Molecular Dynamics of Interfacial Phenomena at Air/ice, Air/water, and Air/salt Water Interfaces

Abstract

The purpose of this research is to investigate the adsorption of organic contaminants, namely polycyclic aromatic hydrocarbons (PAHs) and green leaf volatiles (GLVs), as well as their interactions with reactive oxygen species (ROSs) on atmospheric air/water and air/ice interfaces. In another series of projects, we studied several intermediate and semi-volatile organic compounds from oil (IVOCs and SVOCs, e.g., alkanes with 17-31 carbon atoms), surfactants and dispersants at air/salt water interfaces. These simulations are relevant to understand the fate of these compounds during the recent 2010 Deepwater Horizon (DWH) oil spill. The adsorption of gas-phase aromatics (benzene, naphthalene and phenanthrene), ROSs (O3, OH, H2O2 and HO2) and GLVs (2-methyl-3-buten-2-ol (MBO) and methyl salicylate (MeSA)) on atmospheric air/water or air/ice interfaces was investigated using classical molecular dynamics (MD) simulations and potential of mean force (PMF) calculations. All aromatics, ROSs and GLVs exhibit a strong preference to be adsorbed at air/water or air/ice interfaces. The adsorption of both naphthalene and ozone onto 1-octanol, 1-hexadecanol or 1-octanal coated air/ice interfaces is enhanced when compared to bare air/ice interfaces. Classical MD simulations were performed to investigate the growth of ice from supercooled aqueous solutions of benzene, naphthalene, phenanthrene, •OH, H2O2, or •HO2. All solutes in the supercooled aqueous solutions are displaced to the air/ice interface during the freezing process at both 270 K. In contrast, only a fraction of benzene, H2O2 and •HO2 molecules become trapped inside the ice lattice during the freezing process at 260 K. Our simulations of oil hydrocarbons (IVOCs and SVOCs) and dispersants at air/sat water interfaces were performed in collaboration with experiments from K. T. Valsaraj’s group. We found that n-alkanes (C15 to C20) exhibit a strong preference to stay at both bare and SDS coated-air/salt water interfaces, as opposed to either staying in the gas phase or being dissolved in bulk of salt water solution. Our results suggest that, from the thermodynamic point of view, n-alkanes have a stronger tendency to remain at the air/salt water interface, and thus are more likely to be ejected to the atmosphere, as their chain length increases, and as the SDS concentration increases.