Characterization of ionic liquid-containing Pickering emulsions emulsified by hydrophobic fumed silica

A Pickering emulsion is a dispersion of two immiscible liquids stabilized by solid particles anchored at the liquid-liquid interface. Nowadays, ionic liquid-containing Pickering emulsions are playing a more and more important role in catalysis to immobilize organic acid or base (dissolved in ionic liquids) within emulsion droplets to tune the reactivity of these catalysts and increase the selectivity. In this study, some ionic liquids commonly used for catalytic applications have been selected, characterized from a physico-chemical point of view (density, viscosity, interfacial tension, contact angle) and used to manufacture ionic liquid-in-heptane emulsions (IL/H). The main parameters studied are the influence of the nature of the cation (1-butyl-3-methylimidazolium cation [BMI] and N-butyl-N-methylpyrrolidinium cation [BMPyrr]) and of the anion (bis(trifluoromethylsulfonyl)imide anion [NTf2], hexafluorophosphate [PF6], tetrafluoroborate [BF4], trifluoromethanesulfonate [CF3SO3]), as well as the quantity of silica nanoparticles and the volume fraction of the dispersed phase. The type of emulsion, the size of the drops and the stability to coalescence were systematically determined and analyzed according to the physico-chemical properties of the systems. It is shown that it is possible to manufacture stable IL/H emulsions up to a volume fraction of 30–40% depending on the systems. The main parameter to take into account is the contact angle of the nanoparticles towards the ionic liquid used. This study provides a solid basis for the development of stable emulsions that can be further encapsulated for the implementation of continuous catalytic reactions in a two-phase system.