Transferable Intermolecular Potentials for Carboxylic Acids and Their Phase Behavior

Thursday, November 12, 2009: 9:24 AM
Jackson A (Gaylord Opryland Hotel)

Amir Vahid, Chemical and Biomolecular Engineering, University of Akron, Akron, OH
Neil H. Gray, Chemstations Inc., Houston, TX
J. Richard Elliott, Chemical and Biomolecular Engineering, University of Akron, Akron, OH

Transferable step potentials are characterized for 39 carboxylic acids. The reference potential is treated with discontinuous molecular dynamics, including detailed molecular structure. Thermodynamic perturbation theory is used to interpret the simulation results and to provide an efficient basis for molecular modeling and characterization of the attractive forces. Four steps are used for representation of the attractive forces with only the first and last steps varied independently. The two middle steps are interpolated such that each site type is characterized by three parameters: the diameter, σ, the depth of the inner well, ε1, and the depth of the outer well, ε4. The depths of the attractive wells are optimized to fit experimental vapor pressure and liquid density data. Generally, the vapor pressure is correlated to an overall 43% average absolute deviation (%AAD) and the liquid density to 5%AAD. The deviations tend to be largest for the higher molecular weight acids. These deviations are larger than the errors previously encountered in characterizing organic compounds, but carboxylic acids present exceptional challenges owing to their peculiar dimerization behavior. Simultaneous correlation of vapor pressure, vapor compressibility factor, and phase equilibria of water+carboxylic acids place several constraints on the nature of the potential model, with the parameters of the present model representing a reasonable tradeoff. In the other words, our model represents minimal deviations for vapor pressure, vapor compressibility factor and phase equilibria of all acids simultaneously while varying the parameters σ, ε1, ε4, εCC(dimerizing site bonding energy), εAD(acceptor-donor bonding energy), and KHB(hydrogen bonding volume) for the acid O= and OH site types. The present model is characterized by one acceptor and one dimerizing site on the carbonyl oxygen and one acceptor and one donor site on the hydroxyl oxygen. The acceptor and donor are capable of interacting with water while the dimerizing site is not. With this model, the saturated vapor compressibility factor of acids with seven or fewer carbons is near 0.5 while higher carbon ratios lead to a compressibility factor approaching 1.0.

To compensate for the high vapor pressure deviations of the transferable potential model, a correction is introduced to customize the molecule-molecule self interaction energy. This adaptation results in deviations of 3.1% for vapor pressure of the pure acid database.

In order to validate the behavior of the model for carboxylic acids in mixtures, 33 binary solutions were considered. Acids in this database ranged from formic to hexadecanoic. The average absolute deviation in bubble pressure for aqueous acid systems is 4.4%, 10.5% for acid+acid systems, and 4.7% for acid+n-alkane systems without a customized interaction correction. When applying the correction, deviations were 2.4% for aqueous systems, 2% for acid systems, and 2.8% for acid+n-alkane systems.

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