Influence of the inner droplet fraction on the release rate profiles from multiple W/O/W emulsions

Water-in-oil-in-water (W/O/W) double emulsions were prepared and the kinetics of release of magnesium ions from the internal to the external water phase was investigated as a function of the inner droplet volume fraction, at constant temperature (T = 25 °C). All the emulsions were formulated using Miglyol as the oil phase and the same surface-active species, namely polyglycerol polyricinoleate (oil-soluble) and sodium caseinate (water-soluble). Also, the internal droplet and oil globule diameters were almost identical for all systems under study. The external aqueous phase was a lactose solution at approximately the same osmotic pressure as that of the inner droplets, in order to avoid osmotic water transfer phenomena. We probed two different initial configurations in terms of inner droplet composition: (i) “one-component” systems where all droplets were composed of the same MgCl2 solution, and (ii) a “two-component” systems, where droplets containing MgCl2 and lactose coexisted. In all cases, coalescence of the inner droplets on the oil globule surface was marginal so that magnesium leakage was mainly due to diffusion phenomena. The rate of magnesium release was lowered by increasing the inner droplet fraction, regardless of the initial configuration. Assuming that the transfer of the encapsulated species mainly occurs via thin liquid films, such evolution could reflect the increasing tortuosity of the diffusion pathways with the inner droplet fraction.