Bulk and Interfacial Properties of Brine or Alkane in the Presence of Carbon Dioxide, Methane, and Their Mixture

Ab Initio Characterization of the CO2-Water Interface Using Unsupervised Machine Learning for Dimensionality Reduction

Precise characterization of the supercritical CO2-water interface under high pressure and temperature conditions is crucial for the geological storage of carbon dioxide (CO2) in deep saline aquifers. Molecular dynamics (MD) simulations offer a valuable approach to gaining insight into the CO2-water interface at a microscopic level. However, no attempt has been made to characterize the CO2-water interface with the accuracy afforded by ab initio calculations. In this study, we performed ab initio MD (AIMD) simulations to investigate the structural and dynamical properties of the CO2-water interface, comparing the results with those obtained from classical force-field MD (FF-MD) simulations. Molecular orientation at the interface was well reproduced in both AIMD and FF-MD simulations. Characteristic structural fluctuations of water at the interface were unveiled by applying multidimensional scaling and time-dependent principal component analysis to the AIMD trajectories; however, they were not prominent in the FF-MD simulations. Furthermore, our study demonstrated a marked difference in the residence time of molecules in the interface region between AIMD and FF-MD simulations, indicating that time-dependent properties of the CO2-water interface strongly depend on the description of the intermolecular forces.

Interfacial properties of the brine + carbon dioxide + oil + silica system

Molecular dynamics simulations of the H2O + CO2 + aromatic hydrocarbon and H2O + CO2 + benzene + silica (hydrophilic) systems are performed to gain insights into CO2-enhanced oil recovery (EOR) processes. For comparison purposes, an overview of the previous simulation studies of the interfacial properties of the brine + CO2 + alkane + silica system is also presented. In general, the water contact angle (CA) of the H2O + CO2 + silica (hydrophilic) system increased with pressure and decreased with temperature. The CAs of the H2O + hydrocarbon + silica (hydrophilic) system are not significantly affected by temperature and pressure. The simulated CAs were in the ranges of about 58°-77° and 81°-93° for the H2O + hexane + silica (hydrophilic) and the H2O + aromatic hydrocarbon + silica (hydrophilic) systems, respectively. In general, these CAs were not significantly influenced by the addition of CO2. The simulated CAs were in the ranges of about 51.4°-95.0°, 69.1°-86.0°, and 72.0°-87.9° for the brine + CO2 + silica (hydrophilic), brine + hexane + silica (hydrophilic), and brine + CO2 + hexane + silica (hydrophilic) systems, respectively. All these CAs increased with increasing NaCl concentration. The adhesion tension of the brine + silica (hydrophilic) system in the presence of CO2 and/or hexane decreased with increasing salt concentration. The simulated CAs were in the range of about 117°-139° for the H2O + alkane + silica (hydrophobic) system. These CAs are increased by the addition of CO2. At high pressures, the distributions of H2O normal to the silica (hydrophobic) surface in the droplet region of the H2O + silica system were found to be strongly affected by the presence of CO2. These insights might be key for optimizing the performance of the miscible CO2 water-alternating-gas injection schemes widely used for EOR.

First-principle calculations of sulfur dioxide adsorption on the Ca-montmorillonite

According to the serious problem of sulfur dioxide pollution, montmorillonite is one of the effective ways in gas pollution control because of its excellent absorption properties. One of the fundamental questions is to fully understand sulfur dioxide absorption mechanism of montmorillonite. In this study, using the first-principle methods, we studied the adsorption characteristics of Ca-montmorillonite in the presence of [Formula: see text]. The adsorption energy and elasticity constants as a function of the adsorption capacity were also studied. The calculated results show that bridge site is the most stable adsorption site for [Formula: see text] with the adsorption energy of - 140 meV. As adsorbent, Ca-montmorillonite is a clay with layer-structure, most of bond lengths(such as Al-O, Mg-O, Si-O, and H-O) does not obviously change. As adsorbed gas, the O-S-O bond angle of adsorbed [Formula: see text] change from [Formula: see text] to [Formula: see text]. The volume and adsorption energies of Ca-montmorillonite almost increase linearly with increasing [Formula: see text] adsorption. By calculating the montmorillonite elasticity constants under different adsorption capacity, we found that the elasticity constant C33 which perpendicular to the crystal face, with the maximum changes from 450 to 326 GPa. In addition, Young's modulus,bulk modulus and shear modulus significantly decrease with the increasing adsorption. The calculated results will not only help to understand the physical and chemical of montmorillonite but may also provide theoretical guidance for dealing with the problem of gas pollution.