The role of surfactants in stabilizing liquid-liquid interfaces: their effect on interfacial tension and film drainage times

Liquid-liquid separation is important in different emulsion systems, such as oily bilgewater and water in fuel, and is mainly achieved by coalescence. Effective removal of the dispersed droplets requires understanding of the effect of surfactant presence in these emulsions on their stability. In this work, we highlight this using measurements done across a range of surfactant concentrations, viscosity ratios, and velocities. The first factor studied is the interfacial tension (IFT), where dynamic IFT measurements are performed at two length scales: a millimeter scale using pendant drop experiments and a microscale using microfluidic tensiometry, with systems of light and heavy mineral oil containing SPAN80 surfactant. It was found that the rate of IFT decay decreases with increasing viscosity ratio of the outer to the inner phase. The surfactant diffusivity and interfacial adsorption rates are extracted by fitting a surfactant diffusion equation and an equation of state to the IFT data. The second factor studied is the film drainage time between two coalescing water droplets in oil as well as fuel systems, which is found to increase with decreasing timescales to reach equilibrium IFT.