Prediction of Interfacial Structure and Tension of Binary Mixtures Containing Carbon Dioxide

Accurate prediction of solvent flux in sub-1-nm slit-pore nanosheet membranes

Nanosheet-based membranes have shown enormous potential for energy-efficient molecular transport and separation applications, but designing these membranes for specific separations remains a great challenge due to the lack of good understanding of fluid transport mechanisms in complex nanochannels. We synthesized reduced MXene/graphene hetero-channel membranes with sub-1-nm pores for experimental measurements and theoretical modeling of their structures and fluid transport rates. Our experiments showed that upon complete rejection of salt and organic dyes, these membranes with subnanometer channels exhibit remarkably high solvent fluxes, and their solvent transport behavior is very different from their homo-structured counterparts. We proposed a subcontinuum flow model that enables accurate prediction of solvent flux in sub-1-nm slit-pore membranes by building a direct relationship between the solvent molecule-channel wall interaction and flux from the confined physical properties of a liquid and the structural parameters of the membranes. This work provides a basis for the rational design of nanosheet-based membranes for advanced separation and emerging nanofluidics.

Molecular Insights into the Wettability and Adsorption of Acid Gas-Water Mixture

Sequestration of acid gas in geological formations is a disposal method with potential economic and environmental benefits. The process is governed by variables such as gas-water interfacial tension, wetting transition, and gas adsorption into water, among other things. However, the influence of the pressure and temperature on these parameters is poorly understood. This study investigates these parameters using coarse-grained molecular dynamics (CG-MD) simulations and density gradient theory (DGT). Simulations were carried out at 313.15 K and a pressure range of 0-15 MPa. A comparison was made against H2S-water systems to clarify the effects of adsorption on interfacial tension due to vapor-liquid-liquid equilibrium. The predicted H2S-water interfacial tension and phase densities by CG-MD and DGT matched the experimental values well. The adsorption can be quantified via the Gibbs Adsorption function Γ12, which correlated well with the three-phase transition. On the one hand, pressure increments below the three-phase transition revealed a significant adsorption of H2S. On the other hand, above the three-phase transition, the Gibbs Adsorption capacity remained constant, which indicated a saturation of H2S at the water surface due to liquid-liquid equilibrium. Finally, H2S behaves markedly differently in wetting transition, rather than the involved for CO2 to different molecular layers beneath the surface of aqueous solutions. In this respect, H2S is represented by a first-order wetting transition while CO2 presents a critical wetting. Finally, it has also been found that the preferential adsorption of H2S over the H2O interface is greater if compared to that of CO2, due to its strong interaction with water. In fact, we have also demonstrated that CO2 under triphasic conditions strongly influences the wetting of the ternary system.

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

The shear viscosity, density, and interfacial tensions (IFT) of two systems, namely, brine and brine/n-decane, blended with carbon dioxide (CO₂) were investigated via molecular dynamics simulations over broad ranges of temperature, pressure, CO₂ mole fraction, and brine concentration. The operating conditions for the molecular simulations to be studied are similar to the CO₂ geological storage processes. The effects of temperature, pressure, and concentrations on the viscosity and IFT have been investigated and analyzed. All four influencing parameters affect the shear viscosity and IFT. The pressures and temperatures up to 1000 bar and 573 K, respectively, were used for predicting the viscosity and IFT by considering intermolecular interactions, while salinities up to 32 000 ppm and CO₂ mole fractions between 0 and 0.5 were used in the simulations. Comparisons were made between simulated values and the predicted results of an empirical correlation, both against experimental data. Both monovalent and divalent ions and their mixtures were used in the simulations, and the results showed that monovalent ions impose stronger interactions in the solution than divalents. The results have revealed that the supercritical CO₂'s capability to reduce the IFT of the brine/n-decane interface is remarkable, which makes it a promising agent for underground geological injection for enhanced oil recovery. Also, viscosity and density ratio analysis have confirmed the viability of CO₂ storage in deep saline aquifers, where harsh geothermal conditions of high salinities limit the extent of the experiments. The molecular simulation results are in good qualitative agreement with the experimental data available in the literature for the viscosity, density, and IFT.

Interfacial properties of binary mixtures of Lennard-Jones chains in planar interfaces by molecular dynamics simulation

Binary mixtures of fully flexible linear tangent chains composed of bonded Lennard-Jones interaction sites (monomers) were studied using the molecular dynamics simulation in the NVT ensemble. Their interfacial properties were investigated in planar interfaces by direct simulation of an explicit liquid film in equilibrium with its vapor. A method for the calculation of long-range interactions in inhomogeneous fluids was implemented to take into account the potential truncation effects. Surface tension and the pressure tensor were calculated via the classical Irving-Kirkwood method; vapor pressure, orthobaric densities, density profiles, and Gibbs relative adsorption of the volatile component with respect to the heavy component were also obtained. The properties were studied as a function of the temperature, molar concentration of the heavy component, and the asymmetry of the mixture. According to the results of this work, the temperature loses influence on the surface tension, vapor pressure, and Gibbs relative adsorption curves as the molecular length of the heavy component increases. This suggests that the universal behavior observed in pure fluids of Lennard-Jones chains also holds for binary mixtures. The contribution of the long-range interactions turned out to account for about 60%, 20%, and 10% of the surface tension, vapor pressure, and orthobaric density final values, respectively. This contribution was even larger at high temperatures and for large molecules. Strong enrichment of the volatile component at the interface was observed in the asymmetric mixtures. One of these mixtures even showed a barotropic effect at elevated pressures and a class III phase behavior.