Size dependent droplet interfacial tension and surfactant transport in liquid-liquid systems, with applications in shipboard oily bilgewater emulsions

Many liquid-liquid emulsions, including shipboard oily bilgewater (oil-in-water) and water entrained in diesel fuels (water-in-oil), are chemically stabilized by surfactants and additives and require treatment to destabilize and separate. The interfacial tension (IFT) of surfactant-laden interfaces between the continuous and dispersed phase, as well as the size of the dispersed droplets, are significant factors in determining emulsion stability. In particular, the timescale associated with a dynamic change in IFT due to surfactant transport is indicative of how fast the emulsion will stabilize. In the present work, the dynamic IFT of droplets at micro-scale (∼80 μm) and milli-scale (∼2 mm) is measured with simulated bilgewater with soluble surfactant systems. It is found that the IFT of micro-scale droplets decays faster than that of the milli-scale droplets due to smaller diffusion boundary layer thickness. The change in IFT was also studied for water-soluble surfactants added into the dispersed phase and continuous phase for both milli- and micro-scaled droplets. The results show that the IFT of micro-scale droplets decreases to the equilibrium value faster when the surfactant is in outer phase than in the inner phase, while the IFT does not change significantly for the milli-scale droplets. The observations are explained by the change in diffusion limited to kinetic limited surfactant transport. Finally, the surfactant diffusivities, adsorption and desorption rate constants are calculated using Langmuir's equation. The results presented here provide insight into the fundamental mechanism of the surfactant transport and helps improve mitigation strategies of oil-water emulsions.