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

Abstract

Using 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.

Low‐Field High‐Resolution PFG‐NMR to Predict the Size Distribution of Inner Droplets in Double Emulsions

“…[9][10][11] This method measures signal attenuation arising from free and restricted self-diffusion of molecules, and this last one is dependent on the size of cavities. [5][6][7][12][13][14][15] However, a mathematical model is needed to predict the DSD from the measured signal attenuation. Several modeling approaches were suggested to relate the measured signal attenuation to the selfdiffusion coefficient within spherical cavities, including: Short Gradient Pulse (SGP), [16] which assumes that no motion of the spins occurs during the time that the field gradient pulses are applied, Gaussian Phase Distribution (GPD), [17] which assumes Gaussian distributed phases under the field gradient, and Block Gradient Pulse (BGP) approximation which is based on the solution of the Bloch-Torrey equation under steady gradient.…”

“…This observation can be related to the higher Laplace pressure of smaller droplets. Vermeir et al [14,23] found that exchange occurs from the inner water phase to the oil phase while diffusion to the external water phase was negligible during the NMR measurement.…”

“…The self-diffusion coefficient, D, was approximated for pure water to be 2.5 × 10 −9 m 2 s −1 at 28.5°C at which the NMR measurements were realized. [36] Assuming the system to have two water exchanging components, one is free diffusion and another restricted, the water signal gives rise to a quasi bi-exponential decay as a function of q² (with q = g ) and diffusion delay Δ: [14,20]…”

Low‐field NMR for quality control on oils

“…Multiple emulsions were also measured. Quantification and interpretation of data are highly demanding for this type of emulsions …”

Fat crystals: A tool to inhibit molecular transport in W/O/W double emulsions

“…Taking into account that oil molecules significantly experience restricted diffusion due to a fat crystal network with a minimal solid fat content (SFC) of 50%, it seems reasonable to assume that water and solute molecules will experience comparable SFC‐dependent restrictions on diffusion when travelling through the fat phase. In case of bulk crystallization in the oil phase of an osmotically balanced W/O/W double emulsion, an increasing SFC has been noted to reduce the release rate of encapsulated marker compounds and to reduce the water exchange kinetics between the water compartments . Herzi and Essafi recently demonstrated that the release of magnesium ions was dependent on the SFC provided that the oil phase of the W/O/W globule consisted of a continuous crystal network.…”