Color changing Pickering emulsions stabilized by polysiloxane microspheres bearing phenolphthalein groups

Abstract

The benzoxazine monomer bearing phenolphthalein and alkoxysilane groups was synthesized using phenolphthalein, 3-aminopropyltriethoxysilane, and paraformaldehyde as raw materials via a Mannich reaction. After hydrolysis and condensation of the alkoxysilane groups, the novel polysiloxane microsphere bearing phenolphthalein groups (PMP) was synthesized. Furthermore, a toluene-in-water Pickering emulsion using PMP as a particulate emulsifier was prepared. The existence of phenolphthalein and oxazine structures in PMP were confirmed by Fourier transform infrared (FTIR) spectroscopy. The average diameter of PMP was 208 nm as indicated by dynamic light scattering (DLS), and the PMP shape was spherical as revealed by transmission electron microscopy (TEM). Due to the introduced phenolphthalein structure, the stable toluene-in-water Pickering emulsion presented a color changing behavior within the pH range from 9 to 12. It also exhibited doubly pH-responsive properties: two emulsification/demulsification processes occurred at pH 9 and 12, respectively. Moreover, these processes were verified to be reversible: five and three demulsification/emulsification cycles were repeated with the aid of homogenization.