Water-in-Oil Microstructures Formed by Marine Oil Dispersants in a Model Crude Oil

DOSS (dioctyl sodium sulfosuccinate), Tween 80, and Span 80, surfactants commonly used in marine crude oil spill dispersants, have been mixed into a model oil at a total surfactant concentration of 2 wt %, typical for dispersant-treated oil slicks. These surfactant-oil blends also contained 0.5-1.5 wt % synthetic seawater to enable formation of water-in-oil (W/O) microstructures. Trends in dynamic oil-seawater interfacial tension (IFT) as a function of surfactant blend composition are similar to those observed in prior work for crude oil treated with similar blends of these surfactants. In particular, Span 80-rich surfactant blends exhibit much slower initial dynamic IFT decline than DOSS-rich surfactant blends in both model oil and crude oil, and surfactant blends containing 50 wt % Tween 80 and a DOSS:Span 80 ratio near 1:1 produce ultralow IFT in the model oil (<10^-4 mN/m) just as similar surfactant blends do in crude oil. At all DOSS:Span 80 ratios, surfactant blends containing 50 wt % Tween 80 form clear solutions with seawater in the model oil. Cryo-transmission electron microscopy (cryo-TEM) and dynamic light scattering (DLS) show that these solutions contain spherical W/O microstructures, the size and dispersity of which vary with surfactant blend composition and surfactant:seawater molar ratio. Span 80-rich microstructures exhibit high polydispersity index (PDI > 0.2) and large diameters (≥100 nm), whereas DOSS-rich microstructures exhibit smaller diameters (20-40 nm) and low polydispersity index (PDI < 0.1), indicating a narrow microstructure size distribution. The smaller diameters of DOSS-rich microstructures, as well as the fact that DOSS molecules, being oil-soluble, can diffuse to a bulk oil-water interface as monomers much faster than any of these microstructures, may explain why DOSS-rich blends adsorb to the oil-water interface more quickly than Span 80-rich blends, a phenomenon which has been linked in prior work to the higher effectiveness of DOSS-rich Tween/Span/DOSS-based oil dispersants.