Interactions of hydrophilic silica nanoparticles and classical surfactants at non-polar oil–water interface

The interfacial and bulk properties of submicron oil-in-water emulsions simultaneously stabilised with a conventional surfactant (either lecithin or oleylamine) and hydrophilic silica nanoparticles (Aerosil®380) were investigated and compared with emulsions stabilised by either stabiliser. Emulsions solely stabilised with lecithin or oleylamine showed poor physical stability, i.e., sedimentation and the release of pure oil was observed within 3 months storage. The formation and long-term stability of silica nanoparticle-coated emulsions was investigated as a function of the surfactant type, charge, and concentration; the oil phase polarity (Miglyol®812 versus liquid paraffin); and loading phase of nanoparticles, either oil or water. Highly stable emulsions with long-term resistance to coalescence and creaming were formulated even at low lecithin concentrations in the presence of optimum levels of silica nanoparticles. The attachment energy of silica nanoparticles at the non-polar oil–water interface in the presence of lecithin was significantly higher compared to oleylamine in line with good long-term stability of the former compared to the sedimentation and release of oil in the latter. The attachment energy of silica nanoparticles at the polar oil–water interface especially in the presence of oleylamine was up to five-times higher compared to the non-polar liquid paraffin. The interfacial layer structure of nanoparticles (close-packed layer of particle aggregates or scattered particle flocs) directly related to the free energy of nanoparticle adsorption at both MCT oil and liquid paraffin–water interfaces.

Freeze fracture-SEM of silica-coated oleylamine-stabilised liquid paraffin-in-water emulsions (inset image) showed scattered particle flocs at the droplet surface in line with low magnitude of the attachment energy (E < 500 kT) and poor physical stability.Figure optionsDownload high-quality image (81 K)Download as PowerPoint slideHighlights
► This work aims to investigate synergistic stabilisation of paraffin oil-in-water emulsions using conventional surfactants and hydrophilic silica nanoparticles.
► The attachment energy of silica nanoparticles at the paraffin oil (dissolved lecithin)–water interface was calculated to be 900–4500 kT.
► The attachment energy of silica nanoparticles at the paraffin oil (dissolved oleylamine)–water interface was calculated to be 250–550 kT.
► Calculated attachment energies correlated well with the long-term emulsion stability, which is significantly higher for lecithin/silica stabilised emulsions.