Does Equilibrium Interfacial Tension Depend on Method of Measurement?

In the presence of surfactants, the equilibrium interfacial tension (IFT) between two immiscible fluids can be very dependent on the particular technique used for measurement. This is because the partitioning of surfactants between the two bulk phases and the interface, which ultimately determines IFT, depends on geometric factors such as the volume fractions and specific interfacial areas (i.e., interfacial areas per unit volume) of the two fluids. In this work, the effect of surfactant partitioning on equilibrium IFT is demonstrated both theoretically and experimentally. A novel technique which enables direct measurement of IFT at macroemulsion droplet surfaces, with direct application to emulsion research, is demonstrated.