A Monte Carlo simulation study of the interfacial tension for water/oil mixtures at elevated temperatures and pressures: Water/n-dodecane, water/toluene, and water/(n-dodecane + toluene)

Monte Carlo simulations are performed to predict the interfacial tension (IFT) for water/n-dodecane, water/toluene, and water/(50 wt% n-dodecane + 50 wt% toluene) mixtures at a pressure of 1.83 MPa and temperatures ranging from 383.15 to 443.15 K. Simulations for the binary mixtures are performed in the N1N2pNAT ensemble and those for the ternary mixtures are performed in the N1μ2μ3pNAT ensemble. In order to control the composition of the organic phase for ternary mixture simulations, separate reservoirs for toluene and n-dodecane molecules are utilized, and the minimum and maximum numbers of these molecules in the interfacial box are monitored. Identity switch moves are applied to facilitate the sampling of the spatial distributions of the organic molecules. Calculations of the IFT for water/n-decane and water/benzene mixtures are used to benchmark the force fields and to develop mixing rules. The combination of the TraPPE-UA force field and the TIP4P/2005 water model with Lorentz-Berthelot combining rules yields fairly accurate predictions at moderate temperatures for water/n-decane, but appears to slightly overestimate the IFT at elevated temperatures. For the water/benzene mixtures, the TraPPE-EH//TIP4P/2005 combination significantly overestimates the IFT over the entire temperature range. Subsequent modifications of the energetic Lennard-Jones cross-interaction parameters yield good agreement between experimental and simulated IFT data of water/n-decane and water/benzene. Using a preliminary set of modified cross-interaction parameters for the TraPPE-UA/EH//TIP4P/2005 combination, our predictions for the Industrial Fluid Properties Simulation Challenge yield satisfactory agreement with the benchmark data (mean signed errors of −2.2, −5.1, and −0.1 dyn/cm for water/n-dodecane, water/toluene, and water/(50 wt% n-dodecane + 50 wt% toluene), respectively). A new set of modified cross-interaction parameters developed later based on longer simulation trajectories and a larger set of experimental data is found to reduce the mean signed errors to −0.1, −0.6, and +2.5 dyn/cm, respectively, for these three systems. Although a significant enrichment of toluene molecules at the interface is found for the ternary mixture, this enrichment is not sufficient to reproduce the experimental IFT data.