NMR studies of emulsion microstructure approaching the phase inversion point

[Display omitted] •Emulsion droplet size distributions near the phase inversion point were monitored by NMR.•Results are reproducible with two different NMR spectrometers.•Crude oil emulsions showed an exponential droplet size increase as the inversion point was approached.•The data can be quantitatively described by a model with a single free parameter.•No evidence of o/w/o emulsions was observed near the phase inversion point. Bench-top Nuclear Magnetic Resonance (NMR) pulsed field gradient (PFG) methods were used to measure the droplet size distributions (DSD) of various water-in-oil (w/o) emulsions as their water content was increased towards the phase inversion point (PIP). Such concentrated, opaque emulsions are not readily interrogated with alternative droplet sizing techniques. However, monitoring of the DSD as the PIP is approached is needed to help establish how different potential mechanisms, such as divergence of mean droplet size and/or the formation of multiple emulsions, are involved in the inversion process. Two separate NMR spectrometers were used and found to give consistent results for the emulsion DSDs and their evolution as the PIP was approached. An exponential increase in mean emulsion droplet size was observed for various crude oil emulsion samples prepared by sequential droplet phase (water) addition under shear as the PIP was approached, which is consistent with the mechanism of droplet coalescence exceeding droplet break-up. However, for w/o emulsions consisting of a pure hydrocarbon liquid stabilized by artificial surfactants, no change in mean droplet size was observed as the PIP was similarly approached. The NMR data were also analyzed for the presence of multiple emulsions – these were distinctly evident in several samples but were of the local w/o/w type.