The role of dynamic interfacial phenomena in marine crude oil spill dispersion

Abstract

Marine oil spill dispersants containing the surfactants Tween 80, Span 80, and dioctyl sodium sulfosuccinate (DOSS) have been widely used for decades, but their environmental impact remains controversial. The interfacial science behind their formulation has been studied in this work in order to develop new, equally effective dispersants using nontoxic components. The effectiveness of dispersants containing different Tween 80-Span 80-DOSS (T-S-D) blends was measured using a Stirred Flask Test and correlated with the dynamic oil-water interfacial tension (IFT) produced by each dispersant (Chapter 3). Very low IFT (<10-4 mN/m) was produced by both DOSS-rich dispersants and Span 80-rich dispersants, but DOSS-rich dispersants were significantly more effective and adsorbed to the oil-water interface faster than Span 80-rich dispersants. In order to investigate whether T-S-D dispersants form water-in-oil microstructures which influence dispersants’ interfacial adsorption rates, T-S-D blends were added to a transparent, low-viscosity model crude oil and studied using cryo-transmission electron microscopy and dynamic light scattering (Chapter 4). T-S-D blends formed spherical water-in-oil microstructures in the oil, and the microstructures formed by DOSS-rich T-S-D blends were much smaller than those formed by Span 80-rich T-S-D blends. This may explain why DOSS-rich T-S-D blends adsorb to the interface faster, and thus are more effective, than Span 80-rich T-S-D blends. Blends of Tween 80 and lecithin (L), a biosurfactant which also forms water-in-oil microstructures, were investigated as a substitute for T-S-D dispersants (Chapter 5). The most effective L-T dispersants performed comparably to the most effective T-S-D dispersants in the Baffled Flask dispersant effectiveness test. However, lecithin-rich L-T dispersants were significantly more effective than Tween 80-rich L-T dispersants which produced lower or comparable IFT, even though interfacial adsorption rates of L-T dispersants did not vary as a function of lecithin:Tween 80 ratio. This suggests that interfacial phenomena other than dynamic IFT influence L-T dispersants’ effectiveness. The interface between seawater and crude oil treated with L-T dispersants was therefore studied using light microscopy, cryogenic scanning electron microscopy, and droplet coalescence tests (Chapter 6). Tween 80-rich L-T dispersants caused oil-into-water spontaneous emulsification, indicating rapid dispersant leaching from oil into water. This may explain why Tween 80-rich L-T dispersants are less effective than lecithin-rich L-T dispersants which produce similar IFTs. Conversely, lecithin-rich L-T dispersants exhibited water-into-oil emulsification, indicating that such surfactant blends are stable in the oil and perhaps explaining why some lecithin-rich L-T dispersants are as effective as T-S-D dispersants which produce much lower IFT. Possible mechanisms for the spontaneous emulsification induced by L-T dispersants are discussed, based on images of the spontaneously emulsifying L-T dispersant-treated oil-water interfaces.