Influence of Particle Wettability on the Type and Stability of Surfactant-Free Emulsions

A systematic investigation into the influence of the wettability of spherical, nanometer-sized silica particles on the type and stability of water−toluene emulsions is described. The particles range from hydrophilic to hydrophobic depending on the extent of chemisorbed silane on their surfaces. We show that predictions based on considerations of the energy of attachment of a single particle to the oil−water interface relate directly to the stability of emulsions. A combination of powder immersion, shelf stability, light diffraction, and microscopy measurements is used to characterize the systems. Emulsions stabilized by either very hydrophilic or very hydrophobic particles are large (>100 μm) and unstable to coalescence. Those with particles of intermediate hydrophobicity are submicrometer and stable to coalescence indefinitely. For these, catastrophic inversion of emulsions occurs upon increasing the volume fraction of water, φw, and their stability to sedimentation or creaming increases approaching inversion. The stability of emulsions to gravity-induced separation passes through a sharp maximum upon increasing the particle hydrophobicity, alongside a minimum in the average drop diameter. This is universal and independent of φw and the type of emulsion formed. In one system, an otherwise very stable water-in-oil emulsion is destabilized in an ultracentrifugal field leading to enhanced sedimentation and eventual coalescence.