Molecular Simulation Study of Oil Hydrocarbons, Dispersants and Other Organics in Atmospheric Air/Water Interfaces

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Zhang, Zenghui
Molecular Dynamics; Air/Water Interface; Surfactant; Atmospheric Organics
thesis / dissertation description
The main objective of this dissertation is to study the interfacial properties of organic contaminants, in particular green leaf volatiles (GLVs), oil hydrocarbons [mainly intermediate-volatile organic compounds (IVOCs) and semi-volatile organic compounds (SVOCs)], as well as surfactant species (i.e. DOSS and Span 80) from Corexit dispersants at atmospheric air/water and air/seawater interfaces. All these compounds can significantly contribute to the formation of secondary organic aerosols (SOAs). Furthermore, our simulation work on IVOCs, SVOCs and Corexit surfactants, combining with experiments from the Valsaraj group, suggests that these compounds can be adsorbed on the surface of seawater droplets and aerosolized into the atmosphere. These results suggest that sea-spray aerosols are a significant transport pathway for oil spill matter into the atmosphere, which has not been studied in detail until our studies. Classical molecular dynamics (MD) simulations along with experimental research were conducted to investigate the characteristics of those organic compounds at water and seawater interfaces with air. Our free energy results through potential of mean force (PMF) calculation show good agreement with experimental results of the 1-octanol/water partition coefficients for the four GLVs we studied (i.e. 2-methyl-3-buten-2-ol, methyl salicylate, cis-3-hexen-1-ol and cis-3-hexenylacetate). Furthermore, our PMF results indicate that these GLVs have deep free energy minima at the air/water interfaces, which implies that these compounds have a thermodynamic preference to remain at the air/water interfaces. Likewise, previous studies also suggest that common atmospheric oxidants also prefer to remain at the surface of water droplets. Overall, these results suggest that the air/water interface is the most likely reaction site for GLVs and other atmospheric oxidants. The interactions between n-alkanes from oil and Corexit surfactants near air/seawater interfaces were also studied. PMF calculations show that the n-alkanes, Span 80 and DOSS exhibit strong preferences to stay at the air/seawater interfaces. Our results suggest that n-alkanes have deeper free energy minima when Span 80 is present at the interface, as compared to when DOSS or no surfactants are present. Therefore, these species are likely to be ejected to the atmosphere adsorbed on the surface of seawater droplets, which form in significant quantities in the sea surface.