Interfacial parameters for spans and tweens in relation to water-in-oil-in-water multiple emulsion stability

The term effective minimum concentration (EMC) is introduced to describe the minimum amount of surfactant required to formulate a water-in-oil-in-water (W/O/W) multiple emulsion. The EMC value was determined by monitoring W/O/W multiple emulsion stability and yield as a function of surfactant concentration. The concentration of surfactant forming a monolayer at the oil-water interface in a W/O/W multiple emulsion (EMC*) was also calculated from the surfactant surface excess (Gamma; mu mol/m(2)) and the multiple emulsion interface size or specific surface area (A(s); m(2)/dm(3)). At a Span 80 concentration equal to the calculated EMC* value (1% w/w) a high yield of W/O/W multiple emulsion was obtained. In contrast, the formation of a stable multiple emulsion required a Span 80 concentration of (EMC) greater than or equal to 2.5% w/w. The concentration of hydrophilic surfactant (Tween 20) had no effect on the W/O/W multiple emulsion yield or stability. Calculated interfacial parameters for Span 80 and Tween 20 at the W/O/W multiple emulsion interface were different from values at the planar kerosene-water interface. Results are discussed in. terms of the possible structure of the adsorbed surfactant layer at the oil-water interface in a W/O/W multiple emulsion.