PFG-NMR on W1/O/W2-emulsions: Evidence for molecular exchange between water phases

A major question in the investigation of multiple emulsions of WOW-type is that of stability and ripening, which is often reflected in the droplet-size distribution (DSD). The DSD is an important parameter for judging product properties. Instability in multiple emulsions is often linked to molecular exchange between inner and outer water phases (W-1, W-2), which has to be taken into account. For example, coalescence phenomena and Ostwald ripening affect microscopic and/or macroscopic stability and therefore the multi-dispersity of an emulsion. This is especially important as the inner compartment can act for encapsulating active agents. NMR investigations on emulsions are mainly based on the analysis of diffusion properties of molecules in the different compartments. For determination of the DSD of the inner and outer droplets, it is crucial to know the physical effects influencing the NMR-signal. Depending on these effects, the NMR data are analyzed by different models, depending for example on the occurrence and time scale of molecular exchange between the phases. They are shown to yield correct DSDs. Evidence for the effect of diffusion between phases is given by using the phenomenon of driven diffusion, which allows a direct detection of the exchange by NMR. The exchange is confirmed by confocal laser scanning microscopy.