Physical stability of water-in-oil-in-water multiple emulsions

Date of Completion

January 2002


Chemistry, Pharmaceutical




The significance of inner and outer phase pressure as well as interfacial film strength on W/O/W multiple emulsion stability were evaluated using microscopy and long-term stability tests. Unique metastable structures that had a “dimpled” appearance were observed upon applying a coverslip to samples under certain conditions, such as lipophilic surfactant concentration and internal phase osmotic pressure. The formation of these metastable structures correlated with the real time instability of the W/O/W multiple emulsions investigated and the interfacial elasticity of the hydrophobic surfactant. The formation of the metastable “dimpled” structures and the long-term stability of the multiple emulsions were dependent on the osmotic pressure of the inner droplets and the Laplace curvature pressure as described by the Walstra Equation. The effect of coverslip pressure on multiple emulsions may be useful as an accelerated stability testing method or for initial formulation screening. ^ The optimal surfactant concentrations for W/O/W emulsions stabilized with Span 83 and Tween 80 were determined to be 20% w/v Span 83 in the oil phase and 0.1% w/v Tween 80 in the continuous phase, respectively. Higher concentrations of Tween 80 had a destructive effect on multiple emulsion stability, which correlates with the observation that interfacial film strength at the oil/water interface decreases as the Tween 80 concentration increases. High Span 83 concentrations increased the bulk storage modulus G (solid-like) values and hence enhanced multiple emulsion stability. ^ Water-in-oil (W/O) emulsion stability is important to the stability of W/O/W multiple emulsions. Droplet growth in W/O emulsions due to Ostwald ripening was determined using Lifshitz, Slezov and Wagner (LSW) theory. The experimental values match theoretical rates calculated using the LSW equation, indicating applicability of the LSW theory to W/O emulsions. Ostwald ripening in W/O emulsions was observed to be less sensitive to oil nature, compared to OM emulsions. This is attributed to the smaller solubility difference of water in hydrocarbons than that of hydrocarbons in water. Deviation from the LSW theory was observed after extended aging of emulsions. A small but noticeable increase in the rates of Ostwald ripening of W/O emulsions was observed when surfactant concentration is higher than its critical micelle concentration. ^