Effect of molecular exchange on water droplet size analysis in W/O emulsions as determined by diffusion NMR

Abstract

Using NMR diffusometry, the diffusion of water and tetramethylammonium chloride was recorded in order to determine the water droplet size distribution in W/O emulsions. This study aimed at evaluating the effect of extradroplet diffusion of water on the estimated droplet size distribution upon comparison to the real droplet size distribution. The latter originated from the diffusion behavior of the tetramethylammonium cation (TMA+), which is known to have a much lower permeability through the oil phase as compared to water. Whereas both low-resolution and high-resolution pulsed field gradient NMR revealed that the water droplet size overestimation could be reduced selecting either a lower measurement temperature during diffusion analysis, or a smaller diffusion delay value Δ, still comparison to TMA+ diffusion indicated that artefacts were unavoidable even at low Δ and temperature. In order to correct for this extradroplet water diffusion phenomenon, different data analysis methods were evaluated. The previously described Pfeuffer exchange model could only partly compensate for the effect of extradroplet diffusion on the water droplet size determination. On the other hand, accurate water droplet size analysis results were obtained by correcting the experimentally determined diffusion distances based on Einstein’s diffusion law. As such, reliable data could be obtained by low resolution NMR based on water diffusion at or even above room temperature.