This article reports on the catastrophic phase inversion of Pickering emulsions. Experiments were conducted in a stirred glass vessel where phase inversion was detected by an abrupt change in the emulsion conductivity as the original dispersed phase (water) concentration was increased. The system studied consisted of paraffin oil or toluene, water and HDK-H30 silica particles with a primary particle size of 2-30 nm. The droplet size distribution of the Pickering emulsion at concentrations around the inversion point was measured using Pulse Field Gradient (PFG) NMR. In agreement with the work of Binks [1], the droplet size distribution was consistently bimodal for water-in-oil emulsions and unimodal for the inverted oil-in-water emulsion. Our experiments show that the morphology of Pickering emulsions is unaffected by the viscosity of the oil phase. There is no swelling of the droplet and no restricted diffusion of the continuous phase as inversion is approached, both indicating that multiple emulsions are not formed around the inversion point.

This study represents the first application of PFG NMR to Pickering emulsions and provides insight into the emulsion inversion process.