Emulsions stabilized with mixed SiO₂ and Fe₃O₄ nanoparticles: mechanisms of stabilization and long-term stability

The stabilization of oil-in-water emulsions with SiO2 and Fe3O4 nanoparticles was investigated. The mechanism of emulsion stabilization was altered by changing the mass ratio of negatively charged Ludox HS-30 to positively charged Ludox CL nanoparticles in emulsions. At a mass ratio of nanoparticles between 0.3 : 1.0 and 1 : 1, small nanoparticle heteroaggregates adsorbed on the surface of the oil droplets, giving them resistance to coalescence. The interface film rigidity was enough to prevent droplet deformation. After creaming, the oil-phase fraction in the residual emulsion increased up to dense packing, but did not exceed 0.74-0.78 for a long time. At the mass ratio of (1.5-2.5) : 1.0, the emulsions were kinetically stable to creaming and coalescence due to the formation of a three-dimensional network of nanoparticle heteroaggregates in the aqueous phase. At a mass ratio of Ludox HS-30 to Ludox CL nanoparticles above 3 : 1, large heteroaggregates were formed in the aqueous phase and the rapid separation of the oil and water phases from the emulsions occurred. Such a change in the stability of emulsions was due to a significant alteration of the value of the net ζ-potential of the heteroaggregates. In the case of stabilization with Ludox CL and Fe3O4 nanoparticles, emulsions with nanoparticle networks in the aqueous phase were obtained at pH 8, and these emulsions were stable against creaming and coalescence. Emulsions resistant to coalescence were produced in the systems with Ludox CL and Fe3O4 nanoparticles at pH 4-6 and in systems with Ludox HS-30 and Fe3O4 nanoparticles at pH 2-6.