Interfacial Properties of the Hexane + Carbon Dioxide + Brine System in the Presence of Hydrophilic Silica

Abstract

Molecular dynamics simulations were performed to understand the interfacial properties of brine (up to 5.4 mol/kg NaCl) and brine + silica systems in the presence of CO 2 , hexane, and their equimolar mixture under geological conditions. Simulation results of brine + CO 2 , brine + hexane, and brine + CO 2 + hexane systems agree reasonably well with the theoretical results predicted using the density gradient theory based on the cubic-plus-association equation of state (with Debye−Huckel electrostatic term). In all these systems, the interfacial tension (IFT) increases linearly with increasing NaCl concentration. Here, simulated slopes of the NaCl concentration dependence of IFT are about 1.99 mN/(m mol kg −1 ), under all conditions. We observe a negative surface excess for NaCl, which may explain the increase in IFT with increasing NaCl concentration. The contact angle (CA) of H 2 O + CO 2 + silica and brine + CO 2 + silica systems increases with pressure and decreases with temperature. However, the CA of H 2 O + hexane + silica and brine + hexane + silica systems is nearly independent of temperature and pressure. These CAs are not significantly affected by the presence of CO 2 . An important result is that in all investigated systems, the CA increases with increasing salt content. Our simulated CA is in the ranges of 51.4−95.0°, 69.1−86.0°, and 72.0−87.9°for brine + CO 2 + silica, brine + hexane + silica, and brine + CO 2 + hexane + silica systems, respectively. The density profiles indicate that the positively charged hydrogen atom of the surface hydroxyl group attracts Cl − ions to the surface. In all investigated systems, the adhesion tensions decrease with increasing NaCl concentration.

Interfacial Properties of the Nitrogen + Water System in the Presence of Hydrophilic Silica

“…The interfacial properties of the N 2 + H 2 O system can be further understood by use of the Gibbs adsorption equation: − normald γ = prefix∑ i normalΓ i .25em normald μ i where Γ i and μ i are the surface excess and the chemical potential of the i th component, respectively. The surface excess of N 2 relative to H 2 O (Γ N 2 ) is determined from the density profile ,, and shown in Figure S2. In all studied cases, Γ N 2 first increases and then decreases as a function of pressure.…”

“…It is interesting to compare the interfacial properties of the N 2 + H 2 O + silica system to those obtained for, for example, the H 2 + H 2 O + silica and CO 2 + H 2 O + silica , systems under the conditions studied here (Figure S5). The interfacial properties of the H 2 + H 2 O (H 2 + H 2 O + silica) system were similar to those of the N 2 + H 2 O (N 2 + H 2 O + silica) system.…”

“…In this work, MD simulations of the N 2 + H 2 O and N 2 + H 2 O + silica systems are carried out (in the temperature range of 313–448 K and at pressures up to 50 MPa) using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) program . The details of the simulation procedure used in this work have been described in earlier papers. ,,− The N 2 molecule and silica (Q 3 /Q 4 ) are represented by the INTERFACE model (Tables S1 and S2 in the Supporting Information). The silica surface had 2.4 SiOH atoms/nm 2 .…”

“…Carbon capture and storage (CCS) technology is considered to be critical in controlling CO 2 emissions into the atmosphere and combating climate change. − A wide range of adsorbents (e.g., metal–organic frameworks, carbon nanotubes, polymers, clays, silicas, etc. ) − are of interest to CCS applications. The capillary pressure P c is an important factor that affects the storage of CO 2 in deep saline aquifers and can be described using the Young–Laplace equation: ,, P normalc = 2 γ normalW normalG .25em cos nobreak0em.25em⁡ θ R where γ WG is the fluid–fluid interfacial tension (IFT), θ is the contact angle (CA), and R is the pore radius.…”

“…Computer simulation is a powerful tool for unravelling the bulk and interfacial properties at the molecular level. − ,− ,− In this work, the interfacial properties of the N 2 + H 2 O and N 2 + H 2 O + silica systems at high temperatures and pressures are investigated using molecular dynamics (MD) simulations. Furthermore, density gradient theory (DGT) based on the cubic-plus-association (CPA) equation of state (EoS) is used to predict the interfacial properties of the N 2 + H 2 O system.…”