Predicting multicomponent phase equilibria and free energies of transfer for alkanes by molecular simulation

Configurational-bias Monte Carlo simulations in the Gibbs ensemble have been carried out to calculate (i) the boiling point diagram for the binary mixture of n-octane and n-dodecane at a pressure of 20 kPa, (ii) the 366 K isotherm for the binary mixture of supercritical ethane and n-heptane, and (iii) the free energies of transfer from a helium vapor phase at a pressure of 1 atm to an n-heptane liquid phase for the solutes n-pentane and n-hexane. Results are reported for three force fields (OPLS, SKS, and TraPPE) which all employ the united-atom description and Lennard-Jones nonbonded bead-bead interactions; The simulation methodology in conjunction with the TraPPE force field produces results in excellent agreement with experimental data. The simulated boiling points at 20 kPa are 342.7 +/- 1.6 K (n-octane) and 423.8 +/- 1.3 K (n-dodecane) compared to the experimental values of 349.1 and 431.3 K, respectively. For the ethane/n-heptane mixture the pressure dependence of the composition is very well reproduced. Free energies are calculated directly from the partitioning between the coexisting vapor and liquid phases without the need of multistep thermodynamic integration. The calculated free energies of transfer are -13.3 +/- 0.3 kJ mol(-1) (n-pentane) and -16.2 +/- 0.5 kJ mol(-1) (n-hexane), in good agreement with the experimental values of -13.7 +/- 0.1 and -16.6 +/- 0.1 kJ mol(-1). Our simulation methodology and the TraPPE force field have no pressure-or density-dependent parameters and can be utilized to perform calculations for a wide range of physical conditions which are outside the range of validity of most semiempirical solvation models and for which experiments are sometimes difficult to perform, such as phase equilibria at high temperatures and pressures.