Transferable potentials for phase equilibria.: 1.: United-atom description of n-alkanes

A new set of united-atom Lennard-Jones interaction parameters for n-alkanes is proposed from fitting to critical temperatures and saturated liquid densities. Configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to determine the vapor-liquid coexistence curves for methane to dodecane using three united-atom force fields: OPLS [Jorgensen, et al. J. Am. Chem. Sec. 1984, 106, 813], SKS [Siepmann, et al. Nature 1993, 365, 330], and TraPPE. Standard specific densities and the high-pressure equation-of-state for the transferable potentials for phase equilibria (TraPPE) model were studied by simulations in the isobaric-isothermal and canonical ensembles, respectively. It is found that one set of methyl and methylene parameters is sufficient to accurately describe the fluid phases of all n-alkanes with two or more carbon atoms. Whereas other n-alkane force fields employ methyl groups that an either equal or larger in size than the methylene groups, it is demonstrated here that using a smaller methyl group yields a better fit to the set of experimental data. As should be expected from an effective pair potential, the new parameters do not reproduce experimental second virial coefficients. Saturated vapor pressures and densities show small, but systematic deviation from the experimental data.