Formulation of O/W/O double emulsion model system and its instability mechanism

Abstract: Oil-in-water-in-oil (O/W/O) double emulsions are considered as an advanced design route for oil structuring that achieves the co-delivery of water-soluble and oil-soluble nutrients. However, the O/W/O structure is generally thermodynamic unstable and is prone to rupture during storage or when exposed to environmental stress. In this study, O/W/O double emulsion model systems with commercial surfactants, Tween 80 and PGPR, were formulated using a traditional two-step method for investigating its instability mechanism. By adjusting 1^st and 2^nd homogenization parameters, O/W/O emulsions with different inner oil droplet sizes (0.38-1.2 um) and outer water droplet sizes (0.79-2.20 um) were acquired using Mastersizer and Cryo-SEM. Results showed that all O/W/O emulsions exhibited shear-thinning properties and their migration rate of inner oil (MRIO) increased from ~15% to ~75% after 2-month storage at 20^oC, demonstrating that these model O/W/O emulsions are extremely unstable.

Then, we proposed possible instability mechanisms and verified them based on the interfacial rheology and micellar formation. It was found that residual Tween 80 in the intermediate water phase could decrease the interfacial tension of the PGPR film from 6.72 mN/m to 3.04 mN/m within 30 min, the reason of which was that the low molecular weight of Tween 80 allowed for its fast adsorption onto the PGPR interface. Further, the interfacial moduli were measured as a function of amplitude and frequency using a modified drop shape tensiometry allowing external phase exchanger. It was observed that both elastic modulus Ed’ and viscous modulus Ed’’ reduced significantly after adding Tween 80, suggesting that weak mechanical properties of the PGPR/Tween 80 film might play a critical role in the instability of O/W/O double emulsions. Additionally, the MRIO when using 6% Tween 80 was much higher than that using 3% Tween 80 after 40-day storage, demonstrating that the formed Tween 80 micelle in the intermediate water phase was another reason to cause the instability of O/W/O double emulsions.

Taken together, this study provides fundamental information on the stability of pourable O/W/O double emulsions and thus paves a solid way for enhancing their stability and upscaling their applications in the food industry.