Molecular simulations of benzene and PAH interactions with soot

Molecular mechanics simulations and ab initio calculations were performed to investigate the mechanism of PAH-soot adsorption. Partitioning of benzene, naphthalene, fluorene, phenanthrene, anthracene, pyrene, fluoranthene, benzo[a]anthracene, benzo[k]fluoranthene, benzo[a]pyrene, and benzo[g,h,i]perylene between water and soot was modeled with classical and quantum mechanical calculations in order to determine a method for predicting log(K-d) values. In both cases, the predicted mechanism of adsorption is interaction of the T-electrons in the PAH and soot (i.e., pi-pi van der Waals forces). Solvation energies, the energy difference between the solute in the gas phase and in the model aqueous phase, calculated with molecular mechanics did not follow the observed solubilities of the PAHs. Molecular dynamics simulations overestimate the favorability of PAHs in the aqueous phase. Hence, the partitioning between the aqueous phase and soot does not accurately correlate with observed log(K-d) values. Models of PAH adsorption using structures from molecular mechanics and energies from A initio calculations do produce water-soot partitioning energies that correlate well with observed log(K-d) values. The log(K-d) values for benzene, anthracene, fluorene, and benzo[a]pyrene were predicted based on the correlation between calculated partitioning energies and observed log(K-d) values. Results presented here suggest that partitioning of PAHs onto soot should depend on the size of the PAH, the planarity of PAH molecule, and the aromaticity of the compound. The methodology developed by this research can be used to predict PAH K-d values that have not yet been measured.