Effect of molecular exchange on water droplet size analysis as determined by diffusion NMR: The W/O/W double emulsion case

• A multi-nuclear diffusion analysis of W/O/W emulsions was performed using diffusion NMR.
• The water diffusion signal was recorded by low and high-resolution NMR.
• The water-soluble marker diffusion signal was recorded by high-resolution NMR.
• Signal comparison enables the evaluation of molecular exchange.
• The accuracy of the estimated water droplet size of W/O/W emulsions was evaluated upon signal analysis.

HypothesisThe accuracy of the inner water droplet size determination of W/O/W emulsions upon water diffusion measurement by diffusion NMR was evaluated. The resulting droplet size data were compared to the results acquired from the diffusion measurement of a highly water soluble marker compound with low permeability in the oil layer of a W/O/W emulsion, which provide a closer representation of the actual droplet size. Differences in droplet size data obtained from water and the marker were ascribed to extra-droplet water diffusion.ExperimentsThe diffusion data of the tetramethylammonium cation marker were measured using high-resolution pulsed field gradient NMR, whereas the water diffusion was measured using both low-resolution and high-resolution NMR. Different data analysis procedures were evaluated to correct for the effect of extra-droplet water diffusion on the accuracy of water droplet size analysis.FindingsUsing the water diffusion data, the use of a low measurement temperature and diffusion delay Δ could reduce the droplet size overestimation resulting from extra-droplet water diffusion, but this undesirable effect was inevitable. Detailed analysis of the diffusion data revealed that the extra-droplet diffusion effect was due to an exchange between the inner water phase and the oil phase, rather than by exchange between the internal and external aqueous phase. A promising data analysis procedure for retrieving reliable size data consisted of the application of Einstein’s diffusion law to the experimentally determined diffusion distances. This simple procedure allowed determining the inner water droplet size of W/O/W emulsions upon measurement of water diffusion by low-resolution NMR at or even above room temperature.

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