(691a) 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.