Molecular Simulation of Oil and Dispersant Components in Interfaces Involving Air, Seawater and Oil

Historically, research on chemical dispersants, their interactions with oil, and the effects that dispersant-oil mixtures have on biota has been funded only after oil spills (e.g., Exxon Valdez, Deepwater Horizon) and has been focused primarily on toxicology. However, studies on molecular scale phenomena are important to develop a fundamental understanding of how the components of chemical dispersants and oil interact at different interfaces, such as the air-seawater and the oil-seawater. Here we first report molecular dynamics (MD) simulations of long n-alkanes from oil (C15, C30) and representative components of the Corexit 9500 dispersant [Span80, sodium bis(2-ethylhexyl) sulfosuccinate (DOSS)], at the air/seawater interface. Our simulations show that the n-alkanes have a thermodynamic preference to stay at the air/seawater interfaces; this tendency is further strengthened if surfactant species from Corexit 9500 are present. These results suggest that oil hydrocarbons and dispersant components are very likely to be ejected to the atmosphere in the surface of water droplets, which are formed by processes such as whitecaps and bubble bursting at the surface of the sea.

We also report simulation results of the partitioning of several Corexit components between model seawater and oil phases. These results are relevant to biodegradation and toxicological studies. In a lab setting, researchers simulate the mixing of fresh oil, seawater, and dispersant using a prescribed lab protocol; the resulting aqueous phase (termed chemically-enhanced water accommodated fraction, or CEWAF) is then separated from the oil phase and used for biodegradation or toxicology exposure studies in the lab. However, our preliminary experimental results suggest that CEWAFs prepared using the prescribed lab protocol are representative of only a small subset of environmental conditions, which do not include oil spills in the Arctic (e.g., Exxon Valdez), oil weathering effects, or the direct injection of Corexit in the turbulence of the Macondo wellhead in the Gulf of Mexico. As part of our initial efforts in this topic, simulation results of the partitioning of Corexit components between model seawater and oil phases will be presented and discussed.