Understanding Droplet Bridging in Ionic Liquid-Based Pickering Emulsions

Understanding Droplet Bridging in Ionic Liquid-Based Pickering Emulsions

Denzil S. Frost, Jared J. Schoepf, Elizabeth M. Nofen, and Lenore L. Dai

We have studied the unique bridging behavior of solid-stabilized oil-in-ionic liquid (IL) and water-in-ionic liquid emulsions with respect to particle concentration, particle size, and droplet phase using a confocal laser scanning microscope.  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 droplets rather than residing at the droplet-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 droplets actually promoted phase separation and distressed emulsion stability   The emulsions exhibited three morphology regimes: (1) single, sparingly covered droplets, (2) bridged clusters of droplets, and (3) fully covered droplets.  The degree of bridging was directly proportional to the total potential bridging area, which can be determined from the particle size and concentration.  A “phase” diagram of this behavior is proposed and shown in Figure 1. This type of emulsion diverges from much of the conventional wisdom of oil-water Pickering emulsions regarding the particle self-assembly onto droplet interfaces and liquid film stability.  While the focus here is the bridging regime, we also report interesting observations, specifically, the deformed oil droplets and the transport of excess solid particles into the water droplets, in the fully covered droplet regime. The work identified new self-assembled particle structure and morphology in solid-stabilized emulsions.

Figure 1  “Phase” diagram of the oil-in-water and water-in-oil Pickering emulsions discussed in this paper.  The regimes are denoted as: Region I, single sparingly covered droplets; Region II, bridged clusters of droplets; Region III, fully covered droplets.  In the third phase, sulfate-treated and carboxylate treated particles are represented in green and red, respectively.