Particle self-assembly in oil-in-ionic liquid Pickering emulsions

We have studied polydimethylsiloxane (PDMS)-in-1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) Pickering emulsions stabilized by polystyrene microparticles with different surface chemistry. Surprisingly, in contrast to the consensus originating from oil/water Pickering emulsions in which the solid particles equilibrate at the oil–water droplet interfaces and provide effective stabilization, here the polystyrene microparticles treated with sulfate, aldehyde sulfate, or carboxylate dissociable groups mostly formed monolayer bridges among the oil droplets rather than residing at the oil–ionic liquid interfaces. The bridge formation inhibited individual droplet–droplet coalescence; however, due to low density and large volume (thus the buoyant effect), the aggregated oil droplets actually promoted oil/ionic liquid phase separation and distressed emulsion stability. Systems with binary heterogeneous polystyrene microparticles exhibited similar, even enhanced (in terms of surface chemistry dependence), bridging phenomenon in the PDMS-in-[BMIM][PF6] Pickering emulsions.

Laser scanning confocal images illustrating the formation of heterogeneous particle bridges at oil–ionic liquid Pickering emulsion interfaces. The particle mixtures are (a) amine-treated polystyrene/sulfate-treated polystyrene and (b) carboxylate-treated polystyrene/sulfate-treated polystyrene, respectively. The particles are 1.1 μm in diameter..Figure optionsDownload high-quality image (54 K)Download as PowerPoint slideHighlights
► Oil-in-ionic liquid (IL) Pickering emulsions were investigated.
► Solid particles with certain chemistry bridged the oil droplets but completely avoided oil–IL interfaces.
► The bridge formation surprisingly distressed overall emulation stability.
► Systems with binary solid particles exhibited similar, even enhanced, bridging phenomenon.