Effect of interparticle forces on the stability and droplet diameter of Pickering emulsions stabilized by PEG-coated silica nanoparticles.

HYPOTHESIS: Attractive and repulsive interparticle forces influence the stability and structure of Pickering emulsions. The effect these forces have on emulsion behavior must be better understood to improve Pickering emulsions for subsurface applications, including enhanced oil recovery and aquifer decontamination. Past work demonstrates improved emulsion stability with increasing salinity and reduced electrostatic repulsion, possibly because of interparticle networks. We hypothesize that emulsion stability is similarly improved by reducing interparticle steric repulsion.

EXPERIMENTS: We assessed the effect of interparticle forces on emulsion stability by generating decane-in-water emulsions. We used polyethylene glycol (PEG)-coated silica nanoparticles with different diameters, surface modification, and salinities to modify either vdW, steric, or electrostatic interactions. We measured emulsion stability using centrifugation, imaged emulsion droplets with optical microscopy, and analyzed images with ImageJ to calculate droplet diameters.

FINDINGS: Mildly aggregated particles with 0.5-1.0 μmol/m2 surface PEG exhibit the highest emulsion stability. This optimal surface concentration maximizes a trade-off between particle repulsion and aggregation. Droplet diameters are well explained by an energy balance limited coalescence model, generated by solving DLVO equations. We find that while emulsion stability is influenced by interparticle forces, droplet size is dominated by particle-droplet interactions. These results demonstrate the potential of surface modification to significantly improve emulsion stability.