Pickering Emulsions are dispersions of droplets of one liquid phase in another immiscible liquid phase for which the stabilising substance is a fine solid dispersion. They are encountered in a range of industries including pharmaceuticals and petroleum. With respect to upstream oil production, Pickering emulsions are often encountered as mixtures of oil and water and are increasingly produced with silt acting as the stabilising agent. Inversion of these water-in-oil emulsions is important as phase separation is more rapid for oil-in-water emulsions. Here we consider the inversion of model Pickering emulsions, using magnetic resonance techniques to measure the emulsion droplet size distribution around the inversion point.

Molecular self-diffusion measurement using Pulse Field Gradient Nuclear Magnetic Resonance (PFG-NMR) has commonly been used to obtain the droplet size distribution of emulsions (1). This method relies on the principle of restricted self diffusion of the molecules within the droplet, resulting in less NMR signal attenuation during a PFG experiment for emulsions in comparison with the corresponding pure liquids. This reduced signal loss is interpreted using Tikhonov regularisation techniques to render the droplet size distribution.

This study reports on the catastrophic phase inversion of model 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 gradually 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 PFG NMR. In agreement with the work of Binks and Lumsdon (2), where laser scattering was applied to diluted Pickering emulsions, the droplet size distribution was consistently bimodal for a water-in-oil emulsion and unimodal for the oil-in-water emulsion following phase inversion. However no swelling of the droplets was evident as the inversion point was approached and no significant restricted diffusion behaviour was observed for the continuous phase. These both suggest that no multiple emulsion formation occurred during the inversion process.

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

References

[1] K. G. Hollingsworth, M. L. John, J. Colloid Interface Science 258 (2003) 383

[2] B. P. Binks, S. Lumsdon, Langmuir 16 (2000) 2539