Monte Carlo Simulations of Fluid Phase Equilibria and Interfacial Properties for Water/Alkane Mixtures: An Assessment of Nonpolarizable Water Models and of Departures from the Lorentz–Berthelot Combining Rules

Monte Carlo simulations in the Gibbs ensemble were carried out to determine the mutual solubilities for water/n-alkane mixtures over a wide range of temperatures, pressures, and alkane chain lengths. Combinations of the popular, nonpolarizable SPC/E, TIP4P, and TIP4P/2005 water models with the TraPPE united atom model for alkanes are explored to represent these mixtures. Significant deviations from the experimental data are observed for vapor–liquid and liquid–liquid equilibria where the errors in the predicted mole fraction often exceed a factor of 2. Utilizing a scoring metric based on the logarithmic deviation in mole fraction from experimental data, we find that the TIP4P water model outperforms the SPC/E and TIP4P/2005 models in predicting the fluid phase equilibria for water/alkane mixtures. Three models with adjusted Lennard-Jones parameters for water–alkane cross-interactions reflecting departures from the Lorentz–Berthelot combining rules are also investigated. Although a large increase in the well depth can yield a negative Gibbs free energy of transfer for water from the vapor to alkane-rich liquid phases, the overall performance appears to worsen compared to the models using the standard combining rules. Using the standard Lorentz–Berthelot combining rules, the models yield interfacial tensions that deviate by about 10% from the experimental data, but a large increase in the water–alkane cross-interactions leads to a significant underprediction of the interfacial tension.