Interfacial Rheology as a Tool to Characterize the Dynamics of Spontaneous Emulsification

Water-in-oil emulsions are critical in various fields, including food, agriculture, personal care, and pharmaceuticals. In some situations, spontaneous emulsification occurs in emulsions with high concentrations of oil-soluble surfactants, in which the parent water drops fragment into finer droplets, forming a network near the interface, which exhibits interfacial elasticity. This study investigates this phenomenon using a water/Span 80-paraffin oil system. We measured interfacial shear elasticity and used microscopy to capture the dynamics. The time of onset of the network depends on the contact time between the two liquid phases and the Span 80 concentration, scaling inversely with the square of the bulk concentration of Span 80. Beyond the time of onset of network formation, different trends are observed, depending on the concentration of Span 80. At higher concentrations, the interfacial elasticity shows a maximum, followed by a minimum. This oscillatory trend in the interfacial elasticity is an outcome of the formation of multiple layers of water droplets at the interface. The existence of multiple layers is also observed through an optical microscope. In a more viscous paraffin oil, both the emulsification rate and the elasticity peak are reduced. No spontaneous emulsification is observed when the paraffin oil is replaced with silicone oil. Moreover, spontaneous emulsification is suppressed by the addition of a salt. On the contrary, the addition of Tween 80 in water increases the rate of spontaneous emulsion, resulting in larger droplets and a denser emulsion layer. However, the drops form a very weak network, as indicated by a negligibly small interfacial elasticity.