(125e) Interfacial Rheology of Oil-Water System Undergoing Spontaneous Emulsification

Spontaneous emulsification (SE) finds numerous industrial applications such as drug encapsulation, enhanced oil recovery, personal care products etc. SE involves formation of emulsion even at moderate interfacial tension and with almost no energy input. This phenomenon is driven by migration of surfactant molecules within the system.

Paraffin oil containing a non-ionic surfactant Span 80 was used as organic phase. When this organic phase at concentrations above critical micelle concentration, is brought into contact with water, it results in the spontaneous formation of water droplets at the interface. These droplets present at the interface form a network and impart elasticity to the interface. Formation of the interfacial film is a time dependent phenomenon and to understand time scales involved in this, we have performed temporal interfacial rheology.

Microscopic investigations of the interface also reveal how the structure evolves at the interface. Interfacial elasticity was measured as a function of time as well as surfactant concentration using interfacial shear rheometer equipped with bicone geometry. As the concentration of surfactant is increased, smaller droplets are formed, resulting in higher elasticity. If we keep on increasing concentration of surfactant, formation of multiple planes of the droplets occurs and its signature is observed in interfacial elasticity. Hypothesis of formation of multiple planes is also supported by the microscopic studies performed at the interface. Furthermore, how addition of salt affects the dynamics of this phenomenon was explored. It was observed that appropriate amount of salt addition can restrict the formation of droplets and thereby elasticity.

Over the last few decades, significant efforts have been made to understand the mechanism of spontaneous emulsification but it is still not very clear (López-Montilla et al.). A mechanism based on hydration of surfactant molecules is also proposed in this study.