(335c) Transferable Step Potentials For Nitrates, Phosphates, And Acids

The Step Potential Equilibria And Discontinuous Molecular Dynamics (SPEADMD) model provides a basis for molecular modeling of thermodynamic and transport properties. It is based on Discontinuous Molecular Dynamics (DMD) and second order Thermodynamic Perturbation Theory (TPT). DMD simulation is applied to the repulsive part of the potential, complete with molecular details like interpenetration of the interaction sites, 110 degree bond angles, branching, and rings.1,2 The thermodynamic effects of disperse attractions and hydrogen bonding are treated by TPT. This approach accelerates the molecular simulations in general and the parameterization of the transferable potentials in particular. Transferable potentials have been developed and tested for over 250 components comprising 25 families. These families include thiophene, fluorocarbon, alcohol, amine, aromatic, and ring compounds to name just a few examples.3,4 Methods of predicting the thermodynamic properties are especially valuable when properties are difficult to measure, as for chemicals that are explosive or toxic. This is the case for many nitrates and phosphates.

Nitrates are of special interest in defense and mining applications. New compounds are constantly being sought to enhance stability and reduce toxicity. Being able to leverage a few measurements for a few compounds and predict the properties of isomers and larger molecules is especially valuable. For environmental applications the octanol-water partition coefficient and vapor pressures are of particular interest. We have simulated all the nitrate compounds in the DIPPR database and characterized the interactions for a wide range of molecular structures. Vapor pressures are generally accurate to 15% error.

Phosphates have traditionally played roles as pesticides and nerve poisons. Very few data are available. Nevertheless, we have characterized the phosphate interactions and find accuracy to be near 15% for vapor pressure where data are available. Correlations for octanol-water partition coefficients and preliminary results for correlation of thermochemical properties based on quantum simulation are also presented.

Although the amount of experimental data is relatively large, and the phenomena are qualitatively understood, carboxylic acids present an especially challenging case study for potential modeling. The small acids exhibit saturated vapor compressibility factors approaching one-half in the low temperature, low pressure limit. But they are soluble in both oil and water. These observations necessitate a model that primarily accounts for strong dimerization, but also allows for solvation by other hydrogen bonding species like water. Therefore the acid interaction potential cannot be developed from pure component acid behavior alone. We present a model that matches the saturated vapor density and saturation pressure as well as the acetic acid+water VLE. The transferable potential model is applied to vapor pressure, density, and VLE of carboxylic acids ranging from C1-C8.

Keywords: Nitrates, phosphates, environmental, vapor pressure, octanol-water partitioning.

Reference: (1) Cui, J.; Elliott, J. R. Phase Envelopes for Variable Well-Width Square Well Chain Fluids.J. Chem. Phys. 2001, 114, 7283. (2) Unlu, O.; Gray, N. H.; Gerek, Z. N.; Elliott, J. R. Transferable Step Potentials for the Straight Chain Alkanes, Alkenes, Alkynes, Ethers, and Alcohols.Ind. Eng. Chem. Res. 2004, 43, 1788-1793. (3) Gray, N. H.; Gerek, Z. N.; Elliott, J. R. Molecular Modeling of Isomer Effects in Naphthenic and Aromatic Hydrocarbons.Fluid Phase Eq. 2005, Vol 228-229C, 147-153. (4) Baskaya, F. S.; Gray, N. H.; Gerek, Z. N.; Elliott, J. R. Transferable Step Potentials for Amines, Amides, Acetates, and Ketones.Fluid Phase Eq. 2005, 236, 42-52.