Investigation of multiple emulsion stability using rheological measurements

The stability of water-in-oil-in-water (W/O/W) multiple emulsions was investigated using rheological measurements, droplet size analysis and optical microscopy. Both steady state and oscillatory measurements were carried out simultaneously. The W/O/W emulsion was prepared using a lipophilic surfactant mixture (triglycerol triricinoleate and sorbitan monooleate) and an ABA block copolymer of poly(ethylene oxide)—poly(propylene oxide) (Synperonic PEF127). The composition of the lipophilic surfactant mixture was investigated and it was shown that the triglycerol triricinoleate alone or with small additions of sorbitan monooleate gave the best stability. The effect of the Synperonic PEF127 concentration was investigated using rheological measurements as well as droplet size analysis. This showed that the PEF127 concentration should not exceed 1.2%; otherwise some oil-in water emulsion droplets are produced within the multiple emulsion. Optical microscopy investigations confirmed the results obtained using rheology and droplet size analysis. A multiple emulsion with optimum composition of surfactants remained stable for 223 days at room temperature. The results of these investigations demonstrated that very stable multiple emulsions could be produced provided the system is optimised. The rheological measurements provided a powerful tool to investigate the stability without the need of diluted systems.