Stimulus-responsive particulate emulsifiers based on lightly cross-linked poly(4-vinylpyridine)-silica nanocomposite microgels

Lightly cross-linked poly(4-vinylpyridine)-silica nanocomposite microgel particles have been recently reported to act as pH-responsive particulate emulsifiers [Fujii, S.; Read, E. S.; Armes, S. P.; Binks, B. P. AdV. Mater. 2005, 17, 1014]. In this work, the synthesis and performance of such nanocomposite microgel particles are studied in more detail. Scanning electron microscopy, dynamic light scattering, nitrogen microanalyses, thermogravimetric analysis, aqueous electrophoresis, and acid-base titration were used to characterize the nanocomposites in terms of their particle size and morphology, polymer and silica contents, surface compositions, and critical swelling pH, respectively. Depending on the polarity of the oil phase and the purity of the nanocomposite particles, either oil-in-water or water-in-oil emulsions could be prepared at pH 8-9, but not at pH 2-3. These emulsions were characterized in terms of their emulsion type, mean droplet diameter, and morphology using electrical conductivity, light diffraction, and both electron and optical microscopy. In some cases, rapid demulsification could be induced by lowering the solution pH: addition of acid led to protonation of the 4-vinylpyridine residues, which imparted cationic microgel character to the nanocomposite particles. Cross-linking of the nanocomposite microgel particles is essential for their optimum performance as a pH-responsive emulsifier, but unfortunately it is not sufficient to allow recycling.