PREDICTION OF THE FREE-ENERGY OF DILUTE AQUEOUS METHANE, ETHANE, AND PROPANE AT TEMPERATURES FROM 600 TO 1200-DEGREES-C AND DENSITIES FROM 0 TO 1 G CM(-3) USING MOLECULAR-DYNAMICS SIMULATIONS

Molecular dynamics simulations of united atom Lennard-Jones model for methane, ethane, and propane in TIP3P water have been used to estimate the chemical potentials of aqueous methane, ethane, and propane from 600 to 1200 degrees C and densities from 0 to 1 g cm(-3). Errors in th e simulation itself are negligible. Estimates of the errors in the pre dictions due to the inadequacy of the models show that this method of prediction gives reasonable accuracy. The calculations are not sensiti ve to the dipole moment of the water molecule or to the well depth (ep silon) of the water-methane interaction. The diameter (sigma) of the w ater-methane interaction is the most important parameter. The present predictions for methane are compared with a variety of other methods f rom the literature. An equation with seven adjustable parameters is pr esented which fits all of the simulation results as a function of temp erature, density of water, and number of carbon atoms. This equation s hould allow reasonable extrapolations to predict the properties of but ane, pentane, and hexane. An even simpler equation with only two adjus table parameters is able to fit all of the experimental data in this t emperature and density region if the calculated solute-water second vi rial coefficient fur this model is used. The fit is not quite as accur ate as with the seven-parameter equation, but this equation should be useful for predictions of normal and branched hydrocarbons, Equations of state can give good predictions which are more accurate than those of Shock et al. (Giochem. Cosmochim. Acta 1989, 53, 2157; 1990, 54, 91 5. J. Chem. Sec., Faraday Trans. 1992, 88, 803), but use of molecular dynamics simulations to predict the properties of aqueous nonelectroly tes at very high temperatures is the method of choice for making these predictions.