Emulsions of fluorinated oils stabilised by fluorinated silica nanoparticles

Stabilisation of perfluorohexane (PFH)-in-water emulsions was investigated using (i) fumed silica nanoparticles having different levels of hydrophobicity and (ii) fluoroalkyl-grafted silica nanoparticles of different degrees of fluorophilicity. Hydrophilic silica nanoparticles failed to stabilise these emulsions, whereas particles with intermediate levels of hydrophobicity gave rise to stable water-in-oil emulsions. A series of fluorinated silica nanoparticles were prepared via a silylation reaction between hydrophilic silica nanoparticles and a fluoroalkyl-monochlorosilane. Elemental analysis and 29Si solid-state NMR on the derivatized particles were carried out and grafting densities of the attached fluoroalkylsilyl (F-silyl) were calculated. A series of fluoroalkyl-grafted silica nanoparticles with a grafting density ranging from about 0.4–1 gps nm−2 was obtained. The type and stability of fluorinated oil and water emulsions stabilised by the prepared fluorinated silica were investigated as a function of the particle fluorophilicity, the particle concentration, the oil type and the oil to water volume ratio. The modified fluorosilica particles are more effective in stabilising perfluorohexane-in-water emulsions than the unmodified ones and the emulsion stability increases as the particle concentration increases. The contact angle of water in air on compressed discs of the fluorosilica particles increases as the degree of fluorination increases. Emulsions of perfluorohexane-in-water stabilised by FS-36 silica particles, having 36% F-silyl, were the most stable. The calculated and the measured contact angles of fluorosilica particles at the PFH-water interface were found to correlate with the observed emulsion types and their stabilities. The current work demonstrates that stable Pickering fluorinated oil-in-water emulsions are achievable using fluoroalkyl-grafted silica nanoparticles