Molecular dynamics (MD) simulations were used to study vapor− liquid equilibrium interfacial properties of n-alkane and n-alkane/CO 2 mixtures over a wide range of pressure and temperature conditions. The simulation methodology, based on CHARMM molecular mechanics force field with long-range Lennard-Jones potentials, was first validated against experimental interfacial tension (IFT) data for two pure n-alkanes (n-pentane and n-heptane). Subsequently, liquid−vapor equilibria of CO 2 /n-pentane, propane/n-pentane, and propane/n-hexane mixtures were investigated at temperatures from 296 to 403 K and pressures from 0.2 to 6 MPa. The IFT, liquid and vapor phase densities, and molecular compositions of the liquid and vapor phases and of the interface were analyzed. The calculated mixture IFTs were in excellent agreement with experiments. Likewise, calculated phase densities closely matched values obtained from the equation of state (EOS) fitted to the experimental data. Examination of the density profiles, particularly in the liquid−vapor transition regions, provided a molecular-level rationalization for the observed trends in the IFT as a function of both molecular composition and temperature. Finally, two variants of the empirical parachor model commonly used for predicting the IFT, the Weinaug−Katz and Hugill−Van Welsenes equations, were tested for their accuracy in reproducing the MD simulation results. The IFT prediction accuracies of both equations were nearly identical, implying that the simpler Weinaug−Katz model is sufficient to describe the IFT of the studied systems.

The effects of temperature and pressure on interfacial tension (IFT) of a CO2/n-pentane binary system were investigated using the pendant drop method. To ensure the accuracy and reproducibility of our results, the pendant drop method is first used to measure the IFT of fluid systems whose results are available in the literature and then the validated experimental procedure is applied to determine the IFT of the CO2/n-pentane system as functions of pressure (ranging from 0.18 to 6.07 MPa) and temperature (ranging from 296.15 to 377.55 K). Within the pressure and temperature ranges applied, IFT values varied from 0.94 to 15.2 mN/m, and it is observed that IFT decreases monotonically with isothermally increasing pressure; however, when the temperature increases isobarically, IFT decreases at low pressures but the trend reverses at higher pressures, where IFT shows greater values at higher temperatures than at lower temperatures. Modeling of the experimental IFT results using the parachor equation for the binary mixture is also discussed.

We use the experimental setup and protocols described in Toutouni et al. (Effects of Temperature and Pressure on Interfacial Tensions of Fluid Mixtures. I. CO2/n-Pentane Binary. J. Chem. Eng. Data, 2021, DOI 10.1021/acs.jced.0c01044) to examine the impacts of temperature and pressure on interfacial tensions (IFT) of binary hydrocarbon systems propane/n-pentane and propane/n-hexane. The pressure and temperature vary from 0.2 to 3.5 MPa and 323.15 to 403.15 K for the propane/n-pentane fluid system and from 0.18 to 3.5 MPa and 296.15 to 403.15 K for the propane/n-hexane fluid system. The results indicate that the IFT values of both fluid systems follow the same trends with changes in pressure and temperature. It is observed that when pressure increases isothermally, the IFT values decrease. Furthermore, when increasing temperature isobarically, the IFT values exhibit a decreasing trend at low pressures, but the trend reverses at higher pressures. The measured data also reveal that, at a given pressure and temperature, the IFT value of the propane/n-pentane system is lower than that of the propane/n-hexane system. The difference between the two IFT values decreases as pressure increases isothermally.

The liquid−vapor interfacial properties of hydrocarbons and their mixtures are important factors in a wide range of industrial processes and applications. Determining these properties experimentally, however, is not only practically demanding, but many important properties, such as phase densities and compositions are not directly experimentally accessible, thus requiring the development of theoretical models. Molecular dynamics (MD) simulations, by contrast, are relatively straightforward even for the most complex of mixtures and directly provide all of the microscopic quantities for the studied systems. We have previously applied MD simulations to study the liquid−vapor equilibria of mixtures of hydrocarbons and CO 2 that are particularly relevant to hydrocarbon recovery from geologic formations. In this study, we explore in more detail the robustness of the simulation methods with respect to the choice of the model system parameters, investigate the accuracy of the simulations in determining the key quantities: system pressure and interfacial tension (IFT), and, finally, devise a simple correction for achieving a much closer agreement between the simulated and experimental quantities. We perform extensive MD simulations for three mixtures, propane/n-pentane, propane/n-hexane, and CO 2 /n-pentane, using model systems from 1000 up to 100 000 molecules, and different simulation box dimensions to test for the sensitivity to finite-size effects. The results show that changing the system size and box dimensions does not significantly impact the accuracy of the simulations. Subsequently, we examine the accuracy of the MD simulations in determining the pressure and IFT for two pure hydrocarbon systems, n-pentane and n-heptane. Finally, we propose a simple linear correction formula for the pressures and IFTs obtained from the MD simulations that closely reproduce the experimental values for single components and mixtures of hydrocarbons. Our results enable the MD simulations to provide more accurate and reliable predictions of the interfacial properties, thereby reducing the need for challenging laboratory experiments.

Metrics & MoreArticle Recommendations * sı Supporting Information I n our article we have inadvertently used erroneous values for the calculated pressures, which are generally lower than the correct values. The error does not affect the main results or the conclusions of the study. Figures with the corrected plots are provided below. The Supporting Information, which lists all the numerical values, has also been revised.Additionally, the discussion in the second paragraph of section 3.2 ("IFTs of Binary Mixtures") has to be modified, since the MD simulations no longer appear to underestimate the interfacial tension (IFT) at the low pressures and temperatures. Rather, due to the correct pressure being higher, the IFT under these conditions now appears higher than the experimental values (corrected Figures 4−6), consistent with the results for the pure components (Figure 3). Our conclusions that the comparison is affected by the computed pressure are valid, but the MD overestimates the pressure, rather than underestimating it. The tendency of the CHARMM force field to give high vapor pressures has been noted previously. 1 Likewise, in section 3.3 ("Vapor and Liquid Phase Densities") the mention of the systematic underestimation of the IFT at the end of the second paragraph should be disregarded. With the corrected pressure values the computed densities are in a much better agreement with experiments (corrected Figure 7).