide) (PNIPAM) contains a temperature-sensitive group, which could show hydrophilicity at a low temperature and display hydrophobicity at a high temperature. Meanwhile, its ligands (such as carboxyl groups) can respond to pH changes through protonation and deprotonation: at low pH, protonation increases hydrophilicity and surface charge, while at high pH, deprotonation reduces the charge and increases hydrophobicity, thereby altering the surface activity of the particles and enabling pH-controlled emulsification. In ethyl acetate catalytic application, the reactants (ethanol and acetic acid) are dissolved in the aqueous phase, while the product, ethyl acetate, is dissolved in the organic phase (toluene). Porcine pancreatic lipase (PPL) supported on a microgel catalyzes the esterification reaction, while co-loaded Zn²⁺ stabilizes the enzyme’s conformation. The synergistic action of these two components enables the efficient synthesis of ethyl acetate. Finally, the reactants and products could separate with the separation of water and toluene, and the catalysts could be recycled. In conclusion, these catalysts provide a method for Pickering emulsions and ester catalysis.