Synthesis of bilayer oleic acid-coated Fe3O4 nanoparticles and their application in pH-responsive Pickering emulsions.

Fe(3)O(4) nanoparticles coated with oleic acid bilayer (a diameter about 12 nm) were synthesized. The structure and composition of the particles were analyzed by TEM, FTIR and TGA. The TGA experiments of the bilayer-coated particles show a distinct two-stage mass loss. Partition experiments show that the modified Fe(3)O(4) nanoparticles are affected by aqueous dispersion pH and ion strength. Accordingly, the Pickering emulsions stabilized by modified Fe(3)O(4) particles are also sensitive to pH and ion strength. The phase inversion of the emulsions occurs when 1.00<pH<12.05, and no phase inversion occurs after complete desorption of the second layer surfactants when pH>13.50. The phase inversion of emulsions also can be adjusted by the ion strength. In interfacial adsorption experiments, the hydrophobic Fe(3)O(4) nanoparticles form particle clusters, while the hydrophilic particles form uniform multilayers.