Thermodynamic Properties of Multifunctional Oxygenates in Atmospheric Aerosols from Quantum Mechanics and Molecular Dynamics: Dicarboxylic Acids
Chinghang Tong, Mario Blanco, William A. Goddard III, John H. Seinfeld
Abstract
Ambient particulate matter contains polar multifunctional oxygenates that partition between the vapor and aerosol phases. Vapor pressure predictions are required to determine the gas−particle partitioning of such organic compounds. We present here a method based on atomistic simulations combined with the Clausius−Clapeyron equation to predict the liquid vapor pressure, enthalpies of vaporization, and heats of sublimation of atmospheric organic compounds. The resulting temperature-dependent vapor pressure equation is a function of the heat of vaporization at the normal boiling point [ΔH_(vap)(T_b)], normal boiling point (T_b), and the change in heat capacity (liquid to gas) of the compound upon phase change [ΔC_p(T_b)]. We show that heats of vaporization can be estimated from calculated cohesive energy densities (CED) of the pure compound obtained from multiple sampling molecular dynamics. The simulation method (CED) uses a generic force field (Dreiding) and molecular models with atomic charges determined from quantum mechanics. The heats of vaporization of five dicarboxylic acids [malonic (C_3), succinic (C_4), glutaric (C_5), adipic (C_6), and pimelic (C_7)] are calculated at 500 K. Results are in agreement with experimental values with an averaged error of about 4%. The corresponding heats of sublimation at 298 K are also predicted using molecular simulations. Vapor pressures of the five dicarboxylic acids are also predicted using the derived Clausius−Clapeyron equation. Predicted liquid vapor pressures agree well with available literature data with an averaged error of 29%, while the predicted solid vapor pressures at ambient temperature differ considerably from a recent study by Bilde et al. (Environ. Sci. Technol. 2003, 37, 1371−1378) (an average of 70%). The difference is attributed to the linear dependence assumption that we used in the derived Clausius−Clapeyron equation.
Group Members
Tong, C., Blanco, M., III, W. A. G., & Seinfeld, J. H. (2004). Thermodynamic Properties of Multifunctional Oxygenates in Atmospheric Aerosols from Quantum Mechanics and Molecular Dynamics: Dicarboxylic Acids. *Environmental Science & Technology*, *38*(14), 3941-3949. https://doi.org/10.1021/es0354216