Configurational bias Gibbs ensemble Monte Carlo simulation of vapor-liquid equilibria of linear and short-branched alkanes

We report Gibbs ensemble Monte Carlo simulations of the linear alkane, n-nonane, and its branched isomer, branched 2,6-dimethylheptane; n-eicosane, and its branched isomer 2,6,11,15-tetramethylhexadecane; and an isomer of triacontane, 2,6,10,15,19,23-hexamethyltetracosane (squalane), using the configurational-bias technique to determine the vapor-liquid phase equilibria of these systems. For linear alkanes, quantitative agreement is obtained between simulation and experiment for both critical temperature and density. For short-branched alkanes for which there are experimental data available, the calculated vapor-liquid phase equilibria compare well with experiments. The critical density for branched alkanes studied here from the simulation is in quantitative agreement with experiment, while the critical temperature from simulation is slightly lower. Examination of the vapor-liquid phase equilibria for the applicability of the law of corresponding states shows very minor differences between a linear alkane and its short-branched isomers. In attempting to evaluate the accuracy of the current intermolecular potential model for the equation of state for the branched alkane molecules, we also performed molecular dynamics simulations. The combination of the Gibbs-ensemble and molecular dynamics results suggest that the potential model developed by Siepmann et al. gives reasonable agreement with experiment.