(214e) A Molecular Simulation Study of Dispersants and Oil Hydrocarbons At Atmospheric Air/Salt Water Interfaces

The explosion of the Deepwater Horizon (DWH) drilling rig in the Gulf of Mexico in April 2010 led to the second largest oil spill in U.S. territory and caused substantial environmental damage. Around 4.4 million barrels of crude oil were released during the accident, and a large amount of Corexit oil dispersants were used (1.8 million gallons); therefore, a significant fraction of the oil released and the dispersant used during the DWH accident was able to reach the surface of the sea. According to recent studies, the volatile and some of the intermediate volatile organic compounds (VOCs and IVOCs) that reached the sea surface evaporated into the atmosphere and formed aerosols. However, the fate of other oil organics, such as the heavier IVOCs (compounds with 17-18 carbon atoms) and the semi-volatiles (SVOCs, compounds with 19-31 carbon atoms) that reached the sea surface have not been addressed so far. The hypothesis behind our studies is that these heavier IVOCs, the SVOCs and the dispersants that reached the sea surface would be ejected into the atmosphere by phenomena such as white caps (breaking waves) and bubble bursting on the sea surface. Here, we investigated the properties of n-alkanes (C15, C20 and C30) and three model dispersant compounds (representative of species present in the Corexit dispersants used in the oil spill), at the air/salt water interface by using potential of mean forces (PMF) calculations and classic molecular dynamic (MD) simulations. Our simulation results show that the PMF of n-alkanes and model dispersants show deep minima at the air/salt water interface; these minima become deeper as the chain length of the alkanes increase, and as the concentration of dispersant in the system increases. The fact that the n-alkanes and the dispersant have a strong thermodynamic preference to remain at the air/salt water interface suggest that they are more likely to be ejected to the atmosphere by sea surface processes such as breaking waves and bubble bursting. These simulations agree with our experimental measurements of the ejection rates of oil hydrocarbons and Corexit dispersants. These trends imply that aerosolization through bubble-bursting and breaking waves at the sea surface is a likely transport mechanism for the ejection of spilled oil hydrocarbons into the atmosphere.