Molecular Dynamics Simulations Study on the Shear Viscosity, Density, and Equilibrium Interfacial Tensions of CO2 + Brines and Brines + CO2 + n-Decane Systems

Interfacial Tensions, Solubilities, and Transport Properties of the H2/H2O/NaCl System: A Molecular Simulation Study

Data for several key thermodynamic and transport properties needed for technologies using hydrogen (H2), such as underground H2 storage and H2O electrolysis are scarce or completely missing. Force field-based Molecular Dynamics (MD) and Continuous Fractional Component Monte Carlo (CFCMC) simulations are carried out in this work to cover this gap. Extensive new data sets are provided for (a) interfacial tensions of H2 gas in contact with aqueous NaCl solutions for temperatures of (298 to 523) K, pressures of (1 to 600) bar, and molalities of (0 to 6) mol NaCl/kg H2O, (b) self-diffusivities of infinitely diluted H2 in aqueous NaCl solutions for temperatures of (298 to 723) K, pressures of (1 to 1000) bar, and molalities of (0 to 6) mol NaCl/kg H2O, and (c) solubilities of H2 in aqueous NaCl solutions for temperatures of (298 to 363) K, pressures of (1 to 1000) bar, and molalities of (0 to 6) mol NaCl/kg H2O. The force fields used are the TIP4P/2005 for H2O, the Madrid-2019 and the Madrid-Transport for NaCl, and the Vrabec and Marx for H2. Excellent agreement between the simulation results and available experimental data is found with average deviations lower than 10%.

Quantitative Predictions and Experimental Validation of Liquid-Vapor Interfacial Tension in Binary and Ternary Mixtures of Alkanes Using Molecular Dynamics Simulations

Liquid-vapor interfacial properties of alkane mixtures present a challenge for experimental determination, especially under conditions relevant to the energy industry processes. Molecular dynamics (MD) simulations can accurately predict interfacial tensions (IFTs) for complex alkane mixtures under virtually any conditions, thereby alleviating the need for difficult and costly experiments. MD simulations with the CHARMM force field and empirical corrections for the IFT and pressure were used to obtain the IFT for three binary mixtures of ethane (with n-pentane, n-hexane, and n-nonane) and a ternary system (ethane/n-butane/n-decane) under a variety of conditions. The results were thoroughly validated against experimental data from the literature, and new original IFT data were collected using the pendant drop method. The simulations are able to reproduce the experimental IFT to better than 0.5 mN/m or 5% on average and within 1 mN/m or 10% in the worst case. IFTs for the studied three binary and ternary alkane mixtures were predicted for wide ranges of conditions with no known experimental data. Finally, using the MD simulation data, the reliability of the widely used empirical parachor model for predicting IFT was reaffirmed, and the significance of the empirical parameters examined to establish an optimal balance between the accuracy and broad applicability of the model.

Interfacial behavior of the decane + brine + surfactant system in the presence of carbon dioxide, methane, and their mixture

Molecular dynamics simulations are carried out to get insights into the interfacial behavior of the decane + brine + surfactant + CH₄ + CO₂ system at reservoir conditions. Our results show that the addition of CH₄, CO₂, and sodium dodecyl sulfate (SDS) surfactant at the interface reduces the IFTs of the decane + water and decane + brine (NaCl) systems. Here the influence of methane was found to be less pronounced than that of carbon dioxide. As expected, the addition of salt increases the IFTs of the decane + water + surfactant and decane + water + surfactant + CH₄/CO₂ systems. The IFTs of these surfactant-containing systems decrease with temperature and the influence of pressure is found to be less pronounced. The atomic density profiles show that the sulfate head groups of the SDS molecules penetrate the water-rich phase and their alkyl tails are stretched into the decane-rich phase. The sodium counterions of the surfactant molecules are located very close to their head groups. Furthermore, the density profiles of water and salt ions are hardly affected by the presence of the SDS molecules. However, the interfacial thickness between water and decane/CH₄/CO₂ molecules increases with increasing surfactant concentration. An important result is that the enrichment of CH₄ and/or CO₂ in the interfacial region decreases with increasing surfactant concentration. These results may be useful in the context of the water-alternating-gas approach that has been utilized during CO₂-enhanced oil recovery operations.

Molecular Dynamics Simulations of the Vapor-Liquid Equilibria in CO2/n-Pentane, Propane/n-Pentane, and Propane/n-Hexane Binary Mixtures

Molecular dynamics (MD) simulations were used to study vapor-liquid equilibrium interfacial properties of n-alkane and n-alkane/CO₂ 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 CO₂/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.